An understanding of our atmosphere is fundamental to a pilot,
since it is not just the air we breathe, but the environment we fly in.
This environment can be benign, but equally so very hazardous.
There are many physical processes that work within our atmosphere, and these need detailed analysis.
These processes will be covered in the following lessons,
but for now let us examine what the atmosphere is and what are its main properties.
The word atmosphere is defined as being the gaseous envelope surrounding a celestial body.
In the case of the Earth, it is the collection of gases that are held around the Earth by gravity.
There are well over 20 of these gases, some more abundant than others.
We will concentrate on the four most abundant gases.
By far the most common gas is nitrogen, which accounts for about 78% of the atmosphere.
Oxygen, which is essential to sustaining life and the proper functioning of internal combustion engines,
makes up 21% of the atmosphere.
Just less than 1% is argon, with carbon dioxide making up the rest.
There are many more gases making up our atmosphere,
but these are found in such small quantities that we regard them as trace gases.
These include carbon monoxide, helium, methane, ozone, and hydrogen.
However, one of the most important gases with regards to meteorology is water vapour.
Without water vapour, there would be no weather.
Even though it is found in very small quantities in the atmosphere,
it has by far the greatest effect on our weather.
Clouds and rain are examples of its liquid state,
whereas snow and hail are examples of its solid state.
The relative amounts of the gases stay fairly constant up to about 60 km above the Earth.
Thereafter, gravitational separation alters the composition of the atmosphere.
In order to structure our atmosphere into various segments or layers,
there must be a property of the atmosphere that varies with height.
The variable we use is temperature.
The first of these segments or layers is called the troposphere.
This layer is closest to the Earth,
and its defining characteristic is the marked decrease in temperature with height through the layer.
Because of the effect of gravity, this layer contains over 70% of the total mass of the atmosphere.
The altitude where the temperature no longer decreases with height
defines the upper part of the troposphere.
This upper boundary we call the tropopause.
The thickness of the troposphere, and therefore the height of the tropopause,
is not constant across the Earth.
Over the poles, the height of the tropopause is about 8 km,
whereas over the equator, it is about 16 km.
The average height is 11 km or 36,090 ft.
This is the case at about 45° latitude.
The reason for the height change in the tropopause is simply due to the surface temperature.
Where the Earth is warmest, near the equator, the tropopause is at its highest.
Looking over the poles, where we have very cold air, you can see the tropopause is at its lowest.
We know that temperature decreases with height within the troposphere.
Therefore, the further away the tropopause is, the colder it is.
Over the equator, the tropopause temperature can be minus 80°C,
whereas over the poles, the tropopause temperature is only about minus 40°C.
However, the average temperature is approximately minus 56°C,
which is common in the mid-latitudes, where the tropopause is approximately 11 km.
Having discussed the troposphere and its upper boundary, the tropopause,
we will now look at the next layer in the atmosphere, called the stratosphere.
This layer is characterised by the lack of any significant change in temperature with height.
The stratosphere stops when temperature starts to decrease with height again.
At this point, we find the upper boundary to the stratosphere, which is called the stratopause.
This is usually found at about 50 km above the surface of the Earth.
The temperature of the stratosphere can rise a little in the upper levels due to the presence of ozone gas,
but further explanation on this is outside the scope of the learning requirements.
Returning to our image of the Earth, you can now see the stratosphere and stratopause.
There are several more layers to the atmosphere, but these are outside the learning requirements
for the private pilot's licence, and will therefore not be considered.
Before we leave the structure of the atmosphere,
there is an important point to make about the tropopause that is significant to a pilot.
We stated that the tropopause was where temperature no longer decreases with height.
This causes air to become very stable.
In other words, it limits the vertical movement of air.
This lack of vertical airflow prevents any further cloud development,
and therefore the tropopause signifies the limit of the majority of weather and significant cloud development.
For many purposes, such as calibrations of instruments and testing of aircraft,
an average stage of the atmosphere needs to be represented and modelled.
This is called a standard atmosphere.
In 1964, the International Civil Aviation Organisation, ICAO,
defined the properties of this average standard atmosphere,
and this is referred to as the ICAO standard atmosphere, which is abbreviated to ISA or ISA.
However, the model only extends to 32 km,
which is about 105,000 feet above mean sea level,
so the entire atmosphere is not considered.
ISA assumes that at mean sea level, the temperature is 15°C,
pressure is 1013.25 hPa,
and the density is 1.225 kg per metre cubed.
From mean sea level to 11 km or 36,090 feet,
the temperature decreases by 1.98°C per 1,000 feet or 0.65°C per 100 metres.
From 11 km to 20 km,
the temperature remains constant at minus 56.5°C.
Notice that at 11 km, the temperature no longer falls with height,
and therefore marks the top of the troposphere.
You may remember that we called this point the tropopause.
The layer above this was called the stratosphere.
From 20 km to 32 km,
the temperature rises slowly at 0.3°C per 1,000 feet or 0.1°C per 100 metres.
ISA is only one of several types of standard atmosphere,
but it is the most recognised and the most familiar of them all.
Since all aircraft instruments are calibrated in accordance with ISA,
it is essential to know how much the actual atmosphere on a given day differs from ISA.
This will help quantify errors and improve the accuracy of the readings from these instruments.
The simplest method we use to analyse this difference
is by comparing the real temperature to the standard ISA temperature.
This comparison gives us what is known as the ISA deviation.
In other words, by how much does the real atmosphere deviate from the standard atmosphere?
Let's work through an example to help understand this principle.
In the diagram, we can see an airfield 1,000 feet above sea level.
Remember that in ISA, we stated that mean sea level temperature was 15°C,
and then falls by 1.98°C per every 1,000 feet up to 11 kilometres.
For simplicity, let's assume it is 2°C per 1,000 feet.
Therefore, the temperature at 1,000 feet according to ISA
should be approximately 13°C.
Notice though that on the actual day we are considering,
the real temperature at 1,000 feet is in fact 20°C, which is much hotter than ISA.
A simple calculation tells us that the real atmosphere is 7°C warmer than ISA.
The representation of this information is important.
The best way to understand it is by reading the ISA deviation backwards.
We can say that the real atmosphere is 7°C hotter than ISA.
Let's try another example of an ISA deviation exercise.
Let's say that on another given day, the real temperature at 10,000 feet is minus 15°C.
What is the ISA deviation?
Let's first calculate what ISA says the temperature should be at 10,000 feet.
ISA says that temperature falls by approximately 2°C per 1,000 feet.
So temperature should decrease from sea level to 10,000 feet by 20°C.
But remember that the temperature at sea level in ISA is 15°C.
So 15°C minus a temperature decrease of 20°C up to 10,000 feet
makes the ISA temperature at 10,000 feet minus 5°C.
The real temperature is minus 15°C.
A simple calculation tells us that the real atmosphere is 10°C colder than ISA.
Therefore, the ISA deviation is ISA minus 10.
ISA deviation is used widely in meteorology and aviation in general,
especially when considering aircraft performance and error corrections for instruments.
Because of this, it is important to understand how to calculate ISA deviations.
This concludes the lesson.
Hopefully, we have gained a more detailed view of our atmosphere as a whole.
What will follow will be even greater concentration on the individual elements within the atmosphere,
such as temperature, density, water vapour and pressure.
What will follow?
Study of pressure and pressure variations in the atmosphere is fundamental to an understanding of meteorology.
The forces generated by horizontal pressure differences give rise to horizontal and vertical air movement,
creating winds and clouds.
The vertical variation of pressure gives us the ability to measure the height of an aircraft above a surface.
The instrument that measures this variation is the altimeter.
This lesson aims to give you an understanding of pressure in our atmosphere and its significance to us as aviators.
What is atmospheric pressure?
The air in the atmosphere is made up of molecules.
The atmosphere therefore has mass and, under the force of gravity, exerts a pressure on the Earth's surface.
Put simply, the pressure of the atmosphere at any point is caused by the weight of the column of air
above that point.
Because the force of gravity is greater nearer the centre of the Earth,
most of the mass of the atmosphere is contained in the lower layers near the Earth's surface.
Consequently, air is denser and pressure is greater at the surface than at altitude.
It is important to note that one of the properties of gases is that the pressure they are under is
exerted in all directions.
We will now look at how pressure is measured and represented in meteorology.
The units commonly used to represent pressure are inches of mercury, millibars or hectopascals.
The standard unit being adopted is the hectopascal, but currently the millibar is still used in examinations.
There are two types of instruments used to measure atmospheric pressure.
The aneroid barometer and the mercury barometer.
Changes in atmospheric pressure, that is changes in the force or weight of the atmosphere,
will either depress the mercury in the open reservoir and force more of the mercury up into the tube,
or allow the pressure of the air in the tube above the column of mercury to force the liquid down
the tube into the reservoir.
This displacement is traditionally measured in inches.
The higher the atmospheric pressure, the more mercury is pushed up inside the tube.
The average height of a column of mercury that can be supported by the atmospheric pressure at mean sea level
is 29.92 inches.
This equates to 1013.25 millibars or hectopascals.
The other type of barometer is non-liquid and is called an aneroid barometer.
The aneroid barometer can be less accurate than mercury barometers, but is more sensitive to small changes in air pressure.
An aneroid barometer measures the effective air pressure on a metal chamber from which part of the air has been
removed.
Changes in air pressure make the chamber expand or contract, moving a needle around a dial.
The example shown here is an aneroid altimeter which has a dial calibrated in feet which may indicate height or
altitude.
Other types of aneroid barometers may be scaled in inches, millibars or millimeters.
Shown here is an example of a portable barometer used widely in aviation meteorology to measure pressure at the Earth's
surface.
Let us first analyse the variation of pressure horizontally.
Why, for instance, might we get a high pressure in one area and a low pressure in another?
If on a given day there is a greater amount of air overlying the surface, then the weight of air
will increase and therefore the pressure will increase.
Conversely, if on a given day there is less air overlying a surface, then the weight of air will reduce
and the pressure will decrease.
This gives rise to horizontal surface pressure variations, which are more commonly known as high and low pressures.
Now we need to analyse how we might represent these horizontal pressure variations on a diagram.
This is important because then we will be able to identify if one area on a surface is experiencing higher
or lower pressure than another area.
Because all surfaces are not at the same level, we need to reference our observations to a common horizontal level,
usually mean sea level.
This helps us see exactly what the true horizontal variation in pressure might be.
How are these charts produced though?
Looking at the diagram, we can see various stations reporting their particular mean sea level pressure values.
If we now draw a line connecting all the areas with the same pressure, we end up with a pattern
of lines over the surface.
These lines are called isobars and they join places of equal pressure.
Using these pressure values, we can identify areas of low and high atmospheric pressure.
Lower value isobars indicate a low pressure area.
These systems can range from tens of nautical miles wide to hundreds and even a thousand miles wide.
High value isobars indicate an area of higher pressure.
The isobars themselves can reveal a lot of information.
A force exists that acts from the high pressure to the low pressure.
This force is called the pressure gradient force or PGF for short.
The isobar spacing can tell us the relative strength of this force.
This is important for wind speed analysis.
Closely spaced isobars show a large change in the pressure and therefore a large pressure gradient force.
This is common within low pressures.
Widely spaced isobars show a small change in the pressure and therefore a small pressure gradient force.
This is common within high pressures.
Having worked through horizontal pressure variation, let's now look at how pressure varies vertically.
Earlier on we mentioned that pressure decreases with height in the atmosphere.
In fact, it decreases at a decreasing rate.
In other words, pressure falls very quickly with height near the surface,
but at high altitude the pressure falls much more slowly with height.
This is because most of the air is located close to the Earth's surface.
This is plain to see when the pressure levels are placed onto the diagram as shown here.
However, pressure does not decrease everywhere at the same rate.
This is because temperature affects the pressure lapse rate of the atmosphere.
Cold air has a tendency to cause pressure to change more rapidly with height.
Notice that the pressure lines contract towards the surface.
Warm air, on the other hand, has a tendency to cause pressure to change more slowly with height.
Notice that the pressure lines expand away from the surface.
It is important that you understand this phenomenon because of its significance to altimetry.
The altimeter is calibrated to ISA.
This calibration assumes a pressure change with height in accordance with ISA.
This is shown in the middle of the diagram.
If the pressure were 300 hectopascals or millibars, the altimeter would register a height of 30,000 feet.
However, if the atmosphere were colder than ISA as shown on the left,
the 300 millibar pressure value would be much lower than 30,000 feet.
If the altimeter experiences a 300 millibar pressure, it will read 30,000 feet.
The altimeter is clearly an error. The real height of the altimeter is much less than 30,000 feet.
The altimeter is therefore over-reading.
If the air were warmer as shown on the right, notice this time that the 300 millibar pressure would be
found higher than 30,000 feet.
At this pressure, the altimeter will register 30,000 feet, but it is clearly much higher than this.
The altimeter is therefore under-reading.
In ISA, the pressure change with height at sea level is approximately 1 millibar for every 27 feet.
But since pressure decreases at a decreasing rate, this does change with height.
At 10,000 feet, the rate of pressure change is less, approximately 1 millibar change in pressure every 36 feet.
For ease of calculations, we can say that the average change of pressure with height in the lower atmosphere is
1 millibar for every 30 feet.
This value is useful to use in altimetry when pressure and height calculations need to be quantified.
This ends the lesson on pressure.
The significance of pressure variations horizontally and vertically will be better appreciated in the lessons on pressure systems, wind and
altimetry.
Density is defined as mass of air per unit volume.
Therefore, the density of the atmosphere at any given place may be defined as the mass of air contained in
any given unit of volume.
In other words, how much air is inside a given space.
Density can be expressed in many ways.
One way is by using grams of air per cubic meter, or in standard international units, kilograms of air per
cubic meter.
You may recall from the lesson on the atmosphere that we talked about the ICAO standard atmosphere, or ISA.
At sea level, ISA states that the density is 1,225 grams of air per cubic meter.
So, we have a mass which is 1,225 grams in a volume which is 1 cubic meter.
However, there are other ways in which density can be expressed.
For instance, if we know the standard sea level density in ISA, then we can use this as a benchmark
to compare densities from other areas.
Using this method, we may take the density of 1,225 grams per cubic meter as representing 100%.
If in a given place or at a given altitude, the actual density was only 1,000 grams per cubic
meter,
then this density would be approximately 82% of the ISA sea level density.
We call this expression of density relative density.
We are comparing the prevailing density to the density at sea level according to ISA.
Another way density can be expressed is by using the term density altitude.
This term is seldom understood, but it is essential to understand it, especially when using it to discuss aircraft performance.
Density altitude is simply defined as being the altitude in ISA at which the prevailing density would occur.
This is best explained by using an example.
Let us say we are at sea level and the atmosphere happens to be exactly the same as standard.
The density at sea level in the real atmosphere will therefore correspond exactly to that found at zero feet in
ISA.
However, if the density at sea level in the real atmosphere falls to 1,000 grams per cubic meter,
this new lower density at our location will correspond to a density found at a higher altitude in ISA.
In our example, we can see that the density at sea level in the real atmosphere equates to a density
found at 10,000 feet in ISA.
We therefore say that the density altitude at mean sea level in the real atmosphere on this particular day is
10,000 feet.
The knowledge of density altitude is important in assessing the effect of the changing density on aircraft performance.
We will cover this later, but now let us see how we can actually calculate our density altitude.
There are two simple ways of calculating the density altitude.
One method is by using a flight navigational computer and the other is by using a simple numerical formula.
This formula states that for every 1°C away from ISA, depending on whether the deviation from ISA is positive or
negative,
you add or subtract 118.8 feet to your pressure altitude to find your density altitude.
Let us look at an example.
Let us say the pressure altitude is 2,000 feet and the temperature is 25°C.
This means the atmosphere is 14° warmer than ISA. Therefore the ISA deviation is plus 14°C.
We now see that the density altitude is 3,663 feet.
Having seen the various ways of expressing density, let us have a look at what controls atmospheric density.
Looking at the diagram we can see the density formula.
Notice that in the formula we replace the word density with the Greek symbol ρ.
We will now look at how the variables of temperature and pressure affect the mass or volume and therefore the
density.
Let us first look at how pressure can cause density to vary.
Looking at the diagram we can see a mass within a volume.
As we increase the pressure surrounding the parcel of air, you can see that it gets compressed.
This causes the density to increase.
Conversely, if we reduce the surrounding pressure, the parcel will expand and the density will reduce.
Physical experiments confirm that density is directly proportional to pressure.
The other main variable that controls density is temperature.
Looking at the diagram we can see that as we heat a parcel of air, the particles get excited and
cause the parcel to expand.
This reduces the density.
Conversely, if we cool the parcel of air down it will shrink and the density will increase.
Therefore, density is inversely proportional to temperature.
Having seen the effect of varying temperature and pressure on density, let us now consider the effect of altitude on
density.
Here we have a conundrum.
As altitude increases, temperature falls, therefore density should increase.
But as altitude increases, pressure also falls.
This causes the density to decrease.
So what does happen to density with altitude?
The predominant effect is the change of pressure.
The reduction in pressure reduces the density more than the reducing temperature causes density to increase.
Since pressure is the dominant factor affecting density, it follows that because pressure decreases at a decreasing rate with altitude,
so will density.
Looking at the diagram of ISA, we can see that relative density decreases with altitude at a decreasing rate.
There is one more variable that can alter the density of air.
This variable is water vapour.
This effect is complicated and beyond the scope of the expected level of knowledge needed for the private pilot.
It is sufficient to know that the density of the gas water vapour is less than the typical density of
the gases composed in a sample of dry air.
Therefore, the greater the humidity, the lower the density.
We have so far seen that high altitude, high temperature and high humidity environments all decrease air density.
A knowledge of atmospheric density is vital in understanding the effect density has on aircraft performance.
Lift is directly proportional to density.
Therefore, a reduction in density will cause a reduction in lift.
This is a performance reducing characteristic.
But density affects piston engine performance as well.
The lower the density, the lower the amount of oxygen that will be in a given volume of air.
This oxygen is essential to combustion.
Therefore, a reduction in density will cause a reduction in engine performance.
The combined effect on engine and airframe efficiency of a reduced density needs to be accounted for.
Most density changes are accounted for in performance graphs by the use of altitude and temperature nets as shown here.
The example shown is the landing distance graph for a small single-engine aircraft.
This completes the lesson on density.
Be very careful not to exceed the limitations of the aircraft and accurately account for density using the altitude and
temperature corrections in the aircraft manual.
Throughout this lesson we are going to investigate temperature and demonstrate its importance to meteorology.
Temperature is a measure of the heat energy derived from the agitation of molecules.
The higher the energy the molecules have, the greater the agitation and the higher the temperature.
Temperature can be measured on a variety of scales using many different instruments.
The two most common scales we use to measure the temperature of a substance are Celsius, sometimes called centigrade, and
Fahrenheit.
The Kelvin scale of absolute temperature is rarely used in general aviation meteorology, but you may see it in scientific
formulae.
At absolute zero, or zero degrees Kelvin, all molecular activity has ceased.
However, Celsius is the scale we use throughout the CD-ROM where appropriate.
You may be required to convert from one temperature scale to another.
To convert Fahrenheit into Celsius, simply subtract 32 from the Fahrenheit value and then divide it by 1.8.
To convert Celsius to Fahrenheit, simply multiply the Celsius value by 1.8 and add 32.
You will also find a conversion scale for Fahrenheit and Celsius on your flight navigation computer as shown here.
Finally, to convert Celsius into Kelvin, simply add 273.
The instrument used to measure temperature is called a thermometer.
In meteorology, thermometers which are used to measure the temperature of free air are usually housed in a Stevenson screen.
These are located 4 feet above ground level, allowing the free passage of air through the housing, but preventing direct
exposure to the sun's radiation.
However, there are many different types of thermometer.
The one shown here is a thermograph. It records temperature over time on a rotating drum.
Let's now consider the practical implications of temperature in the atmosphere.
Energy from the sun, or solar radiation as it is called, has a short wavelength.
The Earth's atmosphere is actually almost totally transparent to this incoming shortwave radiation.
The incoming energy passes through the atmosphere with little effect, and therefore does not appreciably warm the atmosphere.
Only when solar radiation encounters clouds or reaches the Earth's surface does it have an impact.
Clouds will reflect part of the incoming energy out to space again. The rest will continue to the Earth's surface.
In clear weather, around 85% of the original energy from the sun will reach the Earth's surface.
At the surface of the Earth, the sun's energy has its greatest effect.
Incoming shortwave solar radiation, or insulation, is absorbed by the Earth's surface, and as it does so, the surface warms
up.
The Earth now acts as a heat source, continually radiating its own heat energy outwards.
Heat energy emitted by the Earth is a lot less intense than the sun, and it has a relatively longwave
radiation.
The atmosphere absorbs the heat energy from the Earth's surface by various means.
The fact that the atmosphere is heated from below and not from above explains why, within the troposphere, temperature generally
falls with increasing altitude.
So what happens to this energy as it is emitted from the Earth's surface? How does it heat the atmosphere?
The most important process is where two of the constituents of the atmosphere, water vapour and carbon dioxide, absorb this
longwave radiation from the Earth.
The Earth then acts as a heat reservoir for the atmosphere, warming it up to a greater or lesser extent
depending on whether it is day or night, summer or winter, cloudy or clear.
The way in which the atmosphere allows solar radiation to pass through it to the Earth, but is then heated
by longwave terrestrial radiation, is referred to as the greenhouse effect, and without it the Earth would cool dramatically.
There are other methods of heat transfer, and we shall look at these in turn.
Conduction is the simple process whereby heat is transferred by direct physical contact between two bodies.
Looking at the diagram, you will notice that during the day the lowest layers which are in direct contact with
the Earth are the warmest.
However, during the night, as the land cools, notice that the air in contact with the land also cools, leaving
a typical night time temperature profile as shown.
The next process we are going to consider is convection. This is defined as the process by which masses of
warmer air are raised into the atmosphere with compensatory downward movements of cooler air.
Because of unequal heating of the Earth's surface, warmer less dense air will rise relative to the cooler heavier air
on either side.
This cooler air in turn will sink.
On hot summer days, this movement is visible through the presence of large vertically growing cloud developments called cumulus clouds.
Some of these may grow up to the tropopause, transferring heat high up into the atmosphere.
Another process of heat transfer is advection. This is similar in principle to convection, but the transfer of heat takes
place on a horizontal plane.
If the warm air is rising, cooler air will move horizontally to replace the rising air. This movement of cold
air is called cold advection.
Finally, the last method of heat transfer is through the absorption or release of latent heat.
This is where heat is liberated when a substance changes state without any change in its temperature.
If condensation occurs, water vapour is converted to water droplets.
This process, which occurs in clouds, actually releases energy in the form of latent heat into the surrounding air.
Conversely, if evaporation takes place, which is when water droplets change to water vapour, latent heat is absorbed from the
air.
The rate at which temperature falls is called the temperature lapse rate.
We have already seen that generally temperature decreases with altitude. This is otherwise known as the environmental lapse rate or
ELR.
You may recall from earlier lessons that ISA stated that the average environmental lapse rate was 1.98°C per 1000
feet, often rounded up to 2°C per 1000 feet.
However, under various conditions, the environmental lapse rate can and does vary significantly from this average.
If the temperature remains constant through a given depth of atmosphere, we describe it as being isothermal. This is shown
here.
You may recall from the lesson on the atmosphere that the tropopause is a place in the upper atmosphere where
isothermal conditions prevail.
We can actually find temperature increasing with altitude. This is because the surface layers are called by the Earth, but
the upper layers, being further away, are not affected.
This temperature increase with altitude is called a temperature inversion.
The variation of temperature across the Earth is an important aspect of meteorology.
This variation can arise from the daily and seasonal influences of the Sun, the nature of the surface, the latitude
and so on.
Each of these reasons will be looked at in turn.
The temperature of the air also often varies greatly in the same location through a 24 hour period.
This is called diurnal variation, or DV for short.
The Sun is at its highest at noon, with the maximum incoming solar radiation occurring at that time.
However, due to lag, or what is called thermal inertia, the surface of the Earth continues to receive more incoming
solar radiation than it gives out.
Therefore, the actual maximum air temperature usually occurs some 2 to 3 hours after maximum solar elevation, usually at about
1500 hours local time.
Similarly, the minimum temperature time is around half an hour after dawn.
Clouds have a big influence on the maximum and minimum temperatures.
You may remember from earlier lessons that we said that clouds reflect incoming solar radiation during the day, thereby reducing
the incoming solar radiation.
This would reduce the maximum daily temperature.
At night, though, the cloud acts in a very different manner.
The energy stored inside them from water vapour and latent heat release actually helps the clouds re-radiate energy back
into the layer of air between them and the surface, much like a blanket.
This has the effect of making the minimum temperature much higher than normal.
The combined effect reduces the diurnal variation of the temperature, but the characteristics of the curve are almost the same.
Notice how the maximum daytime temperature is lower with cloud cover and the minimum temperature, occurring just after dawn, is
higher with cloud cover.
The wind also affects the diurnal variation of the temperature.
By day, the effect is to reduce the maximum temperature.
This is because turbulent mixing of the lower parts of the atmosphere brings down cooler air from above, mixing it
with warmer air near the surface.
Wind also reduces the time that air spends in touch with the ground, lowering the amount of warming that takes
place by conduction.
At night, the wind keeps the temperature up, preventing it falling as low as it otherwise would.
Air near the surface will cool by conduction with the ground.
Look at how turbulent mixing then brings the warmer, unaffected air down from a higher level, averaging out the temperature
through an appreciable depth of the atmosphere.
There is, of course, a large temperature variation across the surface of the Earth.
There are many factors involved with this.
We shall now take a look at the main ones.
Take a look at this diagram.
You should be able to see that when considering a unit cross-section of energy from the Sun, the same
amount of energy is spread across a greater area at higher latitudes than at the equator.
Therefore, any single point at high latitude will receive less energy from the Sun than a point at lower latitude.
Another reason the equator is warmer is because the Sun's energy arrives at 90 degrees to the surface.
This allows more efficient absorption of heat compared to the poles.
These combined effects cause the poles to be significantly colder than the equator.
It is interesting to note that compared to the distance between the Sun and the Earth, the slightly greater distance
between the poles and the Sun compared to the equator and the Sun is negligible.
Because the Earth is tilted on its axis in space, we have seasonal variations in temperature across the Earth's surface.
In the diagram, you will notice that on June 21st, the Sun is directly over the Tropic of Cancer at
23.5 degrees north.
The Sun's rays are striking the Earth's surface at 90 degrees.
Therefore, it is this latitude and not the equator that is the warmest.
The diagram shows the summer season in the Northern Hemisphere and the winter season in the Southern Hemisphere.
However, on September 21st, notice that the solar radiation is striking the equator at 90 degrees.
This is called the autumnal equinox.
On December 21st, the Sun lies directly over the Tropic of Capricorn, which is 23.5 degrees south.
The diagram now shows the summer season in the Southern Hemisphere and the winter season in the Northern Hemisphere.
On March 21st, the Sun is directly over the equator again. This is called the vernal equinox.
The sea takes a long time to react in terms of warming and cooling.
Here we see that there is very little diurnal temperature variation of the sea when compared to the land.
This is because the sea has a large specific heat capacity.
This means that the sea requires more energy to raise the temperature by one degree than the land does.
This effect is noticeable daily and seasonally.
During the day and in the summer months, the land will be hotter than the sea.
But during the night and in the winter months, this situation is reversed.
The sea will generally be warmer than the land.
This difference in temperature over the land compared to the sea is the cause of such things as sea breezes
and land breezes.
These will be looked at more closely in the lesson on winds.
Throughout this lesson we have concentrated on how temperature is measured and more importantly how it varies across the Earth.
This variation plays a very important part in the creation of weather.
Next though, we will look at the types of pressure systems and their associated weather.
The different pressure systems that we find across the surface of the Earth play a primary role in determining the
weather we experience.
Understanding these pressure systems is central to understanding weather itself.
The types of pressure systems we find across the Earth are principally depressions or in other words low pressure areas
or cyclones and their counterpart anti-cyclones or areas of high pressure.
There are also features called coals that we shall look at later.
The first of the pressure systems we are going to look at in detail is probably the most significant to
our weather.
This is the depression or as it is more commonly known area of low pressure.
Depressions will be examined in two aspects.
The first will deal with small scale low pressure areas and the second with much larger scale low pressure areas.
The first types of depression, the small scale lows, can be found almost anywhere on the Earth's surface.
They are created when there is unequal heating of the surface.
Air lying above a warm surface will be heated by that surface through conduction.
The air will then become warmer and less dense than its surroundings.
This process will cause it to rise.
The rising air will travel up through the atmosphere and eventually diverge when reaching higher altitudes.
The total weight of the column of air above the warm surface will reduce as the air diverges and leaves
the area.
This process reduces the total weight of the atmosphere above the surface we are considering.
Thus the surface pressure will fall.
Air around the low pressure at the surface will be drawn inwards.
This might be expected to draw enough air back into the area, filling the low and returning the pressure to
normal.
But friction with the surface slows this inflow of air.
Consequently we see that more air is leaving the area in the upper atmosphere than can be replaced at the
surface.
Thus low pressure is maintained and even deepens as this process continues.
Let us now look at what the weather conditions might be within a lower pressure area.
As the air rises in the centre of the depression, it will expand as the outside air pressure decreases.
This expansion will make the rising air cool as it ascends.
The process is called adiabatic cooling and will be covered in detail later on.
If the air is cooling as it rises, then condensation will take place when the air temperature reaches dew point.
The condensation becomes visible as water vapour turns to water droplets.
If enough droplets are produced we see a cloud.
The clouds that are created will take on a vertical appearance.
We call these cumuliform type clouds.
These types of low pressure can be very intense even though they are only small scale.
Let us examine what the potential hazards may be to aircraft.
The velocity of the rising air can be very significant inside the cloud.
In fact, hailstones up to 1 kg in weight can be suspended inside the cloud.
If the weight of the water droplets exceeds the force of the rising air, then they will fall to the
earth as precipitation.
The onset of precipitation is generally rapid and intense and air will be dragged towards the surface creating very active
downdrafts.
This strong upward and downward flow of air within these small scale lows generates moderate to severe turbulence.
Another phenomenon of significance to aircraft is icing.
Icing occurs when liquid water or super-cooled water held within the cloud freezes onto an aircraft's frame.
The formation of ice dramatically affects the aircraft's performance and flying capability.
This phenomenon will be covered in detail later on in the lesson on icing.
It is important to note that the icing risk can be moderate to severe in weather phenomena associated with small
scale intense low pressures.
One other important weather situation connected with small scale low pressures is the horizontal visibility near the earth's surface.
When air is converging and rising, any impurities near the surface will be drawn up into the upper atmosphere, leaving
much clearer air on the surface.
We can generally say then that in a small scale heat low, the surface visibility should be good, except of
course when there is precipitation.
So far we have considered depressions on a small scale, maybe tens of miles in diameter.
However, under rare circumstances, these depressions can become the most violent weather phenomena on the surface of the earth.
Tropical revolving storms, more commonly known as hurricanes, cyclones or typhoons, can be over 700 nautical miles wide.
Because of their much larger scale, the spin of the earth becomes important, and unlike the small scale low, the
airflow is deflected as it enters the centre of the depression, causing rotational speeds of up to 200 mph.
Such storms are generally found only over the tropical oceans, and require special environmental conditions for them to develop.
Let us now look at the larger scale depressions, otherwise known as polar front depressions.
Polar frontal depressions are created in a very different manner to the small scale heat lows.
Polar front depressions are found along the polar front itself, which lies principally in the higher latitudes at about 40
to 60 degrees depending upon the season.
These depressions typically move from west to east across the globe. The process of their formation is complicated and a
little involved.
Within the temperate latitudes, we have warm tropical air meeting much cooler polar air.
The boundary between these two different air masses is called the polar front.
As with any boundary between two properties of different densities, this will not be a continuous straight line.
At some points along the front we will find kinks, which are smaller regularities along the front.
What is happening in these kinks is that the warm tropical air is intruding into the cooler polar air.
As the image sequence shows, this causes the warm air to replace cold air.
This is an important point.
This image shows a cross section of one of the kinks.
If much lighter, warmer air replaces the colder, heavier air, then the weight of the overlying air is reduced and
the surface pressure will drop.
As the surface pressure drops, air will naturally move along the surface towards the low pressure.
However, because the air movement is on a large scale, the airflow is deflected by the rotation of the Earth.
This deflection is to the right in the northern hemisphere, as shown.
In the southern hemisphere, this deflection is to the left.
Along the polar front, we now identify two areas of interaction fundamental to our understanding of frontal systems.
Along this section of the front, notice that warm air is being forced against the cooler air, the other side
of the polar front.
The warmer, less dense air will rise up and over the cooler air.
The weather feature created in this way, and shown here in cross section, is called a warm front.
Since the warm air is being forced aloft, it expands, cools and condenses, creating cloud along the frontal boundary.
The cloud takes on a horizontal appearance since the slope of the warm front is about 1 in 150.
This horizontal type of cloud is called stratiform cloud.
Returning to the plan view of the polar front depression, you will notice that red semicircles along the front axis
are used to represent the surface position of the warm front.
There is another fundamental feature of the polar front, shown here.
Along this line, colder air is being forced against warm air, the other side of the line.
This colder, heavier air will undercut the warmer, lighter air.
This feature, shown here in cross section, is called a cold front.
As the cold air advances, it forces the warm air aloft.
As this warm air cools, it condenses and clouds are created.
The slope of this frontal boundary is steeper than the warm front, approximately 1 in 50.
The clouds created will still take on a horizontal appearance, and are therefore mainly stratiform cloud.
But there is a fundamental difference along the cold front compared to the warm front.
Notice at this point, the surface position of the cold front lags behind because of friction.
This causes the air to be very unstable at this point.
Unstable air is air which, when disturbed, continues to ascend vertically.
This vigorous vertical ascent will create vertically developed cloud called cumuliform cloud.
Notice, though, that this potentially hazardous cloud is embedded or concealed within the other stratiform type of cloud.
Returning to our plan view of the polar front depression, notice that blue triangles along the front axis are used
to represent the surface position of the cold front.
The last thing to consider are the isobars, which are shown here.
Looking at a typical chart like this one, we can identify polar front depressions and their associated cold and warm
fronts.
There is one other type of frontal feature that can be seen.
Examine the highlighted area and notice the warm and cold front symbols which are found along one frontal axis and
are now coloured pink or purple.
This is called an occluded front and is where the warm and cold features have merged.
Polar front depressions are complicated with a wide variety of cloud and associated weather conditions.
We will deal with these systems in more detail later on in the lesson on air masses and fronts.
Depressions of any kind will change with elapsed time and be of various shapes and sizes.
By analysing the pattern of the isobars, it is possible to identify the shape of the low pressure areas.
If the isobars form a finger-like protrusion away from the centre of the low, we get what is called
an isobaric trough of low pressure.
Trough weather can be as active as and occasionally even more active than that of a main depression.
They can also be associated with fronts like the one shown here.
There is another type of surface trough that is caused by upper air processes.
These are known as trough lines and are represented on surface pressure charts with a solid black line.
These troughs are very significant to pilots because they can have active cumuliform clouds very similar to the cold front
or small depression.
Before you fly, ensure you closely examine the charts for these features.
They can be very hazardous, frequently creating lines of intense thunderstorms.
This brings to a close our study of one type of pressure system, namely low pressure areas.
We pass now to the other major pressure system, high pressure areas or anti-cyclones.
A high pressure at the surface is generated when the weight of a column of air is increased.
This occurs in a number of ways.
The most common process is when there is convergence of air in the upper atmosphere.
This adds air to the area overlying the surface.
Since the amount of air has increased, the surface pressure will rise.
As this happens, air along the surface will naturally want to move away from the region.
However, the outflow of air is slowed due to surface friction.
Whereas in the upper atmosphere, the inflow of air is faster.
This continually increases the total amount of air overlying the area and sustains the high pressure.
The resulting airflow looks like this.
We can summarise this occurrence by saying that air within a high pressure convergence in the upper atmosphere descends and
diverges along the surface.
The descent of air in a high pressure area is sometimes called subsidence.
The downflow of air in high pressure areas is an important process and creates the principal characteristic of weather found
within high pressure systems.
If air is forced down towards the surface, the surrounding air pressure increases.
This causes the air to be compressed.
Compression causes the air to warm up.
This process is called adiabatic warming.
Because air is being warmed while descending, condensation and therefore the creation of clouds is inhibited.
In general, within high pressure areas, there is a lack of any significant cloud.
Although we generally find little significant weather within high pressures, the descent of air does cause one weather problem.
The contaminants within the atmosphere such as smoke and dust are suppressed and held against the earth's surface, causing a
reduction in the surface visibility.
Haze is a common weather feature found in high pressure areas, especially in summer.
However, in winter, as the earth cools, fog is prevalent, together with surface temperature inversions.
Using a typical chart, you can see the high pressure areas to the south and notice the lack of cloud.
One other point to consider is how air flows along the surface within a high pressure area.
We already mentioned that air descends at the centre of an anticyclone and then diverges outwards along the surface.
However, the high pressure areas are of such a large scale that the wind moving outwards along the surface is
deflected by the rotation of the earth.
You may recall this deflected wind to the right in the northern hemisphere and to the left in the southern
hemisphere.
In summary, we may say that air moves around high pressures clockwise in the northern hemisphere.
Notice though, that unlike a low pressure area, the isobar spacing is a lot wider and therefore the winds will
be much lighter in high pressure areas.
By analysing the pattern of the isobars, it is possible to see the shape of the high pressure area.
If the isobars form a finger-like protrusion away from the centre of the high, a ridge of high pressure
is formed.
These features are similar to the main high pressure area but tend to have much less subsidence.
In summer, this allows convection to take place, producing characteristic cumuliform clouds.
Having seen the two main pressure systems, highs and lows, and examined their weather, we need to analyse the last
of the pressure systems, coals.
A coal is quite special in that it represents an area not directly under the influence of either a high
or a low pressure system.
In fact, it is the region in the middle of two lows and two highs.
Note that within this region, the isobars are very widely spaced.
Because of the wide spacing of the isobars, the wind found within a coal is very light and variable.
Other than the wind, the weather within a coal is dependent somewhat on the season.
Generally, over land areas, we can say that in summer, typical coal weather would be thundery, whilst in winter it
is mainly very foggy.
This completes the lesson on pressure systems.
Each type of pressure system has its own associated weather characteristic.
By knowing what pressure system is dominating your area, you can make an accurate prediction as to what the weather
will be like.
Generally, low pressure weather is dominated by lots of cloud and precipitation, whereas high pressure areas have clear skies with
frequent poor visibility from fog or haze.
The relationship between pressure and altitude is fundamental and is what the altimeter will use to determine height.
The altimeter is an aneroid barometer and therefore uses the principle that changing air pressure will cause a partially evacuated
chamber to expand or contract.
This chamber is linked to a pointer over a scale graduated in feet or metres.
We have learnt that pressure will decrease with height.
Therefore, as our aircraft rises up through the atmosphere, pressure will decrease, the chamber will expand, and the altimeter pointer
moves clockwise, showing an increase in height.
Conversely, when the aircraft descends through the atmosphere, the pressure increases, the chamber will compress, and the pointer moves anticlockwise,
showing a decrease in height.
There is, however, one important consideration we must bear in mind.
The altimeter is reading a height, and we have seen that this reading increases as we climb and decreases as
we descend.
But what is this reading measured from? In other words, where is the altimeter assuming the datum level to be?
What we must do is find out at which level the altimeter is assuming zero feet to be.
In fact, this is not an actual geographic location, it is a pressure value.
We can tell the altimeter what pressure value to begin measuring from, but not the physical place.
We do this by means of an adjustable subscale.
The subscale is seen through a small window in the face of the altimeter.
The scale shows a pressure value, either in hectopascals or inches of mercury.
Whatever pressure value is set in the window is the pressure level that the altimeter will start measuring from, that
is, the pressure level that the altimeter will assume to be zero feet.
So, if you want to find out how high you are above the airfield, you must determine what the airfield
pressure is and set this into the subscale.
The altimeter now has its zero datum point set at a pressure that equates to the pressure at the airfield.
This pressure value is commonly referred to as Qfe.
If this pressure was set within the subscale, the altimeter would read zero when on the ground at the threshold
or airfield reference point.
Getting the altimeter to read height above the airfield can be very useful to the pilot if operations require accurate
height above the airfield, such as flying in the circuit or performing aerobatics in the immediate vicinity of the airfield.
Qfe is only really useful within the immediate vicinity of the airfield.
If you were to fly a few miles away, the altimeter will still read the height above the airfield, but
it no longer tells you how high you are above the ground.
This is not very useful and could lead to potential problems, especially if you were unable to see the ground.
If the flight is outside the confines of the airfield, as on a cross-country flight, it would be better
to try and get your altimeter to read height above sea level, because terrain and obstacles are also shown from
sea level.
If we knew our height above sea level, it would make our clearance of obstacles much simpler to calculate.
In order to work out the height above mean sea level, we have to find out what the mean sea
level pressure is.
Calculating sea level pressure from any given point is not easy, because most locations are not at sea level.
A more practical way of calculating height above sea level is to position the altimeter on the airfield and turn
the subscale dial until the altimeter reads the actual height of the airfield above the real sea level.
In other words, we set the elevation of the airfield into the altimeter by adjusting the subscale dial.
Whatever pressure value is then displayed in the subscale is the assumed pressure at mean sea level.
This pressure setting on the subscale is called QNH, and it aims to give the best indication of your height
above sea level.
With QNH set on the subscale, we refer to the indication of the altimeter as an altitude.
Altitude is defined as height above mean sea level.
Horizontal pressure changes cause a unique problem for us.
Look at the diagram and notice that the pressure at both airfields is the same, and that as a result,
the pressure lines or isobars are almost horizontal.
If we wanted to maintain a constant indicated altitude going from the airfield on the left to the airfield on
the right, then we would have to ensure the pressure along the route does not change.
We would therefore have to follow an isobar.
As we go along the route, the indicated altitude and the true altitude above the sea are unchanged.
However, notice what happens if the pressure at the destination airfield were less.
Notice that the isobars now slope downwards.
If we were to repeat the journey keeping the indicated altitude the same, notice that the aircraft would actually descend
towards the mean sea level.
But the altimeter is not indicating our descent because the pressure surrounding the altimeter is unchanged.
In this case, our true altitude is reducing whilst our indicated altitude is remaining the same.
We have a saying to alert us to this problem.
From high to low, look out below.
In other words, whilst flying from an area of high pressure to an area of lower pressure, watch out.
You will actually be descending if you maintain a constant indicated altitude.
Let's look at another situation.
Watch what happens if the destination QNH is higher than at our departure airfield.
Notice that the isobars now slope upwards.
If we were to repeat the same flight and maintain a constant indicated altitude, we would have to follow those
isobars upwards.
This time our true altitude is getting greater, but our indicated altitude is remaining the same.
This situation is not too dangerous since your true height is increasing, but nonetheless it is not accurate flying.
What these principles show is the need to constantly adjust the pressure set into the subscale to account for variation
in horizontal pressure.
This resetting of the subscale is a vital part of aircraft operations.
When taking off or landing at an airfield, the controller will give you the correct QNH, which you must confirm
you have understood by reading it back to him correctly.
You must then adjust your subscale accordingly.
This essential practice ensures your altimeter is not only reading correctly, but is reading the same as altimeters in other
aircraft within the vicinity.
This latter procedure of setting airfield QNH only solves the error on departure or arrival at an airfield.
En route, your altimeter would still be in error.
How do we ensure that en route we have the most up-to-date pressure in the region we are
flying?
This problem is solved by the provision of regional pressure settings called RPS.
The map shown here allocates areas where RPS values are issued.
These areas are known as altimeter setting regions or ASRs.
When flying into these various regions, the altimeter should be reset to the values provided by them.
This will again ensure a more accurate altimeter reading and also ensure everyone else is on the same pressure setting.
However, even if you have the correct setting within a region, on approach to an airfield the controller will still
ask you to set the airfield's QNH because it may be slightly different.
The reason for this difference is that the RPS value is valid for one hour based on the lowest forecast
QNH within the region.
Your airfield will most probably have a slightly higher value of pressure and be more up-to-date than the
RPS value.
On a long flight at high altitudes, changes in the pressure will impact us less since we are usually far
from the ground.
Continually changing the pressure setting in the subscale can be tiresome.
As a result of this, we revert to a constant pressure setting.
This is called the standard pressure setting or SPS and is a pressure setting of 1013 millibars.
In the United Kingdom, whenever we fly below 3000 feet above mean sea level, we set QNH.
But when the aircraft climbs through 3000 feet, we have to change to the SPS.
This changeover altitude is known as the transition altitude.
Flying above this altitude, you must ensure that 1013 millibars is set within the subscale.
From this moment on, we refer to the indication on the altimeter as a flight level or a pressure altitude.
However, the transition altitude is not always 3000 feet on QNH.
For example, within the London TMA, it is 6000 feet.
In some countries, the elevation of an airfield can be greater than 3000 feet.
In this case, the transition altitude is usually a set height above ground level.
Always be sure to check the transition altitude at unfamiliar airfields.
When descending to an airfield, there will be a need to reset the altimeter to QNH again.
The transition level is not fixed and varies over time and place.
As a result, you will be told by air traffic control at what level to change from the SPS to
QNH.
The transition level is always higher than the transition altitude.
The area between these two points is known as the transition layer.
There is one final pressure setting.
It is seldom used and it is unique to very high altitude airfields.
When QFE cannot be selected into the subscale because the values are outside the subscale range,
then the pilot will be instructed to have 1013.2 set in the subscale.
He will then be told what the height of the airfield is with this pressure set into the altimeter.
This pressure setting is known as QNE.
It is defined as the height indicated on landing at an aerodrome when the altimeter subscale is set to 1013
.2 millibars.
So far we have looked at how the altimeter works and how we can best use it to eliminate varying
pressure changes along a route and near the ground.
The variable subscale and the different pressure settings helped us to do this.
However, there is one significant variable which the altimeter cannot cater for – temperature.
In our earlier lesson on pressure we saw that temperature affected the pressure lapse rate.
Our altimeter is calibrated to an ISA temperature and pressure lapse rate profile and is unable to account for the
effect of any deviations from ISA.
Let us recap what we have already learnt on how temperature affects the lapse rate and pressure levels.
Here we can see that if air cools the pressure levels contract and increase the pressure lapse rate.
If we warm the area on the right we can see an expansion of the pressure levels.
Let us suggest that an aircraft were to fly from the area on the right, warm air, to the area
on the left, cold air, at a constant indicated altitude.
We know that to achieve true level flight pressure must not change.
In other words we must follow a pressure level or ISA bar.
However in doing so we are inadvertently following a pressure level that is actually descending.
So in reality we are descending but the altimeter is not showing this.
Therefore the true altitude is decreasing but the indicated altitude is constant.
There is a saying which is used to remind pilots of this potential problem.
We have a saying when flying from hot to cold don't be bold or even more dramatically cold kills.
In cold air the altimeter will over read the true altitude.
This is a potentially dangerous situation especially when visual reference with the surface has been lost.
This temperature effect will reverse if we fly into a warmer region.
Notice how the aircraft is in fact climbing away from the ground.
So this time the true altitude is increasing but our indicated altitude is constant.
In warm air the altimeter will under read your true altitude.
Temperature errors are not the only errors in the altimeter there are many more.
However these are outside the scope of this lesson.
You will be made familiar with them when you study the altimeter in the lesson on aircraft instruments.
Having completed the theory of the altimeter it may be useful to go through some typical altimetry problems and work
through the solutions.
For simple calculations below 5000 feet above mean sea level we use a height change of 30 feet per millibar.
This means that for every 1 millibar change in pressure the altimeter will show a height change of 30 feet.
Looking at the diagram we can see an airfield 1000 feet above mean sea level.
If the QFE were 980 millibars what might the QNH pressure be?
What we have to do is increase the pressure from the airfield to mean sea level but by how much?
If we know that pressure changes by approximately 30 feet per millibar all we have to do is find out
by how much the pressure changes in 1000 feet.
This is a simple calculation.
1000 feet divided by 30 tells us that the pressure change in 1000 feet is approximately 33 millibars.
Now we simply add this to the pressure at the airfield to find the QNH.
In this example the QNH will be 980 plus 33 which equals 1013 millibars.
Let's try another example.
Let us say an airfield 2100 feet above mean sea level has a QNH pressure of 1005.
What will the QFE be?
Since the QFE is the airfield pressure and the airfield is above mean sea level then we know QFE will
have a lower value than QNH but by how much?
If we know that in a height of 30 feet the pressure changes by 1 millibar then we can say
that in 2100 feet we have a pressure change of approximately 70 millibars.
Therefore the airfield pressure will be 935 millibars.
Hopefully from this lesson you have learnt the fundamental principles of the altimeter.
More importantly though we have learnt its limitations and how we can correct for some of these by having various
pressure settings.
An understanding of altimetry is fundamental to flight safety.
You will constantly have to change the subscale to various pressure settings.
Ensure that when instructed to do so you correctly adjust the scale for the pressure given.
Throughout this lesson we are going to look at humidity.
In particular we will concentrate on the different states of water, how water changes between these states and why the
effects of these changes are important to meteorology.
Water has three states or phases.
The solid state called ice, the liquid phase simply called water and the gaseous state called water vapour.
One of the major impacts water has on our atmosphere is when it changes from one phase to another.
During these changes energy is transferred in different ways.
This energy is known as latent heat.
You may recall from the lesson on temperature this is where heat is liberated as a substance changes state.
So energy in the form of heat is either released or absorbed by the change of state.
Let us see what happens when a substance changes state from solid to liquid, in other words during melting.
As we heat the solid it will start to melt.
Energy in the form of latent heat is absorbed from the surroundings and then stored in the liquid.
Let us now examine the change of state from liquid to gas.
This is known as evaporation.
During evaporation latent heat energy is absorbed from the surface and stored in the water vapour.
We can experience this absorption of energy in our everyday lives.
When we have moisture on our skin and it evaporates our skin temperature falls.
This is because heat is being removed from our bodies and absorbed into the water vapour.
In fact when we are too hot our body will produce moisture on the skin called sweat.
This is the body's way of controlling its temperature.
Evaporation from a water surface will occur at any temperature above absolute zero which is minus 273 degrees Celsius
but the rate of evaporation increases as the temperature increases.
This is because at higher temperatures the molecular activity is greater and a change of state is more likely.
However there is only so much water vapour a unit volume of air can hold at any given temperature and
pressure.
This graph shows you how much water vapour can be held at any given temperature.
You can plainly see that the higher the air temperature the more water vapour that can be held in the
air.
When the air is completely full of water vapour and can hold no more the air is said to be
saturated.
No more evaporation can take place because the air cannot hold any more water vapour.
We have established that one way we can make air saturated is by adding water vapour to the air through
the process of evaporation until the air is full of water vapour.
However there is another method to achieve saturation.
You may recall from the previous scene that cold air cannot contain as much water vapour as warm air.
So if we have a parcel of air that is partially filled with water vapour and then cool it down
the air will eventually reach a point where it is holding as much water vapour as it can.
Again the air has become saturated.
Further cooling will cause condensation and formation of cloud.
Condensation is the reverse of evaporation.
It is the phase change from vapour back to liquid.
You will remember that the water vapour had a lot of stored latent heat.
So when it then condenses and changes into water again it releases this latent heat back to the surrounding air.
As we have seen condensation is said to occur when water vapour changes state into visible water like dew, clouds
and fog.
But this process is actually quite complex.
For condensation to occur we need minute particles in the atmosphere for the water to form around.
These are called condensation nuclei and mainly consist of salt particles put into the atmosphere over the sea.
There are other condensation nuclei such as pollutants from industrial sources, forest fires or volcanic eruptions.
These help the condensation process and can aggravate cloud formations.
The change of state of water from liquid to solid is known as freezing.
When the temperature of water drops below zero Celsius it generally turns to ice.
Remember from earlier in the lesson that this involves the release of latent heat.
Here we can see the heat being given up to the surroundings during the freezing process.
However in the atmosphere freezing usually requires the presence of particles.
In this case they are known as freezing nuclei.
So as the temperature of the droplet falls if there are particles in the water the droplet will freeze.
However if they are not present then when the temperature falls below zero it will not freeze.
In this state the water droplets are known as supercooled water droplets.
They are very important in aircraft icing.
There are two other phase changes that we need to mention.
These are less common in the atmosphere than condensation or evaporation but nevertheless are important.
When water goes straight from the solid to gas we call it sublimation.
When water goes from vapour to the solid state directly it is called deposition.
But in meteorology this change is also called sublimation.
Note that both of these phase changes miss out the liquid phase.
Sublimation from ice to water vapour absorbs latent heat
and sublimation or deposition from water vapour to ice releases latent heat.
In meteorology an instrument called a hygrometer or psychrometer is used to measure the amount of moisture in the atmosphere.
Traditionally these instruments have a wet bulb and a dry bulb thermometer.
The wet bulb thermometer is wrapped in a moist muslin cloth.
If the surrounding air is unsaturated water will evaporate from the muslin.
Latent heat will be absorbed from the thermometer and stored in the water vapour.
This lowers the temperature of the wet bulb.
If the air is saturated no evaporation will take place and the wet bulb temperature and dry bulb temperature will
be the same.
Using a scale found on the instrument the difference between the two thermometers
gives a measure of the amount of moisture held in the atmosphere usually expressed as a relative humidity.
Relative humidity is the amount of water vapour a body of air actually contains
expressed as a percentage of the maximum amount it could hold.
If the air is holding as much as it can then we will have a relative humidity of 100%
that is saturation.
If the air is holding only half the amount of water vapour it could actually hold then the relative humidity
would be 50%.
The temperature where the relative humidity reaches 100% that is the temperature at which saturation occurs is known as
the dew point temperature.
Shown here is a parcel of air which is at 15°C and has a certain amount of water vapour inside.
You will recall that when we cool air down we reduce its capacity to hold water vapour.
Look at the image and notice that when we cool the parcel of air its capacity to hold water vapour
reduces
and the relative amount of moisture in the parcel of air increases.
In other words its relative humidity is increasing.
When 10°C is reached notice that now the parcel of air is holding as much water vapour as it can
and is therefore saturated.
The relative humidity is now 100%.
This temperature is the dew point temperature.
If the parcel of air were to cool further condensation would occur because the air can hold no more water
vapour at this temperature.
This condensation is visible to us because clouds and fog will form as visible water droplets condense out of the
saturated air.
This completes the lesson on humidity.
The information you have learnt is vital to our understanding of clouds and precipitation.
An understanding of the dew point temperature is especially important.
However, this will be covered later in the lesson on clouds.
The concept of relative humidity is also very important and will be mentioned many times in the context of other
lessons.
Throughout this lesson we will analyse the concept of adiabatic process and explain its fundamental link to atmospheric stability.
The expression adiabatic process sounds confusing but in fact the concept is relatively simple to understand.
An adiabatic process is where the temperature of a gas within a defined system is changed without any transfer of
heat energy across the boundaries of the system.
The compression or expansion of air within the atmosphere are examples of adiabatic processes.
If a parcel of air were to be compressed, the energy within the parcel of air is squeezed into a
smaller and smaller volume.
This increases molecular activity and raises the temperature.
The temperature change is not caused by any external heat source applied to the parcel of air.
The reverse will take place if air is expanded.
This time the energy of the air molecules is spread through a larger volume, decreasing the molecular activity and reducing
the temperature.
How does this adiabatic process apply to the atmosphere?
You may recall that pressure decreases with altitude.
Therefore, if a parcel of air were to rise up through the atmosphere, the surrounding pressure would drop and the
air would expand.
This would cause the temperature within the parcel to fall.
This is known as adiabatic cooling.
If a parcel of air were to descend within the atmosphere, the surrounding pressure would increase and the parcel would
be compressed.
Therefore the temperature will increase.
This is known as adiabatic warming.
However, we need to quantify the rate at which the temperature of a parcel of air changes with height.
In other words, we need to establish the adiabatic lapse rate.
The rate at which the air will cool or warm depends upon the moisture content.
If unsaturated air is forced to rise or descend within the atmosphere, then the change of temperature within the air
parcel is 3°C per 1000 feet.
This is known as the dry adiabatic lapse rate or DALR.
Shown here is a temperature height graph illustrating the DALR.
However, if the air is saturated, then the change of temperature is less, approximately 1.5°C per 1000 feet.
This is known as the saturated adiabatic lapse rate or SALR.
The temperature height graph now shows both the SALR and the DALR.
The reason that the SALR is lower than the DALR is because of latent heat.
When saturated air is cooled, condensation occurs and latent heat is released inside the parcel of air.
When saturated air is warmed, evaporation occurs and latent heat is absorbed into the water vapour from the parcel of
air.
The adiabatic processes seen here are essential to the understanding of atmospheric stability.
Atmospheric stability is the tendency of air when displaced to either return to its original position or increase its displacement.
This is the tendency of air when displaced to return to its original position or increase its displacement.
This is relatively simple.
If air which is raised through the atmosphere is colder than its surroundings, it will naturally descend to its original
position because it is denser and heavier than the surrounding air.
Such air is stable.
However, if air which is raised through the atmosphere is warmer than the surroundings, then it will increase its displacement
because it is less dense and lighter than the surrounding air.
Let us firstly examine absolute stability or stable air.
This atmospheric condition is said to exist when the environmental lapse rate or ELR is less than the SALR.
In other words, the environmental change of temperature with altitude is less than 1.5°C per 1000 feet.
Looking at the graph, the highlighted area shows that if the ELR is plotted and falls within this zone, the
atmosphere is stable.
Let us understand why using an example ELR of 1°C per 1000 feet.
The diagram shows this ELR on the left hand side with a surface temperature of 20°C and falling by 1°C
per 1000 feet.
If we were to display some unsaturated air and force it to rise to 1000 feet, the temperature would fall
at the DALR which is 3°C per 1000 feet.
Therefore, by the time the air was displaced to 1000 feet, the parcel of air would be at a temperature
of 17°C.
Compare the temperature of the parcel to the surrounding environmental air temperature at that level.
You can see that the parcel of displaced air is colder than the environmental air.
If the displacement force was removed, the parcel would tend to return to the surface because it is denser and
heavier than the surroundings.
Even if we were to displace the parcel of air to 2000 feet, notice now that the difference in temperature
between it and the environment would be even greater.
Therefore, the tendency to return to the surface would have increased.
Let us now consider the displacement of a parcel of saturated air.
You may recall that saturated air, when displaced, will change its temperature by 1.5°C per 1000 feet.
If we displace the saturated air to 1000 feet, the temperature will reduce to 18.5°C.
Notice that just as with the unsaturated air, it is colder than the surrounding environmental air and will tend to
return to the surface.
Again, even if the displacement were continued to 2000 feet, the temperature difference would increase, creating a greater tendency to
return.
This explains that whenever the environmental air cools at a rate less than 1.8°C per 1000 feet, we have
a stable atmosphere.
For example, an isotherm is where temperature is constant with height, and the ELR would be here.
This explains why the tropopause is said to be very stable, limiting vertical airflow.
Inversions create very stable atmosphere. An inversion is where temperature increases with altitude, as shown here.
The effect of a stable atmosphere is suppressive.
In other words, the atmosphere resists vertical displacement, preventing any air from rising significantly through the atmosphere.
This resistance prevents vertical cloud formation like cumuliform cloud.
If any cloud were to be produced in this kind of atmosphere, it would predominantly be stratiform type cloud.
One other important effect about a stable atmosphere is the surface visibility.
Because of the atmosphere's resistance to vertical motion, any pollution such as dust or smoke will be trapped near the
surface and accumulate, creating very hazy conditions with resulting poor surface visibility.
This completes our study of a stable atmosphere and its associated conditions.
Let us now examine the reverse of this condition, absolute instability or an unstable atmosphere.
This atmospheric condition is said to exist when the environmental lapse rate, or ELR, is greater than the DALR.
In other words, the environmental change of temperature with altitude is greater than 3°C per 1000 feet.
Looking at the graph, the highlighted area shows that if the ELR is plotted and falls within this zone, the
atmosphere is unstable.
To aid our understanding, let us consider an example ELR of 4°C per 1000 feet.
The diagram shows this ELR on the left-hand side with a surface temperature of 20°C and falling this time
by 4°C per 1000 feet.
If we were to display some unsaturated air and force it to rise to 1000 feet, remember that the temperature
would fall by the DALR, which is 3°C per 1000 feet.
Therefore, by the time the air was displaced to 1000 feet, the parcel of air would be 17°C.
Compare the temperature of the parcel to the surrounding environmental air temperature at that level.
You can see that the parcel of displaced air is now warmer than the environmental air.
If the displacement force was removed, the parcel would tend to continue to rise because it is less dense and
lighter than the surroundings.
Even if we were to displace the parcel of air to 2000 feet, the difference in temperature between it and
the environment would be even more.
Therefore, the tendency to continue to rise increases.
Let us repeat this situation, but this time displacing a parcel of saturated air.
You may recall that saturated air, when displaced, will change its temperature by 1.5°C per 1000 feet.
If we displace the saturated air to 1000 feet, the temperature will reduce to 18.5°C.
Notice that just as with the unsaturated air, it is warmer than the surrounding environmental air and will tend to
continue to rise.
Again, even if the displacement were continued to 2000 feet, the temperature difference would increase even more, creating a greater
tendency to rise.
This explains why whenever the environmental air cools at a rate greater than 3°C per 1000 feet, we have an
unstable atmosphere.
With an unstable atmosphere, whether the air was saturated or not, the moment air was displaced, it would continue to
rise and accelerate upwards into the atmosphere.
This generates significant vertical air currents and as a result, if any cloud were to be produced, we would expect
to see cumuliform type clouds.
The clouds themselves can present a hazard to flight, as we have already learnt from the pressure systems lesson.
However, the active air currents can be used to beneficial purposes for glider pilots, who use these updrafts to gain
altitude.
There is an additional effect that an unstable atmosphere has.
Because of the active uplift of air, surface pollution will be raised away from the surface and moved up into
the atmosphere, increasing and improving horizontal surface visibility.
On warm, clear summer days, as the surface temperature rises rapidly, the ELR will increase creating an unstable atmosphere and
active cumuliform clouds can frequently be seen.
This is specially so in the tropics, like Florida for example.
Returning to the graph, there is one outstanding area which needs to be considered.
If the ELR were between 1.5°C per 1000 feet and 3°C per 1000 feet, then the atmosphere would be
described as being in a state of conditional instability.
This implies that the stability of the atmosphere is now dependent on additional conditions, because so far it was solely
the value of the ELR which determined whether the atmosphere was unstable or stable.
Let us try and understand what this additional condition is by using an example.
Let us assume an ELR of 2°C per 1000 feet as shown on the graph.
Looking at the diagram again, you can identify the example ELR on the left hand scale.
Let us displace some unsaturated air into this atmosphere.
Notice that when the displaced unsaturated air reaches 1000 feet, it would have cooled to 17°C.
Notice that it is colder than the surrounding environmental air at that level and will tend to return to its
original position.
This leads to a stable atmosphere.
However, if the air was saturated and it was displaced to 1000 feet, it would cool to 18.5°C which
is warmer than the environmental air at that level.
The air now has a tendency to continue to rise.
This of course leads to an unstable atmosphere.
As you can see, the stability of the atmosphere now depends upon its humidity.
Looking at the graph again, if the ELR falls between the SALR and the DALR as shown, the stability is
dependent on the humidity.
Saturated air will become unstable and unsaturated air will become stable.
The last type of atmospheric stability that can be observed is neutral stability.
This is described as being the condition of the atmosphere in which a parcel of air, if disturbed, will tend
to remain at the displaced location.
This can only happen when the environmental lapse rate is the same as the adiabatic lapse rate of the displaced
air, that is, either 3°C per 1000 feet or 1.5°C per 1000 feet.
If the ELR is 3°C per 1000 feet as shown and unsaturated air were displaced, then notice that the displaced
air cools at the same rate as the environment and therefore will remain at the location to which it has
been displaced, not tending to return or rise.
This situation will also occur if the ELR is 1.5°C per 1000 feet.
Notice again that if saturated air is displaced, it cools at the same rate as the ELR, again creating a
situation of neutral stability.
It is important to use this graph as a tool to understand the stability of the atmosphere.
Whichever zone the ELR falls into dictates the stability of the atmosphere and creates the associated conditions pertaining to that
stability condition.
This completes the lesson on stability.
Hopefully you have understood how the different stability states are achieved and what corresponding weather conditions are to be expected.
In this lesson we will look at the phenomenon of turbulence and wind shear, how they are created and their
impact on aviation.
Turbulence means disturbed or rough air which disturbs the aircraft's attitude but allows the aircraft still to maintain its flight
path.
Wind shear is a similar phenomenon but far more severe.
It is not rough air as such but airflow that has marked variations in speed and or direction.
Wind shear differs from turbulence in that it will displace the aircraft abruptly from its intended path such that substantial
control action is required to maintain heading and speed.
One of the most common causes of turbulence is the surface over which the airflow is moving.
The layer of air that is disturbed is known as the friction layer.
Above this friction layer the airflow is unaffected by the surface.
The boundary of the disturbed upper flow is called the friction level.
Turbulent air near the surface is created in two principal ways.
Mechanically by objects like trees, hills and buildings and thermally through the effect of warm air rising.
We call these effects mechanical or thermal turbulence respectively.
It is not just near the surface that we find disturbed air.
It is also found around significant cloud developments.
Looking at the diagram we see cumuliform cloud created by rising air.
This vertical flow of air will disturb the free air creating turbulence within the cloud.
The vertical extent of the cloud is therefore a very good indication of the intensity of the vertical airflow and
therefore the intensity of the turbulence.
For this reason cumuliform clouds are treated with caution in aviation.
Cumulonimbus clouds are the most visually obvious in terms of their capability of generating turbulence.
However we don't just find turbulence within the cloud itself.
If we now consider the bigger picture we can identify downdrafts at quite some distance from the cloud.
These currents can also disturb the free air likewise causing turbulence.
Very large cloud developments like cumulonimbus will have very active downdrafts and must be treated with extra caution.
There is one added concern with large cloud developments particularly of the cumuliform type.
When precipitation falls from clouds it tends to drag air down with it creating downdrafts within and underneath the cloud.
If the amount of air descending from the cloud is significant enough then a phenomenon known as a microburst or
macroburst is experienced.
When the air descends beneath the cloud it may come into contact with the ground and then spread out.
Because of the very large changes in the direction and speed of the wind both vertically and horizontally this gives
rise to very dangerous low level wind shear.
There is another area where the airflow can be significantly disturbed to generate significant turbulence and potentially very hazardous conditions.
Looking at the diagram we can see a mountain with horizontal airflow from the left.
As the air flows over the mountain it starts to oscillate in a wave-like pattern.
These oscillations are known as mountain waves, lee waves or standing waves.
To create the oscillations certain conditions must prevail in the atmosphere.
Firstly the air must be very stable otherwise the airflow when reaching the mountain would rise vertically and destroy the
oscillation pattern.
Another condition which must be met is that the wind speed at the level of the ridge must be greater
than 15 knots and increase with altitude.
The direction of the wind in relation to the mountain's orientation is another crucial factor.
There must be a significant proportion of the wind close to the perpendicular of the mountain axis.
The diagram shows an example of a mountain range aligned north-south.
The direction of airflow that is needed to help create the mountain wave must either be from the left or
the right.
If all these conditions are met the mountain wave will be created.
Often these oscillations may become visual.
When the air is forced to rise at this point it will cool adiabatically, condensation may take place and cloud
may form.
However when the air is forced to descend at this point it warms adiabatically and any cloud dissipates.
Notice the shape of these clouds.
Particular features of this cloud are that they have a lens-like shape and that they are often stationary,
the cloud continuously forming at the leading edge and dissolving on the trailing edge.
These clouds are called lenticular clouds.
There are other visual clues of the mountain wave for the pilot.
One of these is the cloud that shrouds mountains making it difficult for the mountain to be seen from the
air.
These clouds are called cap clouds.
There is one other cloud that is probably the most significant since it implies the location of the most severe
turbulence.
If we look at the flow of air underneath the first wave crest we can observe a rotation of air.
This zone is the most dangerous since the wind direction is changing constantly within a very confined area.
The clouds that develop here are called rotocloud or roll clouds.
There is another situation in the atmosphere that can give rise to turbulence and wind shear.
In the lesson on temperature and pressure systems we said that there may be times when we can experience temperature
inversions.
You may recall that inversions are where temperature increases with altitude.
These tend to be confined to the very lowest layers of the atmosphere close to the surface.
If the temperature inversions are strong enough then as a result of the changing density we can get significant changes
in wind direction and speed through the inversion.
This leads to wind shear and as pilots we need to be made aware of them.
Be extra cautious if your destination airfield issues an MTI.
This stands for a marked temperature inversion.
Receiving an MTI should prompt you to be extra cautious of wind shear around the airfield and on the approach.
Though beyond the scope of this course we will make brief mention of mixing in the lower layers of the
atmosphere creating inversions.
Where mixing occurs temperature falls more rapidly with height.
But at the altitude where there is no mixing the temperature is a little higher than within the mixing layer.
These inversions like any other type need careful consideration because of the wind shear they can create.
Turbulence of any kind is something we wish to avoid.
It is especially dangerous to aircraft on the approach or during the landing phase of flight since the aircraft has
low forward speed, low altitude and high amounts of drag.
Fly with great caution, respond promptly to turbulence and monitor your airspeed closely.
Use a slightly higher takeoff or landing speed to ensure a greater safety margin from your stalling speed.
But be wary, this will increase the landing and takeoff distance.
From this lesson you will hopefully be more informed about turbulence and wind shear.
This will help you to be prepared and more able to make correct decisions about avoidance.
In this lesson we are going to look at clouds, their formation and characteristics and how they relate to aviation.
We shall also look at precipitation and how it is produced.
To begin with we will examine how clouds are measured and reported.
Cloud amount is measured in octaves or eighths of the sky.
If half the sky were covered with cloud, as shown here, then we would report the cloud amount as 4
octaves.
In aviation weather reports, cloud amounts are normally expressed as few, which is a covering of 1 to 2 octaves,
scattered, which is 3 to 4 octaves,
broken, which is 5 to 7 octaves,
and lastly overcast, which is a complete cloud coverage, or in other words 8 octaves.
Cloud base is defined as the lower zone in which the type of obscuration perceptively changes
from that corresponding to clear air haze to that corresponding to water droplets or ice crystals.
The cloud base is usually given as the height of the cloud above aerodrome level,
but on area forecast charts, cloud bases are usually shown above mean sea level.
How do we ascertain cloud base height?
There are several ways. The most obvious one is passed to ground stations by aircraft in flight reports.
Another way is by using a balloon with a known rate of ascent
and timing the release until the balloon disappears into the cloud.
Currently, laser cloud base recorders are used at most military or larger civilian aerodromes to measure cloud base heights accurately.
The definition of cloud tops is fairly simple. It is noted by a sharp reduction in the relative humidity.
On a convective day, when we have cumulus clouds as we see here,
the bases of the individual clouds are usually at a fairly constant level,
whereas the tops are at a multitude of heights above ground level.
The cloud growth will stop when the air within the cloud is prevented from rising
and therefore prevented from cooling any further.
This will happen when the temperature of the air within the cloud reaches the same temperature as the surrounding air.
In other words, when the rate of cooling of the rising saturated air, the SALR,
is such that it will eventually be the same as the environmental air, the ELR.
We are now going to look at cloud types.
The simplest method of classification is based on the shape of the cloud.
These are stratiform, cumuliform and ciriform.
Stratiform is a layered type of cloud of great horizontal extent but little vertical extent.
Cumuliform cloud is heaped type cloud, normally with large vertical extent compared with its horizontal extent.
Lastly, ciriform is a fibrous cloud, wispy or hair-like in its form.
This is only found at high levels in the troposphere.
Clouds are also identified by the height at which they usually occur.
There are three distinct levels within the troposphere.
The first cloud level is the low level cloud.
The clouds classified as low level are those which are found principally between the surface and 6500 feet.
These are stratus, stratocumulus, cumulus and cumulonimbus.
The suffix nimbus implies rain bearing.
However, cumulus and cumulonimbus may extend from low level to higher levels, especially cumulonimbus.
The next cloud level is the medium level cloud.
The clouds classified as medium level are usually found between 6500 feet and 23000 feet above ground level.
These medium level clouds are always denoted by the prefix alto.
These are altostratus and altocumulus.
Nimbostratus is also a medium level cloud but it can extend into both the lower and upper level.
The last level is the high level cloud.
High level cloud can be found anywhere from 16500 feet to the tropopause level.
The clouds found at these heights are given the prefix cirro.
These are cirrostratus, cirrocumulus and cirrus.
To find out about each of these clouds in a little more detail, click on the relevant cloud type.
Once you are ready to proceed, please click the forward continue button.
Cirrostratus is a transparent whitish cloud veil of fibrous or smooth appearance, totally or partially covering the sky.
It is made up of ice crystals and lies between 18000 and 45000 feet.
It is a further warning of an approaching frontal system and usually produces a large halo around the sun or
moon.
Cirrocumulus is quite a rare cloud.
It is a thin white patch, sheet or layer of cloud with no shading, composed of very small elements in
the form of grains or ripples and more or less regularly arranged.
It is generally found between 20000 and 30000 feet and consists of ice crystals.
Cirrus is the highest of all the clouds within the troposphere.
It is composed entirely of ice crystals.
They are detached clouds with the form of white delicate filaments in patches or narrow bands.
They are fibrous or hair-like, sometimes even having a smooth sheen texture.
Cirrus clouds can be found between 16500 feet and 45000 feet.
They often herald the approach of a warm front.
Altostratus is grey or bluish cloud sheet or layer which can be fibrous or uniform in appearance,
sometimes covering the whole sky giving a ground glass effect to the sun or moon.
Altostratus is usually found between 8000 and 15000 feet and can be from around 2000 to 8000 feet thick.
Altocumulus is white or grey or both and appears in patches or sheets of cloud.
It is composed of rounded masses which can appear fibrous or diffuse.
Its base usually lies between 8000 and 15000 feet with tops up to 20000 or even 23000 feet.
Nimbostratus is a dense rain-bearing cloud.
It is usually grey but often very dark with its boundaries made diffuse by continuous precipitation of rain or snow.
It is thick enough to blot out the sun.
Stratus is generally a grey cloud layer with a fairly uniform base which may give drizzle or snow grains.
When the sun is visible through the cloud its outline becomes clearly discernible.
Stratus does not produce a halo phenomenon.
The depth of this cloud is no more than 1000 to 150000 feet thick and is usually the lowest of
all clouds.
The main hazard associated with stratus is that it often covers higher ground resulting in hill fog.
Strata cumulus cloud is probably the most common form of cloud.
It appears grey or whitish or even both but almost always has distinct dark parts.
Strata cumulus can be seen as patches, sheet or in a continuous layer.
This type of cloud is usually no more than 2000 to 3000 feet thick but can be 5000 to 6000
feet deep on occasions.
Cumulus is the most common form of heaped or convective cloud.
It is generally dense and with sharp outlines developing vertically in the form of mounds, domes or towers of which
the upper parts often resemble a cauliflower.
The sunlit parts of these clouds are mostly brilliant white but their bases are relatively dark and nearly horizontal.
This cloud can appear benign and may only be a few thousand feet thick but cumulus clouds can grow rapidly
under the right conditions and can reach over 20000 feet.
Be very wary of tall growing cumulus clouds if you encounter them in flight.
Cumulonimbus clouds are of great significance to the aviator. They are vigorous convective cloud cells.
They look like large cumulus clouds of great vertical extent in their initial stages but develop large characteristic anvil tops
as the upper part of the cloud hits the tropopause which can be as high as 45000 feet.
The base of cumulonimbus clouds is often very dark with ragged cloud appearing beneath the main cloud cell.
The risk of icing and turbulence associated with this type of cloud is always severe.
These clouds have tremendous vertical support and as a result heavy hail and rain showers can be experienced.
Other hazards include lightning and static discharge which may lead to erroneous instrument readings.
Clouds are simply made up of either tiny droplets of liquid water or at high altitudes tiny ice crystals.
For cloud to form, water vapour must be condensed into liquid water.
You may recall from the lesson on humidity that this happened when saturated air was cooled.
So to generate clouds in the atmosphere, air must be cooled to dew point.
This can happen in various ways.
Air can be cooled by passing over a cooler surface than that from which it originated or it can be
cooled by mixing.
Usually though, most cloud is created by being lifted and cooled adiabatically.
Each of these ways will be looked at in turn.
In the lower atmosphere, air can be cooled and cloud created by turbulent mixing.
We saw this in the lesson on turbulence.
We see again here that the environmental air temperature is falling with altitude at approximately 1 degree Celsius per 1000
feet.
Notice the temperature at 2000 feet is 18 degrees Celsius and at 3000 feet is 17 degrees Celsius.
If there is mixing taking place up to 3000 feet, notice that the air temperature at 2000 feet and 3000
feet is now less than before.
This cooling can, if there is sufficient moisture, create cloud.
However, the production of cloud will only occur where the air has been cooled below its dew point.
No cloud is found above the mixing layer because the temperature has not fallen and an inversion exists.
This cloud is usually layered type cloud, in other words, stratiform type.
You may recall we said earlier that cloud can be created from air that is being lifted and cooled adiabatically
to dew point.
Let us now consider the various mechanisms for lifting air to higher levels.
But first of all, these parcels of air need a trigger to set them rising and displace them from their
present position.
One of these triggers is convection.
During the day the Earth's surface will heat up, some surfaces heating up more than others.
The air overlying these warmer surfaces will be less dense than the surroundings and therefore lighter.
This air will rise and as it does so will cool adiabatically.
When the air is cooled to dew point, condensation will occur and clouds will form.
These types of clouds are convective clouds and are the vertically developed clouds which we call cumuliform.
This method of cloud creation normally takes place on a very small scale, perhaps a few miles across,
but the extent of convection may sometimes be much greater.
If the air is very dry, in other words it has a low dew point temperature,
then the air will have to rise to a significantly higher altitude in order for it to be cooled to
the dew point.
Therefore a relationship exists between the cloud base and the dew point temperature of the air.
An approximate way of calculating the height of the cloud base in feet
is to subtract the dew point temperature from the dry bulb temperature and multiply the answer by 400.
This rule of thumb will give the height of the cloud base in feet.
So if the dry bulb temperature is plus 12 and the dew point is plus 5 degrees, then 12 minus
5 equals 7.
Multiplying this by 400 gives a convective cloud base of 2800 feet above ground level.
There are other ways in which the air may be lifted and the air cooled adiabatically.
If an area on the Earth's surface is experiencing an accumulation of air such as two masses of air converging,
then the air will be forced to rise.
This forced ascent will cause the air to cool.
Condensation may take place and cloud will form.
Again in the same way as convective cloud, this cloud will have a vertical appearance and will be of cumuliform
type.
Remember these clouds are potentially hazardous so are best avoided.
These convergent zones are shown on charts as solid black lines like the one shown here.
If warmer air comes into contact with colder, denser air, then the warmer air will be forced to rise.
The rising warm air will cool and condense creating cloud.
This method of uplift is called frontal uplift and creates frontal cloud.
There are two types of fronts associated with this, warm fronts and cold fronts.
The warm front shown here is when warm air is forced against colder air and rises up and over the
colder air.
The cold front is created when cold air is forced against warm air and undercuts the warm air, forcing it
to rise and again creating clouds.
The presence of high ground will also on many occasions force the uplift of air and hence the formation of
cloud.
However, the type of cloud created depends on the stability of the air.
You may recall that in an unstable atmosphere lifted air will continue to rise after the lifting force is removed.
Looking at the diagram, we can see the air coming from the left and being forced up the side of
the mountain, but at the peak of the mountain because of the instability, the air continues to rise.
Because the air is rising vertically, the cloud would be cumuliform type, possibly even cumulonimbus cloud.
It may be difficult to see this cloud as it may be embedded within the other cloud around the mountain.
However, if the air is stable, it will tend to sink down the other side of the mountain.
This time the cloud created would hug the mountain, creating a cap cloud as shown.
Under special circumstances, the stable conditions can create standing waves and the associated lenticular and rotor cloud.
There is an additional effect that is caused by stable air flowing over a mountain.
This is known as the Fern effect and gives rise to the fern wind.
The term fern wind describes any wind which is blowing down a mountainside and is warmer and drier than the
air at the same height on the upwind side.
Looking at the diagram, we see moist air blowing up one side of the mountain.
When saturation is reached, the air will cool at the saturated adiabatic lapse rate of 1.5°C per 1000 feet
and cloud and precipitation may form.
However, when the air descends on the leeward side, it now starts to warm and becomes unsaturated, warming on the
descent by the dry adiabatic lapse rate of 3°C per 1000 feet.
Notice that the air on the leeward side is warmer than on the windward side.
The cloud shape also helps us to notice this effect since the cloud base is generally quite high on the
leeward side but because of the loss of moisture through condensation and precipitation is much lower on the windward side.
A good example is found on the southern slopes of the Alps as air flows into the Mediterranean during the
winter months.
For the rest of this lesson we are going to examine precipitation.
First of all, let us see how precipitation is produced.
We need condensation to occur but more importantly we need condensation nuclei.
As the air cools, water vapour condenses into visible water droplets around these nuclei as shown here.
However, these droplets are very small.
If they were to fall to the surface, they would only generate very fine drizzle.
The cloud which commonly produces this is Stratus cloud.
Remember that this cloud has very little vertical strength and does not have the necessary vertical airflow to support larger
droplets.
The small condensation droplets that are produced can grow however through various methods.
Within a cloud, droplets collide fairly frequently.
These collisions cause droplets to join and form a larger droplet.
These will eventually fall under their increased weight.
This process is called coalescence.
The size to which droplets grow will depend mainly on the strength of the upcurrents within the cloud.
Powerful vertical air flows will support much larger droplets and produce heavier rain.
Clouds such as Nimbus Stratus and especially Cumuliform type clouds will have greater vertical forces and give heavier, more intense
precipitation.
The final mechanism for producing rain is the most efficient.
It is called the Bergeron process.
This is quite complex.
Once ice crystals are created, they have a relatively large fall speed and hence drop out towards the ground quickly.
But they will grow further by accretion because any water droplets which collide with the ice crystals will freeze on
contact.
If they then start to melt, they become sticky and can amalgamate into large clusters of ice crystals to form
snow.
Hail is formed when rain droplets are swept quickly to the top of the cloud, causing them to freeze.
The small hailstone will then fall again before being swept back upwards.
The hailstone will gather water by collision with other water droplets, again adding to the size.
This may happen several times, adding more and more ice to the hailstone.
Hailstones grow layer by layer by this mechanism.
Hail can become very large indeed, making it a hazard to aviation.
Hailstones can weigh up to one kilogram.
To support a one kilogram hailstone, the updrafts must be very violent
and therefore clouds in which this process is taking place must be avoided at all costs.
You will be aware that there are many different types of precipitation, such as rain, hail, snow etc, but these
fall in different ways.
Depending on its type, precipitation may fall steadily over a long period of time or sharply as a shower.
Showers develop in unstable atmospheric conditions from large cumulus clouds and cumulonimbus clouds.
These clouds contain large updrafts of air within them, helping to support large droplets of precipitation.
Hence, showers of rain tend to consist of large rain droplets.
The convective cell drifting over the ground means the shower will have a relatively sharp onset and cessation, a feature
of this type of precipitation.
They can be of rain, snow or hail.
Dynamic precipitation falls from layered cloud as opposed to showers from convective cloud.
It is characterised by its more gentle onset and gradual cessation and duration of several hours.
This type of precipitation can be of drizzle, rain, snow or sleet or occasionally ice pellets, which are similar to
hail but come from layered cloud.
Dynamic precipitation can be described as being intermittent, in other words, falling from time to time for usually less than
60 minutes,
or continuous, which is fairly self-explanatory and strictly lasts for 60 minutes or more at any one location.
The intensity of precipitation is described according to the following criteria.
Slight rain is described as less than 0.5 mm per hour, whereas heavy rain is greater than 4 mm
per hour.
Notice that these figures relate to temperate latitudes only, because in the tropics, extremely heavy rainfall may be experienced, which
is not covered here.
Snowfall is represented in cm of accumulation per hour.
In this lesson we have learnt that clouds are our signposts in the sky, letting us know what is happening
in the atmosphere.
They can inform us of potentially hazardous flying conditions.
A stable atmosphere gives rise to stratiform clouds and potential dangers posed to aircraft are small.
However, an unstable atmosphere is very problematic, giving rise to potentially very dangerous weather conditions such as hail, lightning, icing
and turbulence.
Cumuliform clouds, and in particular cumulonimbus clouds, are the most visual indication of these threats.
In this lesson we are going to look at thunderstorms, concentrating mainly on their formation and the potential hazards they
pose to pilots.
Well-developed cumulonimbus clouds are required for thunderstorms.
Windshear is a similar phenomenon, but far more severe.
It is not rough air as such, but airflow that has marked variations in speed and or direction.
Windshear differs from turbulence in that it will displace the aircraft abruptly from its intended path, such that substantial control
action is required to maintain heading and speed.
An unstable atmosphere, in other words an environmental lapse rate greater than the saturated adiabatic lapse rate, and this must
extend to well above the freezing level.
Sufficient water vapour to form and maintain the cloud, and a trigger action to produce the initial ascent of the
air.
There are several trigger mechanisms which will cause air to ascend and begin vigorous convective activity.
The first of these is through convection from surface heating.
Because the surface of the earth heats up unequally, air lying adjacent to warmer surfaces will become less dense than
air above colder surfaces, and this less dense air will rise.
Air can also be triggered to rise by being forced up the sides of mountains and hills, this is called
orographic uplift.
If two air masses were to move together, air will again be forced to rise.
This type of trigger action is known as convergence.
Lastly, air will rise due to frontal uplift.
This occurs when warmer, less dense air is forced up and over colder, heavier air.
There are generally two different types of thunderstorms.
Those that occur in a single air mass, most common in summer, and frontal thunderstorms, which are the more common
type in winter.
In non-winter months, thunderstorms can be caused by convection, convergence, or orographic uplift.
This air mass or heat type thunderstorm is most frequent over land, and generally forms by day.
Although mainly isolated, this type of thunderstorm does on occasions form in organised lines or large groups, which we shall
discuss later.
Thunderstorms also occur on cold fronts or very occasionally on occlusions.
They are often embedded in layered cloud, which makes them particularly hazardous to aviation.
This type of thunderstorm forms mainly over the land, and is more common in winter months.
The next issue we must consider is how air mass thunderstorms develop.
There are three main stages involved.
The initial stage or cumulus stage, the mature stage, and finally the dissipating stage.
Let us examine each in turn.
The initial stage is characterised by strung updrafts and by the rapid growth both horizontally and vertically of cumulus clouds.
Strung updrafts produce clouds of great vertical extent.
Air is drawn into the cloud from beneath and from the sides.
This stage usually lasts for around 20 minutes and produces a cloud sometimes 5 nautical miles across and 25,000
feet high.
The mature stage is characterised by strung up and downdrafts and precipitation.
Rain, hail or snow can be produced, and as these fall, strong downdrafts occur.
Downdrafts can reach 2,000 feet per minute, bringing cold heavy air down to lower levels within and below the
cloud.
These downdrafts are frequently colder than the surrounding air beneath the cloud, causing the air to sink at a faster
rate and present a major hazard to aircraft caught beneath the cloud.
The upcurrents within the cloud can reach 10,000 feet per minute, and the cloud top may grow upwards by
as much as 5,000 feet per minute.
At this stage, the cumulonimbus will begin to develop the characteristic fibrous anvil top.
The rising and falling water droplets, hail and snow, produce a build-up of static electricity.
The top of the cloud becomes positively charged and the lower part negatively charged.
The build-up eventually leads to lightning discharge and the characteristic boom of explosively heated air, which is thunder.
Microburst and gust fronts are other features of the mature stage.
Microbursts are created by the cold downflow of air beneath the cloud, spreading out when it reaches the surface.
These are very dangerous. The rate of sink of the air sometimes exceeds the climb performance of some types of
aircraft, thereby forcing them to the ground.
A good visual clue to pilots is the presence of furger.
This is precipitation which is falling but not reaching the ground, and looks like tendrils hanging beneath the cloud, as
shown here.
The leading edge of this outflowing colder air beneath the storm is known as a gust front.
It marks a rapid change in wind direction and speed, with a noticeable temperature drop.
Gust fronts can be made visible as the colder air undercuts the warm air, forcing it to rise and creating
a cloud.
These are known as roll clouds.
The dissipating stage of a cumulonimbus starts as the anvil top becomes fully formed.
This stage is characterised predominantly by downdrafts.
However, updrafts can still prevail at the top of the storm, feeding the growing anvil.
Precipitation at this stage is usually heavy.
Lightning and thunder are also still likely at this stage, which may last for between one and a half and
two and a half hours.
Once developed, these types of storms move in the same direction as the winds, at around 10,000 feet.
The individual thunderstorms, although on occasions quite large, do not last any longer,
because the downdrafts caused by the precipitation suppress the upcurrents necessary to maintain the cloud.
The two effects therefore cancel each other out.
The storm effectively dies once precipitation commences.
The next section in this lesson will concentrate on the hazards which thunderstorms pose to pilots.
Select each named hazard on the screen to learn more about it.
When you have viewed them all, click the forward button.
Severe turbulence can occur in and around any cumulonimbus clouds, and particularly around thunderstorms.
The violent up and downdrafts within the cloud are specially hazardous.
The downdrafts and associated wind shear are particularly dangerous during takeoff and landing,
since the aircraft is in a configuration of high drag and low forward speed.
Wind shear can result in a loss of control, as well as reducing airspeed to stalling speed.
Structural damage from turbulence can occur to airframes,
and in severe cases lead to terminal failures of the wing fillet.
Severe icing may occur in cumulonimbus or thunderstorm cloud between 0 and minus 45 degrees Celsius,
but can be particularly significant in the range minus 3 to minus 10 degrees Celsius.
High concentrations of large supercooled water droplets can lead to severe clear icing on the airframe very quickly indeed.
To avoid this, do not fly into the cloud above the freezing level.
Carburetor or engine icing can occur at temperatures between minus 10 and plus 30 degrees Celsius
in the high humidity associated with any cloud.
Hail can be encountered at any height in and underneath a thunderstorm.
Even within a few miles of the storm itself, you must expect hail.
Hail can also be experienced beneath the anvil outside the cloud.
Severe damage can be caused to the skin of the airframe.
Any damage to an aircraft will significantly reduce its performance,
particularly if the damage is to the leading edge of the wing.
Lightening is most likely to occur 5,000 feet either side of the freezing level
in a part of the cloud that usually has a temperature between plus 20 and minus 10 degrees Celsius.
Lightening within the vicinity of an aircraft can cause the following effects.
Temporary blindness, erroneous compass reading,
the ADF becoming unreliable, often pointing into the storm itself,
skin damage if the aircraft is struck by lightning,
ignition of the fuel vapour if the fuel tanks are nearly empty.
Large pressure variations can occur in and around cumulonimbus and thunderstorms.
Rapid pressure fluctuations can lead to errors in indicated height of up to plus or minus 1,000 feet.
Inaccurate height indications and severe downdrafts are potentially very dangerous conditions.
If the altimeter is fluctuating, don't fight it. You could overstress the aircraft.
It is more important to maintain attitude rather than altitude inside the storm.
Aviation authorities have produced guidelines on avoidance distances using radar-equipped aircraft.
Here is a table showing recommended distances from cumulonimbus activity.
Most private pilot flights will take place well below flight level 200 and no radar equipment will be carried.
Therefore, light aircraft should avoid thunderstorms by at least 10 nautical miles horizontally and 5,000 feet vertically.
Thorough pre-flight planning is particularly crucial in weather where strong convection is expected.
Low-level significant weather charts as well as TAFs should give a good indication for timings and location of cumulonimbus
or thunderstorm development.
If this lesson has achieved its objective, you will now understand more about thunderstorms, how they are generated and more
importantly how to recognise and avoid them.
They are very dangerous and it is a frightening experience to be caught in one.
Treat them with caution and before you fly analyse weather reports and forecasts, carefully looking especially for any signs of
thunderstorm activity.
In this lesson we're going to look at wind and concentrate on those winds which blow within the first few
thousand feet above the surface of the Earth.
We'll examine what creates wind, what the forces involved are and how these change as the wind interacts with its
environment, particularly the surface over which it flows.
What is wind though? Put simply, it's the sustained horizontal flow of air over the Earth.
To begin with, let's examine the measurement of wind and its units.
Wind is usually represented as a velocity indicating both wind direction and wind speed.
Let's take a look at direction. The direction of movement of any object is usually given in degrees from north.
This scale starts from zero, which is north, all the way through to 360 degrees, which is another way of
expressing north.
Wind direction is given not as the heading of the wind but rather as an indication of where the wind
is coming from.
For example, a wind blowing towards the east is called a westerly wind and would therefore be shown as having
a direction of 270 degrees.
You may already know that the Earth has a true north pole and a magnetic north pole.
Most wind directions given by meteorologists will be referenced to true north.
When using such wind directions, be careful to adjust them using the relevant variation, so that you have their direction
relative to magnetic north.
There are rare occasions when the reported winds are given as magnetic wind directions and not true wind directions.
These are typically whenever wind is reported by an air traffic controller from a tower, like on take-off and
landing, and in the Recorded Aerodrome Terminal Information Service or ATIS.
Changes in the direction of the wind are expressed using the terms backing and veering.
A wind that is veering is changing direction in a clockwise manner.
For example, a wind that was 090 degrees and is now 180 degrees would be described as veering.
A wind is said to be backing if the direction change of the wind is anticlockwise.
For example, a wind that was 270 degrees and is now 180 degrees would be described as backing.
You may recall that we said that wind is reported as a velocity, that is, both its direction and speed
are given.
The standard unit of speed is the metre per second.
More generally, however, the unit adopted for operational purposes in aviation is the knot.
One knot is one nautical mile per hour.
Measuring wind velocity is quite simple.
The most commonly used instrument is the cup generator and wind vane, which is shown here.
The three cups capture the wind and, as a result, rotate about a spindle.
The direction of the wind is measured by a remote transmitting wind vane.
These instruments are used around airfields to calculate the approximate surface wind velocity for aircraft either taking off or landing.
The definition of the surface wind is one that blows at 10 metres above ground level.
Consequently, these instruments are placed 10 metres above ground level and clear of any nearby obstacles that may unduly disturb
the flow of air and give misrepresentative readings.
On some weather charts you may see wind speed and direction being coded.
Shown here is an example of how these are presented.
The circle denotes the point of observation or the location of the reading.
The direction is shown by a straight line coming from the circle.
This line points towards the direction from which the wind is blowing.
Shown here is a wind from 270 degrees, in other words, a westerly wind.
The wind speed coding is attached to the line emanating from the circle.
A thin straight line or feather represents 10 knots.
A short line or feather represents 5 knots.
The triangle shows a speed of 50 knots.
In this example we read the speed as 65 knots.
The study of airflow is complex and in order to understand it we must assume certain conditions and construct simplified
models.
The first of these is the geostrophic wind.
Firstly, we'll explain and list the facts we know about the geostrophic wind and expand on each one as it
is mentioned.
The geostrophic wind is subject to two forces.
These forces are in balance and work opposite to each other.
What are these forces though?
The first force is the pressure gradient force.
You may recall from earlier lessons that this is the force that acts from high to low pressure.
Closely spaced isobars tell us that the pressure gradient force is strong.
This situation is common in low pressure systems.
Widely spaced isobars tells us that the pressure gradient force is weak.
This is common in most high pressure systems.
It's the pressure gradient force that determines the strength of the wind.
If the pressure gradient force is very large then there will be a rapid movement of air and the resulting
winds will be very strong.
However, if the pressure gradient force is very weak then the air will move slowly resulting in light winds.
The force which balances the pressure gradient force is the Coriolis force.
Coriolis force arises from the rotation of the Earth on its axis in space.
In the northern hemisphere the Coriolis force acts at 90 degrees to the wind direction, deflecting it to the right.
This results in a wind diagram like the one shown.
The two forces are acting opposite to each other and in balance.
The pressure gradient force pulling air to the left and the Coriolis force pulling it to the right.
The pressure gradient force is said to blow parallel to straight isobars.
The geostrophic wind is said to blow parallel to straight isobars.
But how do we know which way it blows along those isobars?
Looking at the diagram we know that the pressure gradient will cause the air to flow towards the low.
But as the air moves the Coriolis force in the northern hemisphere deflects the air to the right,
causing the wind to blow parallel to the isobars.
Byes Ballots came up with an easy law to help you remember this.
He stated that with your back to the wind in the northern hemisphere the low pressure will be on your
left.
Let's now consider wind speed.
To calculate the speed measure the spacing between two isobars and use that spacing on the geostrophic wind scale to
read off the speed.
Remember to use the latitude on the geostrophic wind scale that corresponds to the latitude at which you took the
isobar spacing.
We've now learnt how to calculate the direction and speed of the geostrophic wind.
But we also need to know at what height it blows.
You may remember from the lesson on turbulence that a layer of air moving close to the surface is disturbed
and slowed due to the friction between the air and the surface.
This layer is called the friction layer.
Friction then is another force acting on the wind.
We may conclude that because the geostrophic wind only has two forces there can be no geostrophic wind within the
friction layer.
You may remember that the height of the friction level changes depending on the time of day and the nature
of the surface.
The friction layer normally extends up to about two or three thousand feet.
It's above this height that the geostrophic wind blows.
The concept of the geostrophic wind is very useful to us. It can act as a benchmark to help us
understand other effects.
Looking at the image of geostrophic wind in the northern hemisphere we can see the two forces opposing each other
and in balance, with the Coriolis force acting at 90 degrees to the right of the wind direction.
The geostrophic wind is the blue line pointing straight up.
We now need to consider why the geostrophic wind does not continue to blow within the friction layer.
Firstly we know that in the friction layer the speed of the wind is reduced due to friction.
However when speed reduces so does the Coriolis force.
Notice now that the forces are not balanced. The pressure gradient force is now the stronger force and will cause
the wind to be deflected as shown.
This deflection is purely as a result of the friction. The greater the surface friction the more the surface wind
is deflected from the geostrophic.
You see from the diagram that over land the deflection is approximately 30 degrees and the speed at the surface
about half that of the geostrophic wind.
Over the sea, which is smoother, the deflection is only approximately 10 degrees and the surface wind speed about two
thirds of the geostrophic wind.
Look at the image and notice that the surface wind has backed and decreased with respect to the geostrophic wind.
Therefore as you climb from the surface to the geostrophic wind level the wind speed will increase and veer.
This is a fundamental diagram to remember.
When using the diagram it's important to understand how the speed and direction of the wind varies from the surface
to the friction level where we find the geostrophic wind.
If we now go back to our surface pressure chart that we saw earlier we can see the geostrophic wind
blowing parallel to the isobars.
But notice that at the surface the airflow is slightly across the isobars and towards the low pressure.
As you can see air flows anticlockwise around low pressures and clockwise around high pressure in the northern hemisphere.
There is one more major variation in surface wind that we need to consider.
This is the change from day to night.
In other words the diurnal variation of the surface winds.
Again we refer to the geostrophic wind and its forces to understand what's happening.
At approximately 1500 hours the surface wind speed will be at its highest.
This is due to thermal mixing and higher level faster winds.
Therefore the surface wind will only be a little slower than the geostrophic wind and the Coriolis force will only
be slightly less than that of the geostrophic wind.
But at 0600 hours the surface wind is at its slowest.
This is because of the lack of any thermal mixing.
Since the surface wind is so slow the Coriolis force is significantly less than the geostrophic wind.
Therefore the surface wind at 0600 will be deflected by a greater amount from the geostrophic wind.
Put simply the surface winds will move between these two extremes depending on the time of day.
From 0600 onwards the surface wind will increase in strength and move towards the 1500 position.
But after 1500 hours the wind will slowly return and move towards the 0600 position.
The winds we've discussed so far are the winds which blow on a large scale all over the earth.
The rest of this lesson deals with more localised winds.
The first of these are land and sea breezes.
Land and sea breezes are local winds that can be found along coastlines, particularly in the tropics.
Land and sea breezes can be observed only when the general wind conditions are slack
and when there are no major pressure systems dominating the coastal regions.
The diagram shows a section across the land and sea during the day.
The land will heat up faster than the sea and will therefore be warmer.
As a result the air above the land surface will be warmer and less dense than that above the surface
of the sea.
This will cause a localised reduction in pressure over the land and air will start to be drawn in from
over the sea.
This air will be heated and start to rise.
The airflow from the sea to the land is known as a sea breeze and its strength will be at
a maximum at about 1500 hours.
This flow of air creates a circulation pattern up to approximately 1000 feet.
Let's look at what will happen at night.
The diagram shows a section across the land and sea during the night.
The land will cool down faster than the sea.
As a result the air above the sea surface will be warmer and less dense than that above the land.
This situation will cause a localised reduction in pressure and air will start to be drawn in from over the
land towards the sea.
This air will be heated and start to rise.
The airflow from the land to the sea is known as a land breeze and its strength will be maximum
just after sunrise where we have the minimum land surface temperatures.
Again this flow of air creates a circulation pattern up to approximately 1000 feet.
Land and sea breezes are particularly important to coastal airfields.
Shown here is an airfield near the sea with the runway parallel to the coastline.
During the night and day a constant crosswind will be experienced across the runway.
There are two other types of localised winds.
Again these can only be observed when the general wind conditions are slack and when there are no major pressure
systems dominating the region.
They are called anabatic and catabatic winds.
In the diagram we see a sloping terrain on either side of a valley.
During the day the land will warm and in turn warm the air adjacent to the surface.
This air will be less dense and start to flow up the valley sides.
These upward flowing winds are called anabatic winds.
Sometimes clouds on top of the ridges are signs that anabatic winds are present.
Always treat mountain ridges with extra caution regardless of the clouds that may or may not be present.
During the night we can see the reverse effect.
The sloped surfaces will cool faster than the air in the valley and the air overlying them will become denser.
As a result it will sink down the sloping terrain and collect in the valley bottom below.
The wind generated by this sinking air is called a catabatic wind.
Throughout the night cold air will continue to collect in the valley.
When a new point is reached this can lead to the formation of valley fog.
The last wind that needs discussion is the fern wind.
This wind is caused when stable air flows over a mountain and descends on the other side.
As air is forced over a mountain it will cool at the dry adiabatic lapse rate of 3 degrees per
thousand feet.
If the dew point is reached then cloud will form.
From then on if forced ascent continues it cools at the saturated adiabatic lapse rate of 1.5 degrees per
thousand feet.
There may be precipitation produced on the windward slope which reduces the water content of the air mass.
However, because the air is stable it will flow back down the leeward side of the mountain and as it
descends it will warm adiabatically.
At first it warms at the saturated adiabatic lapse rate until the cloud disperses.
And then at the dry adiabatic lapse rate as it descends further.
Because there's been a loss of water vapour from the air the cloud base is higher on the leeward side
than on the windward side.
Therefore the descending air will begin warming at the dry adiabatic lapse rate at a higher level.
This means that when the air reaches the valley floor again it's gone through a warming process due to the
loss of precipitation over the mountain.
A good example is found on the southern slopes of the Alps as air flows into the Mediterranean during the
winter months.
This completes the lesson on low level winds. Ensure you know the difference between the geostrophic and surface winds.
Surface winds are always best understood when comparing them to the geostrophic, so be sure to remember how they differ
in direction and speed.
Whenever you examine a pressure chart to find the winds to help in flight planning, don't forget also to bear
in mind the local effect of hills, valleys and sloping terrain.
These may significantly alter the wind from what is suggested by the surface pressure charts.
Visibility within the atmosphere is of great importance to pilots and aviation in general.
Visibility is a crucial factor in all VFR flights and has significant influence on the legality of landing or take
-off.
Meteorological optical range, or METVIZ, is described as the greatest horizontal distance at which a dark object can be recognised
by an observer with normal eyesight.
Alternatively, if it is dark, visibility is a measure of how many lights of a specified candle power can be
seen.
Put simply, visibility is a measure of the atmospheric clarity or obscurity.
It is relatively simple to estimate visibility.
By day, reference is made to suitable objects at known distances from the observing point, perhaps a hill or a
building, such as a church or a factory chimney.
At night, it is more difficult. The observer uses lights, a glow from an urban area or perhaps an outline
of a hill to distinguish the visibility.
There are also several instruments that can be used to measure visibility more accurately.
These are the Gold Visibility Meter, the Visiometer and the Transmissometer.
There are two main obscurations in the atmosphere whose presence will restrict light.
The first is the presence of water droplets. These can either be suspended in the form of cloud, mist or
fog, or be falling as precipitation, for example as snow, rain or drizzle.
The second main obscuration in the atmosphere is the presence of solid particulates, for example dust, smoke or sand.
We can also have a combination of solid matter and water droplets, sometimes referred to as smog.
You may recall from the lesson on stability that a stable atmosphere prevents the vertical movement of air.
A stable atmosphere is mostly the product of a temperature inversion.
Solid particulates generated by industrial and agricultural activities will be trapped close to the Earth's surface, causing them to build
up and reduce the visibility.
Layers of particulate-laden air are frequently seen when flying, and a stark boundary is sometimes visible.
Such a situation is common around cities under the influence of high pressure.
If the visibility is less than 1000m and the relative humidity is near 100%, fog exists.
With a met visibility of 1000m or more and an upper limit usually of 5000m, although this upper limit does
vary occasionally, we have missed.
The relative humidity must be 95% or more.
Haze or smoke is caused by solid particulates and so the humidity will be less than 95% and is
frequently in the order of 50-60%.
Haze or smoke is reported with any visibility below 5000m when the humidity is not high enough to report mist
or fog.
Let's now consider the visibility reducers in a little more detail, namely precipitation and suspended water droplets.
Precipitation reduces the visibility whenever it occurs, but some types of precipitation cause a greater reduction than others.
The worst of these is snow, which can reduce the visibility very swiftly, sometimes to values of 50-200m in
a heavy shower, as shown here.
Drifting or blowing snow has an even greater effect on the visibility.
For private pilot flying, where visual reference to the ground and obstacles is vital, flying in any snow shower should
be avoided.
The reduction of visibility in rain will also depend on its intensity.
Visibility as low as 1000m or less can occur in heavy rain.
Drizzle, on the other hand, can reduce visibility to a greater extent than rain due to the very large number
of droplets present in drizzle.
Visibilities of 500m or less are not uncommon.
Probably the most important reduction of visibility is that caused by fog.
There are five types of fog that have been identified.
These are radiation fog, hill fog, advection fog, frontal fog and arctic or steam fog.
Firstly, let us examine radiation fog.
Radiation fog is caused by the loss of heat from the Earth's surface at night.
The air at low levels is cooled by conduction, and if enough cooling takes place, the air in contact with
the surface will reach its dew point temperature and become saturated form fog.
Certain conditions must prevail for the land to cool by the required amount.
Firstly, we need a clear sky.
Remember that clouds act in a way that can trap heat near the Earth's surface.
These two requirements are usually met when high pressure prevails.
Secondly, there must be sufficient moisture in the air so that cooling will result in saturation, so therefore the relative
humidity needs to be high.
Thirdly, we need light winds of around 2 to 8 knots.
If there is no wind, the condensation nuclei necessary for condensation to occur cannot be suspended.
If the wind is too strong, mixing occurs with warmer air from higher up and prevents sufficient cooling.
Radiation from the sun reaching the Earth will generally lead to the thinning and eventual dispersal of radiation fog through
evaporation.
As the sun warms the ground and the air in contact with it, convection and stirring of the lower layers
of air will take place.
An area of radiation fog will be cleared mainly from below, but also from the fog top and around the
edges.
Increasing wind will cause mixing and warming of the lower layers. This effect will lift the fog into low stratus
cloud.
Hill fog forms when moist stable air is forced to rise over high ground.
The moist air will then condense to create low cloud, which covers or shrouds the high ground.
It can be localised or very extensive indeed, and is a major hazard to low-flying aircraft.
Visibility in this type of fog will usually be less than 200 metres.
Advection fog is formed when warm, moist air moves across a cold surface.
The temperature of the surface must be such that the air moving over it is cooled to below its dew
point.
For this to happen, the wind speed must be around 15 knots and the air must have a high relative
humidity.
The cold surface can be either land or sea.
Advection fog is common around coastlines and is sometimes called sea fog.
Dispersal of advection fog usually occurs as a result of drier air moving into the area or by an increase
in wind speed,
which has the effect of lifting the fog into low stratus cloud.
Frontal fog forms just ahead of some warm and occluded frontal systems.
The evaporation of the water into this cooler air causes it to reach saturation and form fog.
The belt of frontal fog can extend as much as 200 miles ahead of the front itself.
The fog will clear once the front has passed.
Runway visual range, or RVR reports, are designed to give more accurate visibility readings along the runway.
These are only reported when either the horizontal net visibility, or the RVR, is less than 1500 metres.
Reports are taken every 15 or 30 minutes, depending on the amount of air traffic.
The instruments used to measure RVR are called transmissometers and there are usually three placed alongside the runway,
one at both ends and the third in the middle of the runway.
The readings given by these instruments are actually called instrumented runway visual range, or IRVR.
RVR readings can be given by air traffic control to pilots or they can be found in the pre-flight
briefing documents such as METARS and SPECIs.
Shown here is a typical RVR reading.
The prefix R is the identifier for RVR information.
The next is the runway designator, in this example, runway 24L.
The value 1100 is the touchdown zone RVR value in meters. There are some other details we must know.
RVR values less than 50 metres are reported as M0050.
RVRs greater than 1500 metres are reported as P1500.
There may be a 10 minute trend reported. U means the visibility has increased over the last 10 minutes.
D means it has decreased.
And N means no change.
The letter V should be included if there are great variations in the measured RVR.
Here, the RVR has varied between 450 metres to 600 metres during the 10 minutes preceding the observation.
Oblique or slant visibility is the surface distance ahead you can see when operating at a specific height.
If you are flying within a haze layer, you will experience a restricted visibility.
This is shown by the part of the visibility sphere in front of the aircraft.
At the height shown, it is not possible to see the ground. Oblique visibility is zero.
However, if the aircraft descends, then the pilot would be able to see the ground, and he would be able
to see more of the ground ahead of him.
So, within a haze layer, descend to improve your oblique visibility.
Conversely, if operating above the top of the haze layer, the oblique visibility is increased by gaining altitude.
If there is a shallow fog layer covering an aerodrome, the pilot flying an aircraft overhead might think that the
visibility is quite reasonable.
However, lower down on final approach, conditions will appear much worse as the pilot has now got to view through
a greater amount of fog.
This is dangerous and may lead to confusion and disorientation.
That completes the lesson on visibility and on the cause of the various types of fog.
60% of all weather-related air accidents and fatalities occur in conditions of impaired visibility.
Be very aware of the danger of poor visibility.
As a private pilot, you have to maintain visual reference with the ground. Without it, you may quickly become disorientated.
In this lesson on icing, we aimed not only to understand the cause of aircraft and engine icing, but also
the dangers it poses to aircraft operations.
We'll also learn how to recognize and subsequently avoid icing conditions in the atmosphere.
Firstly, let's examine why icing is so dangerous, in particular, airframe icing.
What's the danger of airframe icing?
An aircraft's wing has been carefully designed to create lift and reduce drag.
Ice accumulation will significantly affect the shape of the aerofoil and as a result, lift will be reduced.
The greater the ice accumulation, the greater the loss of lift.
Ice can also disrupt the flow of air over the airframe, causing extra drag.
Another performance factor to consider is weight.
Significant ice accretion can increase the weight of the aircraft. To oppose this extra weight, we need to increase lift.
And as we've just stated, ice on the airframe will have the effect of reducing lift.
The combination of these effects increases the stall speed of an aircraft and reduces its stability.
In flight, this spells danger.
Icing also has adverse effects on aircraft instruments.
It can block vents and ports, and so, in icing conditions, be wary of instruments which rely on these vents
and ports for accurate readings.
Aircraft icing occurs because of the presence of supercooled water droplets inside clouds.
These exist as large or small droplets, or both, depending on the outside air temperature and the type of cloud
in which they are found.
Supercooled water droplets are water droplets which are still in a liquid state, even though the temperature is below zero
degrees Celsius.
This situation prevails only when there is nothing for the droplets to freeze onto, that is, when there are no
freezing nuclei.
Supercooled water droplets can exist in the atmosphere at temperatures as low as minus 40 degrees Celsius.
As you can imagine, there is one surface which, when presented to the supercooled water droplets, will cause them to
freeze.
This is, of course, an aircraft.
The type of airframe icing depends on the size of the droplets present within the cloud.
Let's examine the small supercooled water droplets first.
These can be found at temperatures between minus 15 degrees Celsius to minus 40 degrees Celsius.
Since they are so small, the moment they come into contact with the leading edges of an aircraft they will
freeze instantly.
As the diagram shows, over time the ice will accumulate on the leading surfaces of the aircraft.
This type of icing is known as rime ice.
It's characterised not only by the way it builds just on the tip of the leading edges, but also by
its colour and texture.
It's opaque or milky in appearance.
This is because air is trapped inside the ice because the freezing process occurs so rapidly.
It also has a rough sandpaper-like texture.
This is because small droplets freeze straight away and accumulate to give a granular growth.
It looks and feels very much like the ice that grows on the inside of domestic freezers.
The large supercooled water droplets are found at temperatures between zero degrees Celsius and minus 15 degrees Celsius,
and they freeze in a very different way.
Because of their size, they don't freeze straight away.
When the droplet first strikes the leading edges of the aircraft, only that part touching the aircraft will freeze instantaneously.
The remainder of the droplet can then flow backwards, slowly freezing as it comes into contact with the skin surface.
This build-up is characteristic of this type of icing called clear or glazed ice.
Clear ice looks like glass and is mostly transparent, making it difficult to spot.
It's clear because there is no air trapped inside.
The freezing process is slow enough to allow the air to escape.
This icing is very dangerous. It can rapidly accumulate on the wings and is more difficult to remove.
On most occasions, though, it's unlikely that you will experience just one type of icing.
It's far more likely you'll experience both forms.
This type of icing is called mixed ice and incorporates both rime and clear ice characteristics.
There's a third type of icing called rain ice or freezing rain.
This type of icing is the most hazardous to aircraft operations.
In order for it to form, warm air must overlie very cold air which is below zero degrees.
This is only possible on a warm front, like the one shown here, or an occluded front.
The cloud and rain is being produced inside the warm air as it rises up and along the front.
But notice that at this point the rain is falling into air below zero degrees and it may become super
-cooled.
If an aircraft were to be flying in this area, the rate of accumulation of ice is so severe that
within a very short time the aircraft will not be able to sustain flight, making diversion a necessity.
Be cautious when flying through warm fronts or occlusions, especially if the air ahead of the front is very cold.
If you start to encounter freezing rain, turn away and leave the area immediately.
There is one more type of icing.
This is called hoar frost and it's the only icing that can develop clear of cloud.
This icing is identical to that which can develop on car windscreens on frosty nights.
It can develop on an aircraft in flight or on the ground.
On the ground on clear nights, when the surface temperature falls below zero, water vapour can turn directly to ice
and hoar frost can be seen on the surface as the ice crystals are created.
It's important that prior to flight this ice be removed, since it can interfere with the airflow over the aircraft.
This kind of icing can occur in flight in clear air when an aircraft rapidly climbs through an inversion.
If the initial air temperature is below zero, then the skin of the aircraft will also be below zero degrees.
But when it climbs and enters the warmer air, the air travelling over the skin is cooled and sublimation can
occur, creating ice crystals and frost.
This can also be observed in a rapid descent from high altitude, where the aircraft's skin temperature is still below
zero.
As the lower warmer air flows over the skin, it again can be cooled to freezing point.
If your windshield is suffering from this icing, then the danger is that you will not be able to maintain
visual reference with the ground.
We mentioned earlier that both the outside air temperature and the cloud type determine the size of the supercooled water
droplets.
Cumuliform type clouds frequently have strong vertical updrafts of air moving within them.
They can therefore support significant numbers of large, supercooled water droplets.
As a result, a pilot can expect moderate or even severe clear ice within these clouds, especially in flight just
above the zero degrees Celsius level.
In cumulonimbus clouds, the updrafts are even greater, and so the icing risk is always severe.
Avoid these clouds, not just because of the icing risk, but because of the other dangers posed by them.
In layered cloud the icing risk is usually much lower because of the lack of vertical air development.
Even if the temperatures were suitable for the formation of large, supercooled water droplets,
the cloud has little capability to support them, and even less capability of supporting large quantities of them.
As a result, we can generally expect only small droplets, and therefore only light rime ice.
The cloud which is an exception to this is nimbostratus.
It can have the upcurrents to support the large droplets, making the icing risk moderate or even severe.
Having considered the danger of airframe icing, let's now consider another type of icing that you may experience.
This is called induction icing.
Known here is a simple induction system.
The first of this type of ice is called impact ice.
This occurs as snow or supercooled water droplets sticking or freezing instantaneously when they come into contact with the walls
of the intake chamber and air filter.
It can eventually block the air intake completely, causing the engine to stop.
Any type of piston engine can suffer from this type of ice build-up.
Fuel-injected aircraft have an alternate air supply to feed the engine when the normal air intakes are blocked.
When the relative humidity is higher than 50% and the temperature lies between about minus 10 degrees Celsius to
plus 30 degrees Celsius,
another type of ice, called carburetor ice, can build up within the induction system.
This can be caused in two ways.
As air passes through the venturi, shown here, static pressure decreases and the temperature of the air will fall, possibly
causing ice to form within the venturi.
The throttle butterfly valve has a similar effect.
On low power settings the butterfly valve is almost closed and restricts the airflow.
This acts like the venturi and a drop in temperature can occur, causing ice to build up and possibly jam
the throttle.
The effect of the vaporization or evaporation of fuel as it leaves the fuel jet, shown here, may combine with
this to aggravate the situation.
This process causes latent heat to be absorbed and thereby reduces the temperature, possibly leading to ice formation.
The combined effect of these icing processes will cause a piston engine to be starved of air, causing it to
run rough and possibly stop.
However, there is a way to combat this potentially dangerous situation.
By selecting carb heat we can divert warm air from around the engine compartment into the induction system to help
melt the ice and prevent starvation of the engine.
However, as hot air passes through the engine performance will initially deteriorate further because of its reduced density.
This may appear rather alarming, but don't worry, this is a sign that the hot air is doing its job.
Similarly, ice and liquid water which may pass through the engine may again cause it to run rough, but as
before, this is a sign that it's working.
After a few moments the engine will start to operate normally again.
This completes the lesson on icing. We've dealt with some very important issues that a pilot must be aware of.
Icing is dangerous. Being an informed and educated pilot is the best way to counter that danger.
Above all, expect airframe icing in cloud whenever flying above the freezing level.
At the private pilot level you should stay well clear of any cloud, especially convective clouds.
Lastly, when on low power setting in the descent for instance, especially when the humidity is high, always be prepared
for a rough running engine caused by carb icing.
Remember that the application of carb heat may momentarily cause the situation to worsen.
Keep the carb heat applied and the engine performance will improve.
We may define an air mass as a large body of air in which the temperature and humidity are more
or less constant.
These bodies of air originate from source regions where they originally developed their characteristics because, given time, air will pick
up the characteristics of the surface that underlies it.
For example, if the surface of the earth is cold and wet, then the overlying air, given enough time, will
also become cold and wet.
Conversely, if the ground is warm and dry and the air overlying it remains there for long enough, it too
will become warm and dry.
In order for the air to obtain the characteristics of the earth's surface, the air must remain over the source
region for quite some time.
We can observe this phenomenon in areas of high pressure or anti-cyclones where the winds are light and the
air mass is stationary.
We classify air masses in two ways, by temperature and humidity.
In the classification of temperature we use the description polar, which is cold air from polar sources, and tropical, which
is comparatively warmer air from tropical origins.
There is a third, called arctic, which does not necessarily imply it is colder than polar, but rather it describes
its true origin, which is nearer the North Pole.
There are two classifications denoting the humidity of air masses. The first is maritime, which implies a moist air mass
whose source region is the sea.
The second classification is continental, which implies a dry air mass whose source is a continental land mass.
We use a combination of the temperature and humidity characteristics to label and list the air mass types.
In temperate latitudes around the United Kingdom these are polar maritime, polar continental, tropical maritime, tropical continental, arctic, and finally
returning polar maritime.
Under certain conditions these air masses will affect the United Kingdom.
In order for them to do this they must move from their source region. However, once air masses move, they
become modified by the surface they are moving over.
They will normally either become warm or cool as they move. This will dramatically change the air mass and create
unique weather characteristics.
If an air mass moves to a warmer area, for example towards the equator, it will be warmed from beneath.
Its environmental temperature lapse rate will then increase and the air will become unstable.
Vigorous ascent of air may take place, and if there is moisture in the air, cumuliform clouds would develop.
Conversely, if air moves from its source region to a colder part of the globe, for example towards the poles,
it will be cooled from below.
Its environmental lapse rate will decrease and the air will become stable.
If there is moisture present in this type of air mass, low level stratiform clouds will develop.
Now we have looked at the movement of the air masses, let us consider each air mass in detail and
see what happens when they arrive over the United Kingdom.
Click on the relevant air mass to discover how it influences UK weather.
Then, once you have seen each one, click the forward button to continue with the lesson.
Polar maritime air approaches the British Isles from the northwest.
Its source region is the North Atlantic, but when moving south it will pass over warmer surfaces.
This will cause it to become increasingly unstable as it approaches Europe.
As a result of the instability of this relatively moist airflow, we tend to get cumulus and occasionally cumulonimbus development,
so showers can be expected, perhaps even with hail and thunder in certain circumstances.
Daytime flying conditions are quite bumpy caused by the instability and the overturning of the atmosphere.
However, visibility is generally excellent outside of the precipitation zones.
Showers usually die out in land overnight as the land cools, often leading to clear skies, inversions and sometimes radiation
fog.
If polar maritime air moves southward before swinging to the United Kingdom on a northeast track, we call it returning
polar maritime air.
It is distinct enough to have a classification all of its own.
As the initial track is southwards, the air mass becomes unstable with plenty of moisture gathered on its passage over
the ocean.
However, when it then heads towards the United Kingdom, it can become stable in its lower layers as it travels
slightly polewards and cools a little from below.
However, the rest of the air mass, for example the middle and upper parts, are still unstable.
The cloud type which forms in returning polar maritime air is generally stratiform in the lowest layers, with potentially unstable
convective cloud in the middle layers.
Tropical maritime air is the most common air mass affecting Europe.
Its origin is the mid-Atlantic around the Azores, and so it approaches from the southwest cooling from below on
its northward track, thus increasing its stability.
Since it is the lower layers which cool, only low cloud is produced with poor visibility and advection fog, especially
on windward coastlines.
Inland conditions are often slightly better, but still relatively poor with extensive low cloud.
Inland in summer, however, there is usually enough heat from the sun to lift and occasionally break the cloud into
shallow cumulus cloud.
Tropical continental air approaches the United Kingdom from the southeast.
This air mass mainly affects the area in the summer and is responsible for the highest temperatures.
It is dry usually with cloudless skies, although hazy conditions often result, giving moderate to poor visibility.
The polar continental air mass affects Europe in winter only.
Originating from Siberia or northern Russia, it is a stable, very dry and cold air mass.
Polar continental air has little moisture since its origin is over an extensive land mass.
It brings cloudless but bitterly cold conditions to much of the country.
If there is sufficient track across a sea area like the North Sea, there will be enough warming from beneath
and picking up of moisture to cause convection to take place.
On occasions, this convection can be extremely vigorous, with snow showers resulting on eastern coastal areas.
Arctic air comes down to the United Kingdom in strong northerly air flows.
The air quickly becomes unstable as it moves south across warmer and warmer sea, bringing snow showers to northern Scotland.
Having completed the section on air masses, let's now focus on fronts.
Fronts are transition zones between air masses of different temperature, namely polar and tropical.
You may recall from the pressure systems lesson that a frontal boundary exists between polar maritime air and tropical maritime
air, as you can see here.
This is called the polar front.
We will concentrate on the polar front for the remainder of this lesson.
This frontal boundary is very fluid, and when warm air moves northwards to replace heavy cold air, pressure is reduced.
As air along the surface moves towards the low pressure, it is deflected to the right by Coriolis force.
This flow of air creates two unique frontal features, a cold and warm front.
Along this part of the polar front, cold air is being forced against warm air and undercutting it.
This is called a cold front. It is shown here in cross section.
Along this part of the polar front, warm air is being forced up and over the colder air the other
side of the front.
This is called a warm front and is shown here in cross section.
To get a better perspective of what the whole system looks like in cross section, for example the warm and
cold front,
let's briefly return to the polar front depression again, looking at the plan view.
These systems are found between 35 degrees north and 65 degrees north and move from west to east across the
Atlantic and Pacific.
What we need to understand in more detail is what weather conditions we might expect if such a system were
approaching the British Isles.
The fronts will normally arrive in sequence, the warm front first, followed by the cold front.
On the plan view, we can draw a line which cuts through both cold and warm fronts, and we can
then look at the cross section as shown here.
The warm front will be the first of the fronts to influence the weather as the system moves from left
to right.
The slope of a typical warm front is 1 in 150. It is very shallow and the cloud that develops
is mainly stratiform type.
However, the first clouds noticed by an observer on the ground will be very high altitude cirrus, then cirrostratus.
Eventually, as the front gets closer, thicker altostratus will be observed and eventually, where the front meets the ground, the
nimbostratus will prevail.
The whole sequence may take up to 12 hours to pass over a location.
The cloud ahead of the warm front can extend for about 600 nautical miles ahead of the surface position of
the front.
Notice how the main area of precipitation starts some 200 to 300 nautical miles ahead of the surface front as
the base of the cloud gradually lowers.
This can persist for many hours as the front gets closer.
As the precipitation increases closer to the ground position of the front, visibility will reduce and eventually frontal fog may
be observed.
We've considered the cloud ahead of a warm front, but what happens to the other elements of the weather?
First, let's look at the wind. To see this, let's return to the plan view.
Ahead of the warm front, notice that the wind is fairly light, coming from about 250 degrees.
But, as the front approaches it does change. It slowly increases in speed and blows from about 200 degrees.
So with the approach of the warm front, the wind backs slightly and strengthens.
To summarise, these are the conditions that you would expect to observe at the surface with an approaching warm front.
The cloud will increase and the base will lower.
Precipitation will set in, becoming persistent and moderate in intensity.
The wind will back and increase in strength.
The visibility will deteriorate.
The pressure will fall steadily.
The temperature will remain at a fairly steady low value.
The dew point will usually be steady and fairly low.
Next, we shall consider the weather expected when the surface warm front arrives, as shown here on the plan view
and here on the cross section view.
The cloud will be low and remain fairly constant.
Precipitation will be continuous and moderate, perhaps heavy.
Looking at the plan view, you will notice the wind changes direction quite markedly from here, just before the front,
to here, just after the front.
We see here a sharp veer in the wind.
Any sudden change in wind direction near the surface needs to be planned for when you are flying because of
the changes in heading that this will necessitate to maintain a fixed track.
Visibility will be poor, with widespread hill fog developing.
The pressure will continue to decrease slightly.
The temperature and dew point will suddenly rise as we enter the warm tropical air.
This warm air between the cold and warm fronts is called the warm sector.
As we have just learnt, the warm sector lies between the fronts.
This is a stable air mass and the weather is usually fairly consistent throughout this section.
It is characterised by a fairly uniform base of extensive stratus, or stratocumulus cloud, with the possibility of light drizzle.
The wind will be steady and often strong.
The visibility can be poor, with extensive fog over high ground.
Looking at just the plan view, you will notice that in the warm sector, as we approach the cold front,
the pressure continues to fall slowly.
Lastly, the temperature and dew point will remain fairly high and steady throughout this warm, moist air mass.
Now let's look at the cold front, which will follow behind the warm sector as the system moves from left
to right.
A cold front is almost a warm front in reverse, but note that the average slope is a little steeper,
usually 1 in 50.
The cold air advances and undercuts the warm air, forcing it aloft, causing the air to cool adiabatically and condense.
The main cloud is still stratiform since the slope is still very shallow.
However, as you may recall from the pressure systems lesson, the nose of cold air here creates very unstable conditions
and air rises very rapidly, creating large cumuliform clouds ahead of the surface position of the front.
This makes the passage of the cold front quite sudden with little warning that it is approaching.
These cumuliform clouds can create dangerous squall line thunderstorms.
It is highly recommended that you never fly through a cold front at low level, no matter what your experience
or capability.
Watch the weather immediately ahead of the cold front very carefully too.
To summarise then, these are the conditions that you would expect to notice at the surface with an approaching cold
front.
Expect embedded cumuliform cloud within the stratiform cloud, giving steady and showery precipitation.
The wind will veer sharply with gusts and squalls are also a possibility.
Before the front arrived, the surface pressure will fall but will rise once the front has passed.
In the precipitation, the visibility will be poor but will quickly show signs of improving.
Lastly, the temperature and dew point will usually fall sharply as we cross into the cold air behind the front.
Once the cold front has passed, the weather will be dominated by the conditions behind the cold front, as shown
here.
In this area, notice that cold polar maritime air is moving towards the equator and is therefore travelling over warmer
surfaces.
You may recall that this creates instability and therefore isolated cumuliform clouds will develop behind the cold front.
The pressure will continue rising, with the visibility good, though it will deteriorate temporarily in the showers.
Remember, even though the dangerous conditions of the cold front have passed, do not presume that the conditions will improve
significantly.
You may still find significant weather after the cold front, as we have shown.
Having seen the weather expected with the passage of the polar front depression, we will now examine how it is
possible to calculate the movement of the fronts themselves.
The speed of movement of a warm front can be estimated using a simple technique.
All you need is a weather chart with a geostrophic scale on it.
Measure the isobaric spacing on the front like this.
Use that measurement on the geostrophic wind scale and then multiply the answer by two thirds.
This gives the approximate speed of the front.
Having seen how to calculate the speed, let us look at the direction.
The front will move in the direction of the symbols, but more accurately it will move parallel to the isobars
behind the front.
The speed of movement of a cold front can be estimated in a similar manner to the warm front technique.
Again, measure the isobaric spacing on the front like this and use the geostrophic wind scale.
This gives an approximate answer for the speed of the front.
The direction of the cold front will be parallel to the isobars behind the front.
Of the two types of fronts we have seen, the cold front is the faster moving, and it will eventually
catch up with the warm front as we see here in plan and profile views.
When the two fronts meet, they merge and create what is known as an occluded front.
Notice that to identify this new front, both the warm and cold front symbols appear together on the frontal line.
However, as you will now see in the cross section, they can merge in two ways.
If the cold front goes up and over the warm front like this, it is called a warm occlusion.
However, if the cold front goes underneath the warm front like this, then it is called a cold occlusion.
The weather of the occlusion is a combination of the warm and cold front weather.
This is typically long periods of continuous rain interspersed with heavy showers.
Occlusions in the United Kingdom are responsible for most of the major flooding events over recent years.
There is one last front to examine before we complete this lesson.
Let us return to the polar front boundary we observed at the beginning of the lesson.
There is little interaction between the warm tropical maritime and polar maritime air, then no kinks or disturbances will be
observed.
This is described as a stationary front and the cold and warm front symbols will be shown like this to
signify the static boundary.
Note though that the winds will blow parallel to the frontal position as shown.
That completes the lesson on air masses and fronts.
It is a very important part of aviation meteorology and it is vital to understand the various points that have
been brought out through this lesson.
The passage of fronts has significant implications for private pilots.
Pressure and temperature changes can cause altimeter altitude errors of up to a thousand feet or more.
Crossing any front will inevitably lead to flying on instruments and combined with wind changes this could be dangerous for
an inexperienced pilot, even one with an instrument rating.
When fronts are present pre-flight planning needs to be especially thorough and detailed if the fronts are to be
avoided or negotiated.
METARs are aviation routine weather reports.
The code METAR stands for meteorological aerodrome report.
These are coded messages of the actual weather conditions at a stated time and place.
METARs are usually issued every half hour during aerodrome operating hours.
We shall examine the METAR coding section by section, slowly building up our understanding by using a sample METAR report.
The first group that appears in the METAR is the identification of the type of report.
The four digit IKO location indicator of the issuing airfield is shown next.
In our example we will use Kidlington, Echo, Golf, Tango, Kilo.
The next group is the date and time of the actual weather observation.
The first two digits represent the date,
followed by the time in hours and minutes according to universal time coordinates.
Notice though that the code Z for Zulu time is used instead of UTC.
The observed wind information is shown next.
Firstly, the direction of the wind given in degrees true round up or down to the nearest 10 degrees.
Next, the wind speed in knots which is mean over a 10 minute period.
However, if a gust is observed at least 10 knots more than the mean, then a gust figure comes after
the mean wind.
This gust figure is preceded by the letter G as shown.
The next group may or may not appear depending on the directional variability of the wind.
It is shown after the main wind group as indicated here.
It signifies the extremes in the direction of the wind during the past 10 minutes.
The letter V will appear between these two extremes.
If the wind is calm, then this coding is used.
Visibility in the metta is represented by the next group.
When there is no marked variability in visibility by direction, the minimum visibility is usually given.
Up to 10 km, this is measured in metres.
For example, 6000 means that the minimum horizontal visibility is 6000 metres as measured from the observing point.
Once the visibility reaches 10 km or more, the code figure is 9999.
Visibility of less than 50 metres is indicated by the code 0000.
In some instances, we can also find the runway visibility information.
This is known as Runway Visual Range, or RVR.
It is only given when either the horizontal visibility or runway visual range is less than 1500 metres.
This group starts with the letter R,
and then goes on to give the runway in use.
Followed by the threshold visibility in metres.
In our example at Kidlington, let us consider a met visibility of 1400 metres with an RVR on runway 30
of 1100 metres.
If the RVR visibility is more than the maximum reportable value, then code P is used in front of the
visibility value.
A letter can sometimes come after the RVR visibility to indicate the trend that has been experienced.
A U means that the visibility has increased by 100 metres or more in the last 10 minutes.
A D that it has decreased in that same time period.
And an N that there is no distinct tendency.
The next section of the METAR deals with weather groups.
This signifies what weather is actually prevailing at or near the aerodrome at the time of the observation.
This is a complete table showing you what codes can be used to describe the weather.
The first column shown here represents the intensity or proximity of a weather phenomenon.
These are
minus, which means light,
no qualifier, which means moderate,
plus, which means heavy and lastly
BC, which means in the vicinity but not at the observing point.
The next column adds detail or describes the actual weather phenomenon which we will see later.
Three of the most common that you are likely to see are
BC, which means patches and is frequently used to describe fog,
SH, which means showers,
and TS, which is thunderstorm.
The last three columns represent weather phenomena.
We shall now look at each of these columns in turn and explain their usage.
The first of these columns considers precipitation.
This group is fairly self-explanatory.
It contains all the precipitation weather such as drizzle, rain, snow, hail etc.
The next column looks particularly at those weather phenomena which are classified as obscurations.
These include mist, fog, smoke, ash etc.
The final column contains all other weather phenomena which have not already been included.
This group mainly consists of the more unusual weather events that are rarely reported but are important nonetheless.
This completes the whole table of the weather groups of the META.
Let's now look at how some of these symbols and letters are used.
Let's analyse the code which indicates a heavy shower of rain.
We use the following symbols and letters to represent this.
Heavy,
Shower,
of rain.
The definition of a thunderstorm occurring at a location is that thunder has been heard within the last 10 minutes.
Therefore, just because lightning is seen, it doesn't necessarily mean that there is a thunderstorm at any one place.
Whenever a thunderstorm occurs, the letters TS are used.
If there is no precipitation, then only those letters will be used.
However, if there is precipitation of some kind, another two letters are inserted after the TS.
For example, if there is rain observed from the storm, then TSRA will appear.
However, hail may be observed, so you may see TSGR or TSGS.
GS meaning Small Hail.
Cloud is the next information in the META.
There are several cloud prefixes which are used to describe the amount of cloud in each layer.
We saw in the lesson on clouds that we subdivide the cloud coverage into eighths, called octaves.
Depending on the cloud coverage, cloud amounts may be classified as
Few, meaning one to two eighths of cloud.
Scattered, meaning three to four eighths.
Broken, five to seven eighths.
Or overcast, meaning complete cloud coverage, eight eighths.
The height of the cloud will follow immediately after the amount.
This will appear as a three-digit height in hundreds of feet.
Cloud heights in a META are always measured from aerodrome level on QFE.
For example, six octaves at 1,900 feet would appear as
And eight octaves at 500 feet would be abbreviated to
The only cloud types that are specified in the META are the significant convective cloud.
These are cumulonimbus and towering cumulus.
Shown here is an overcast layer of cloud but with a convective cumulonimbus cloud
at about 2,000 feet above ground level, producing a heavy shower of rain.
If there is no cloud observed at the airfield, then the code SKC, meaning sky clear, is used.
If the sky is obscured for reasons other than cloud cover, so that the cloud cannot be determined easily,
then the code VV is used in place of the cloud information to report the vertical visibility in hundreds of
feet.
Shown here is an example of an obscured sky because of fog.
A vertical visibility of 200 feet has been reported.
If the vertical visibility cannot be assessed, then three forward slashes will replace the number of feet.
As shown here.
The term CAVOK is frequently used in the META code to denote that cloud ceiling and visibility are OK.
If CAVOK is used, it will replace the visibility, RVR, weather and cloud groups.
There are four criteria which must be met in order for CAVOK to be used.
The first of these is that visibility must be 10km or more.
Next, the lowest cloud ceiling must be no lower than either 5000 feet or the highest minimum sector altitude, whichever
is the greater.
No cumulonimbus must be present and there must be no significant weather.
If these criteria are met, then the abbreviation CAVOK will appear in the META code.
The temperature and dew point will be the next group in the META code.
This is simply a two digit number for the air temperature, a forward slash, followed by another two digit number
for the dew point.
These are measured in degrees Celsius.
In this example, the air temperature is plus 10 degrees Celsius and the dew point is plus 2 degrees.
If either are negative, they will have the prefix M.
Now the dew point is minus 2 degrees Celsius.
The next important feature of the META is QNH.
The QNH will be represented by the letter Q,
followed by a four digit number representing the actual pressure value.
If the QNH is less than 1000 mbar, then the value will be preceded by a zero.
For example, a QNH of 991 mbar would appear as
Q0991.
It is important to note, however, that none of the other pressure values will appear in a META, only the
QNH.
The QNH is always rounded down for safety reasons.
The group entitled Recent Weather is used only when significant operational weather has occurred in the past hour or since
the last META, whichever is shorter.
The group always begins with RE and is included if a weather phenomenon has recently ceased or has decreased in
intensity.
RE actually stands for Recent.
If there was a thunderstorm during the hour, but by the time of the observation the rain was light,
the present weather would be minus RA with RETS as the recent weather group.
Although not currently issued at UK airfields, wind shear information can be reported in the META.
This will simply be denoted by the letters WS, followed by the details of the report.
A trend forecast is valid for two hours after the time of the observation and is attached to the end
of the META.
It gives the changing conditions expected. The change can be indicated either as
Becoming or
Temporarily. These may be followed by a time period in hours and minutes.
However, these time periods may be preceded by FROM, TIL or AT.
Let's look at an example. This trend is suggesting that temporarily from 10.20 Zulu to 12.20 Zulu, the
visibility will reduce to 1000 metres in heavy showers of rain.
If there is no change forecast, then the word NOSIG is used to imply no significant changes expected in the
next two hours.
An equal sign will appear on the META to denote the end of the message.
A variation on the META is a special report. This will have the same format as a META,
but will be denoted by the abbreviation SPECI at the very start of the message.
A specific report will be issued when the weather conditions significantly change in the period between routine observations.
This can be used to indicate either an improvement or deterioration in the weather.
With practice, you should be able to decode META reports accurately and speedily.
Make sure you do not just look at the META for your departure or destination airfields,
but also at others along your route, and in particular those upwind, in order to get a better picture of
the conditions that could be with you soon.
Always be wary. Conditions will and do vary from what you may have interpreted from the META, so be cautious
and assume the worst.
TAFs are different from the META in that they describe the forecast conditions at an aerodrome, and usually cover a
period of between 9 and 24 hours.
9-hour TAFs are usually issued every 3 hours, and 12-24 hour TAFs every 6 hours.
The TAF is issued separately from the METAs, but many of the codes and groups in the META are also
used in the TAF.
We shall only consider the main differences in this lesson.
Shown here is an example TAF.
The first obvious difference is that the reported identifier is now a TAF, not a META.
The ICAO location indicator of the airfield issuing the report is the same as that for the META. Again, we
use Kidlington.
Remember that the META was just an indication of the weather at a specific time.
However, the TAF aims to give a forecast over a period of time, so the TAF will have a different
time group to the META.
The first of these is the time of origin. The date first, and then the time in hours and minutes
in Zulu.
However, the validity time comes next. Again, this will start with the date, then the start time in hours, and
then the end time in hours.
So here we have a validity period from the 13th at
0700 Zulu to 1600 Zulu the same day, in other words, a 9 hour TAF.
The decoding of the wind is the same as for the META, and the example shown here is a mean
wind direction of 310 degrees true at 15 knots.
Visibility in the TAF follows the same coding rules as the META, except that only one minimum visibility value is
shown.
In our example, the visibility is 8000 metres.
Much like visibility, the weather coding for the TAF is the same as the META, and the table shown here
is to refresh your memory.
However, cloud coding is slightly different than the META.
If there is no cumulonimbus, or any cloud below 5000 feet or below the minimum sector altitude forecast, and CAVOK
or SKC does not apply, then the code NSC is used.
This stands for No Significant Cloud.
Another difference is that only cumulonimbus clouds are ever reported in TAFs, whereas in the META towering cumulus and cumulonimbus
clouds were reported.
The main TAF information ends at the cloud group.
You may recall though that the META had a lot of extra information after the cloud groups, as shown here.
TAFs however, do not have any temperature and dew point, Q&H, recent weather, wind shear or runway state information.
Only significant changes of weather follow the cloud group.
This part looks at how the significant weather changes are coded within the TAF period.
The first change indicator is FM, meaning FROM.
This is followed by a time group in hours and minutes, as shown here.
This means that from 12.20 Zulu certain weather changes will occur.
These weather changes will supersede the corresponding weather prior to this period.
In this example, from 12.20 Zulu till the end of the TAF period, the wind will change to be
270 degrees true at 17 knots with a minimum visibility of
4000 metres and some broken cloud at 1000 feet.
This is just one type of weather change indicator that can be used.
The change cannot be accurately forecast to the hour and minute, then another code is used.
The change group BECOMING, followed by a four-figure time group, indicates the expected beginning time of a permanent change
in the forecast conditions.
These may be regular or irregular and starting at an unspecified time within the period given.
By the end of the time period given, the change would have occurred and the conditions suggested will now prevail.
Let's look at the example.
This means that at some time between 0900 Zulu and 1100 Zulu,
the visibility will permanently become 5000 metres in light rain.
Notice that the wind information has not changed. It will still be 310 degrees true at 15 knots.
The other change indicator is TEMPO, meaning temporarily.
This is different than the BECOMING change indicator in that this change may occur at any time within a specified
time period and not be a permanent change.
These temporary fluctuations are expected to last less than an hour each time, but in aggregate no more than half
the time period given.
Let's look through this example.
Between 1200 Zulu and 1400 Zulu, the visibility will fall to
4000 metres, with the weather being
thunderstorms and moderate rain.
There will be 5 to 7 octaves of cumulonimbus cloud at 1000 feet.
However, after 1400, the weather will return to the base conditions specified here.
A probability indication can be used to assess the likelihood of occurrence and is shown by the code PROB.
This is expressed as a percentage of either 30%, which is considered low probability, or
40%, which is a high probability.
Probability can be followed by a time group, as shown here, or by an indicator, such as becoming or temporarily.
One last thing must be covered before we finish the lesson.
When a TAF requires an amendment, the amended forecast may be indicated by inserting an AMD after the TAF identifier,
as shown here.
Hopefully from this lesson you will now be able to accurately decode the TAF message.
Remember, it is a forecast of the expected conditions and therefore essential to your met briefing prior to your flight.
If you use TAFs correctly, you will be in a better position to make accurate and informed decisions about your
flight and the expected conditions of the aerodromes you visit.
In the United Kingdom, the Spot Wind Chart is referred to as Form 214.
It is an excellent briefing tool, allowing you to quickly identify the wind velocity and temperature at different altitudes and
locations in and around the UK.
Form 214 should be used as an everyday reference to flights within the UK and Republic of Ireland, up to
an altitude of 24,000 feet.
At the top of every chart, there will be information on validity times.
Since there are four issues of this chart every day, it is important to ensure it covers the time period
of your planned flight.
The chart is a forecast of conditions expected at a given time and date.
In our example chart, we see that it is forecast to 14 February 2003 at 1500 UTC.
But valid three hours prior to this, that is, from 1200 UTC, and three hours after, that is, until 1800
UTC.
The chart itself shows a map of the UK and Ireland overlaid with information boxes.
The data within each box is specific to a location or spot on the map.
The coordinates of these locations appear at the top of each box.
Let's have a closer look at one of these spot locations.
The coordinates for the spot are shown here in degrees and minutes.
More simply though, notice that the box overlies the intersect point of the latitude and longitude coordinates it pertains to.
Underneath the coordinates are four columns.
The left-hand column gives the altitude in thousands of feet above sea level.
The next column shows the wind direction.
This is given in 10 degree intervals and reference to true north.
Be careful then to account for variation when using this wind for cross-country flight planning.
Occasionally the abbreviation VRB will be used to denote a variable wind direction when wind speed is below 5 knots.
Next follows the wind speed in knots in 5 knot increments.
The last column shows the static air temperature to the nearest whole degree Celsius with either a positive or negative
prefix.
In using this chart for flight planning and flight briefing you may need to interpolate the data if your altitude
is between the altitudes in the information box
and also if your departure and destination airfields are situated between the given spot locations, as they almost certainly will
be.
For example, let's assume you are planning to fly the route shown.
You elect to fly initially at 3,000 feet altitude, and then climb up to 4,000 feet before passing
over the Cheveot hills and the southern uplands.
Both these altitudes fall between the 2 and 5,000 feet rows on the Form 214.
For the first half of the route you would plan for a wind from 075 degrees at 17 knots.
For the second half of the route you would assume a wind from 140 degrees at 10 knots.
At mid-route you will expect a wind of about 105 degrees at 12 knots.
Obviously the winds will almost certainly vary along the route, and also with time.
However, Form 214 will give you a good estimate of the wind conditions you can expect to encounter along the
route.
This information can then be used in planning your navigation.
Although the Form 214 has a lot of information pertinent to flight planning,
you will still need to supplement this with other forecast charts such as significant weather charts and the relevant TAFs
and METARs.
Low-level significant weather charts are a vital aid to successful flight planning for flights that take place between the
surface and 10,000 feet.
These charts indicate in a precise and very structured format the meteorological phenomena which are anticipated.
Form 215
In the UK the chart is commonly referred to as the Form 215, or just 215.
They are produced every 6 hours, 365 days a year for low-level aircraft flying over the UK and adjacent
areas.
The 215 is issued approximately 3 hours before the start of the validity period, and each chart is valid for
6 hours.
A precise period of validity is clearly identified at the top of each forecast.
In this example we have a chart which was issued at 0310 UTC on 8 January 2002,
and is valid between 0600 UTC and 1200 UTC on 8 January.
And is valid between 0600 UTC and 1200 UTC on 8 January.
In this example we have a chart which was issued at 0310 UTC on 8 January 2002.
Below the chart title and validity information are two schematic maps.
The left-hand map is a snapshot at a single forecast time, showing the positions of any fronts and with
areas of weather clearly divided into zones.
The time at which this forecast chart is valid is clearly displayed beneath the chart.
In our example we have a chart which is valid on 8 January at 0900 UTC.
The chart uses standard weather symbols, many of which you may be familiar with.
Shown here is a list of codes and symbols, and it might be worth spending time looking through the chart
and remembering some of the symbols and codes.
In this example we have a map which appears in the top right-hand corner of the chart,
and below the chart is a synoptic outlook chart which is valid six hours after the expiry of the main
chart information we just saw.
In our example, this outlook chart is valid on 8 January at 1800 UTC.
Any anticyclonic and cyclonic centres are identified on both of the maps by a small cross or circle, and the
letters H and L respectively.
Returning to our original chart, let us now focus in detail on the left-hand map.
As previously mentioned, frontal boundaries and zones of weather are clearly displayed.
These weather zones are bound by scalloped lines, and in some cases the edge of the map area.
Zones are clearly distinguished by an encircled zone number.
In our example, here we have the map area divided up into five different zones.
Each zone is linked to the text contained in the main body of the chart by the zone number.
The movement of fronts and zone boundaries is identified by arrows which indicate the direction of movement.
The numbers which appear beside the arrows indicate the speed at which the feature is moving in knots.
If a front or zone boundary is stagnant or moving at less than 5 knots, then the word SLOW is
indicated on the chart.
The height of the 0°C isotherm or freezing level is highlighted on the chart using small boxes.
This example shown here is a freezing level at 3,000 feet.
Let us now look at the text information on the low-level significant weather chart.
As we saw earlier, the forecast information is linked to the different zones on the chart by zone numbers.
The forecast text which applies to each zone is divided into individual sections below the chart maps,
and the zone number is indicated in the top left hand corner of each section of text.
However, there may be subdivisions based on the frequency of occurrence of specific weather phenomena, as we shall now see.
The text which appears in each section, zone 1 in our example, follows standard abbreviations.
Each section is divided into four different columns of information.
The first column describes the frequency or distribution of a weather phenomenon.
The abbreviation GEM is always used to indicate the general conditions which can be expected to occur in each zone.
However, variations may exist within the zone that are different from the general conditions.
More detail is provided underneath the general information using the abbreviations for
In these cases, additional information is included.
In our example, these are the general conditions.
But isolated within zone 1, we may expect these conditions to prevail.
The second column of information contained in the low-level significant weather chart
relates to the visibility which is forecast to occur with each weather phenomenon.
In our example, generally within zone 1, we can expect 25km visibility.
But in isolated areas, this will be reduced to 7km.
The third column of text relates to the actual weather phenomena.
Many of the weather phenomena are indicated on this chart by using codes that are identical to those seen in
the TAFs and METAs.
In our example, in zone 1, generally there is no significant weather.
But within this zone, there will be isolated occurrences of rain showers.
The last column of information on the chart relates to cloud forecasts.
Cloud amounts are always displayed in octaves and sometimes a range of amounts is given.
Cloud types are always forecast using standard cloud type abbreviators
and cloud hides are always displayed in feet above mean sea level.
The first line of cloud information which corresponds to the general conditions which are forecast to occur in zone 1
indicates that 2-4 octaves of cumulus and stratocumulus are expected to occur with a cloud base of 2,500
feet
and with a cloud top of 5,000 feet.
In addition, 0-3 octaves of altocumulus cloud is forecast with a base of 7,000 feet and a top
of 10,000 feet.
The abbreviator SFC is used to signify the surface or ground level and LYR signifies a cloud layer.
The final lines of text which appear immediately below the cloud information refer to warnings.
These warnings highlight particular aspects of the forecast which require special consideration.
They are largely self-explanatory but involve further abbreviations which are highlighted here.
Some of these abbreviations we have already seen. These are simply included again here as a reminder.
Right at the bottom of the chart is a brief line or two of text which summarises the outlook for
6 hours after the expiry of the main chart.
This outlook does not go into precise detail about cloud heights and weather phenomena
but gives an indication of how the synoptic situation is expected to change.
This outlook forecast links in with the outlook map at the top of the chart.
The understanding which you will have attained during this lesson will provide you with sufficient background knowledge
to understand the variety of low-level significant weather charts produced around the world.
Even though there are similarities between most of these charts there are some differences.
However, we hope that the level understanding that you have now achieved will help you use them in an effective
manner.
This chart represents one of the best self-briefing tools for a general aviation pilot
and should form an integral part of any pre-flight weather briefing.
World Area Forecast System Significant Weather Charts provide information about critical weather at medium and high-level altitudes.
These charts include the forecast locations of embedded cumulonimbus clouds, medium and severe clear air turbulence,
medium and severe icing, jet streams and other important meteorological phenomena.
Let us firstly turn to the high-level charts.
Each chart has a clearly defined legend which contains the following information.
The name of the World Area Forecast Centre or WAFSI which produced the chart, London or Washington.
A reminder that the forecast is for a single fixed time.
The ICAO letter which is used to designate the area.
The range of flight levels through which the chart is valid.
The time in UTC and the date on which the chart is valid.
WAFSI Significant Weather Charts are produced every 6 hours and are valid for 2400 written 0000, 0600, 1200 and 1800
UTC every day.
Since each chart is valid for one single fixed time, it is the responsibility of the user to interpolate between
charts to establish what can be expected at intermediate times.
WAFSI Significant Weather Charts which are produced by WAFSI London contain one additional legend.
This highlights the flight levels between which clear air turbulence or CAT can be expected.
In the legend, each area of CAT has a number assigned to it and this number corresponds to the same
number which is attached to an area on the chart.
This area is enclosed by a dashed line.
The legends which are assigned to the medium-level Significant Weather Charts are similar to those on the high-level
charts.
Apart from the text line which clearly highlights that these charts are valid between flight level 100 and flight level
450.
Let's now have a look at the different phenomena which may appear on the WAFSI Significant Weather Charts.
Thunderstorms are not explicitly displayed on the charts as such.
However, they are assumed whenever a cumulonimbus cloud is forecast.
Thus, whenever the code CB is seen, you must assume thunderstorm activity.
The area on the chart to which this refers is sectioned using a scalloped line, as seen here.
However, it's important to remember that only embedded cumulonimbus are displayed on WAFSI charts.
Open cell or isolated cumulonimbus is relatively easy to spot in flight.
If there is any mention of cumulonimbus or CB, you must assume moderate or severe turbulence, ice, hail and the
chance of a thunderstorm.
Included in the information about the cumulonimbus and thunderstorm activity is an indication of the level of activity of the
cumulonimbus within the scalloped area on the chart.
ISOL is used to indicate that cumulonimbus will be isolated within that area.
OCTNOL is used to indicate that cumulonimbus may be occasionally encountered within the area.
WIDESPREAD implies that cumulonimbus are forecast to be widespread within the scalloped area.
And finally, FRQ is used to imply that cumulonimbus are expected to be frequent.
Indication is given in the cumulonimbus boxes of the flight levels between which the cumulonimbus can be expected.
If any of these flight levels are outside the height range of the chart, then XXX is used.
In this example, the tops of the activity are forecast for flight level 410, but the bottom of the cumulonimbus
activity is below the level of the chart.
Remember that this was flight level 250 for high level charts.
You'd have to consult the middle level chart to see the base of the cumulonimbus activity.
The next phenomenon which we will look at is tropical cyclones, otherwise known as hurricanes in the Atlantic Ocean.
These severe storms are easily recognisable on significant weather charts by this revolving symbol.
The symbol is always accompanied by the World Meteorological Organisation name of the storm, Olga on our example.
Tropical cyclones are associated with intense areas of cumulonimbus activity and they need to be very carefully monitored.
If moderate or severe turbulence is forecast, then this is clearly displayed on the chart by the way of the
following symbols.
Remember that whenever cumulonimbus is forecast, we must automatically assume moderate to severe turbulence.
In addition, on the medium level significant weather charts, areas of moderate and severe turbulence are often associated with other
types of medium level cloud.
These areas are highlighted by way of the turbulence symbols which are included within the cloud boxes assigned to every
scalloped cloud area.
On high level charts, only cumulonimbus clouds are shown and you may recall that moderate or severe icing is also
automatically assumed.
However, on medium level charts, other clouds are shown and these may give rise to icing conditions.
These are the symbols used to denote moderate or severe icing conditions.
On the chart, notice that the range of flight levels in which the icing risk is forecast is given.
Widespread sandstorms or dust storms are highlighted on the significant weather charts only when these phenomena are forecast to significantly
obscure visibility between the flight levels for which the chart is valid.
The symbol which is used on the chart is identical for both phenomena and is shown here.
As we learnt earlier, the only clouds which are displayed on high level charts are embedded cumulonimbus.
However, on the medium level charts, rather more cloud information is displayed.
Any cloud types which are likely to have a significant icing or turbulence risk are clearly highlighted.
In our example here, the cloud text box points to two scalloped cloud areas and indicates that broken amounts of
cumulus and altocumulus are forecast within these regions.
Moderate turbulence can be expected between flight levels 250 and XXX, which is less than flight level 100.
Moderate icing can be expected between flight levels 250 and 150.
Significant frontal systems are highlighted on significant weather charts.
It's important to appreciate that not all frontal systems are shown, only those which could have a significant impact on
aviation by way of associated weather and air mass change.
Standard frontal coding is used to indicate the surface positions of the fronts and directions and speeds of movement are
displayed by virtue of arrows.
These arrows indicate the direction of movement and the numbers indicate the speed of passage in knots.
In our example, we see a cold front moving in an eastward direction at a speed of 20 knots.
Tropopause heights are included on the high level significant weather charts.
The heights shown actually indicate the coldest temperature in the troposphere.
These tropopause heights are indicated at a large number of spot locations across each of the high level charts.
Tropopause heights are presented in flight levels in a rectangular box.
When several tropopauses are present in the atmosphere, either an H or an L is added to the tropopause box
to indicate whether the upper or lower tropopause height is displayed.
In these cases, the rectangular box is expanded to include the additional symbol, and the box looks more like an
arrowhead pointing up or down.
Jet streams are identifiable on the charts by black solid lines, which have wind barbs indicating jet speeds at spot
points, and the flight level at which the maximum speed is attained.
The arrowhead on the end of each jet streamline indicates the direction of movement of the flow.
Increases or decreases of speed in increments of 20 knots along the length of the jet stream are indicated by
either an additional set of wind feathers or, if there is little space on the chart, two parallel lines perpendicular
to the direction of flow of the jet.
In this lesson, we have looked in depth at the different elements of the significant weather charts. It's important that
you understand them and appreciate that they represent the best fixed time forecast of various phenomena which are of considerable
interest to aviation.
They should become an important element of successful flight planning when flying above flight level 100.
A SIGMET is an abbreviated plain language message which concerns the occurrence and or expected occurrence of significant weather which
may affect the operational safety of aircraft.
A SIGMET is valid for 4 hours, whilst a Volcanic or Tropical Cyclone SIGMET is valid for up to 6
hours.
The messages are issued from designated meteorological watch offices which have a responsibility for the Flight Information Regions or FIRs.
SIGMET information for subsonic cruising levels are simply identified as SIGMET.
Those with significant information for supersonic or transonic cruising levels are coded as SIGMET SST.
SIGMETs are written using abbreviated plain language. Most of these abbreviations you will have seen before in the low-level
significant weather charts. A full list is displayed and you may wish to spend a few moments looking through it.
We will examine how a SIGMET message is constructed by looking at the individual components of an actual SIGMET message.
The first item in the SIGMET is the location indicator of the air traffic unit serving the Flight Information Region
or control area to which the SIGMET message refers, in this case LFMM.
LFMM stands for Marseille Flight Information Centre. This is followed by the word SIGMET which acts as an identifier.
After the identifier the sequence number is shown next. In this example the sequence number is 2.
This sequence number corresponds to the number of SIGMET messages issued for the Flight Information Region since 0001 UTC on
the day of issue.
The sequence of SIGMET messages is sometimes indicated by a combination of both figures and letters.
The next items are the date and time groups indicating the period of validity in UTC of the SIGMET message.
In this example the message is valid on the 25th of the month from 0700 UTC to 1100 UTC.
The last item on the first line of the SIGMET is the location indicator of the meteorological watch office which
produced the message, which is then followed by a hyphen to separate the preamble from the next line of text.
At the beginning of the second line we have the name of the FIR or control area for which the
SIGMET is issued. In this example we have FIR Marseille.
This is followed by the name and description of the weather phenomenon which is the reason for issuing the SIGMET.
This will be abbreviated plain language from the table of abbreviations seen earlier in the lesson.
Here we have severe turbulence observed.
Following the name of the phenomenon there is an indication of whether the information is observed and expected to continue
using the abbreviation OBS or forecast using the abbreviation FCST.
If relevant the time of observation in UTC will also be given.
Our SIGMET shows OBS at 0645 which indicates that the severe turbulence was observed at 0645 UTC.
The next group of information relates to the location and altitude of the observed or forecast phenomenon.
Where possible, reference is made to latitude and longitude, locations or well known geographical features.
Examples of these are forecast top at FL360, north of 52°N and Gander Oceanic at FL280.
In our example, we see that within the Marseille FIR, severe turbulence was observed at 0645 UTC located between the
Pyrenees and the Massif Son trial, between ground level and FL080.
Movement or the forecast movement of the weather phenomenon is provided for by reference to one of the eight points
of the compass.
Speed is given in kilometres per hour, knots or stationary if appropriate.
We have STNR in our example. This indicates that the severe turbulence is stationary.
Finally, an indication is made of any changes in intensity of the phenomenon using the abbreviations for intensifying, weakening or
no change.
Our example shows no change.
We can now put this all together. The SIGMET reads
The second SIGMET message issued by the meteorological watch office in Marseille, France since 0001 UTC.
The message is valid from 0700 UTC to 1100 UTC on the 25th of the month.
The area covered by the SIGMET is the Marseille FIR. Severe turbulence was observed at 0645 between the Pyrenees and
the Massif Son trial from the ground to FL080.
The phenomenon is stationary with no change expected.
SIGMETs for volcanic ash clouds or tropical cyclones include an additional line of text.
This line contains a brief outlook forecast beyond the period of validity specified earlier in the message.
The outlook will include a trajectory of the ash cloud or positions of the tropical cyclone centre as appropriate.
It is important that you familiarise yourself with the structure and abbreviations used in SIGMET messages.
The correct interpretation of SIGMETs is essential for successful flight planning and in-flight safety.
The airmet is a forecast designed for low-level aviation using abbreviated plain language either in spoken form or text
form.
It is applicable for aircraft normally operating up to FL100.
There is an airmet forecast telephone service for use by pilots who do not have access to meteorological information disseminated
by fax,
or airmet forecasts disseminated in text form by teleprinter to aerodrome briefing rooms.
In the UK, airmets are produced for different geographical areas as shown here.
The regional airmets for areas Scotland, North England, including Northern Ireland and Wales, and lastly South England, are produced four
times a day.
The subregional airmets for areas Central, Southeast and Southwest are produced three times a day, whilst the cross-channel airmet
is produced just twice a day.
Let's now take a look at a typical UK airmet.
The example which we will now look at is from a Scottish region airmet.
The first line of the airmet is a description of when the forecast is valid.
The next section of text is a description of the meteorological situation.
Comment is usually made of any fronts and their forecast movement, together with any significant air mass changes.
Common, concise plain English is used to describe the meteorological situation.
After the general text of the met situation, there is a line or two of text which details any strong
wind warnings which are expected.
These are included when the wind is expected to exceed 20 knots.
In the example we have a westerly surface wind situated west of 4 degrees west with occasional gusts from 35
to 45 knots.
Below the strong wind warning is the more general wind forecast for the airmet region given at different levels above
mean sea level.
At the end of each wind description is a forecast of the temperature which can be expected at that flight
level.
PS denotes temperature above 0 degrees Celsius plus and MS denotes a temperature below 0 degrees Celsius minus.
Immediately below the section of text which describes the winds is a forecast of the freezing level.
Sometimes this varies spatially across the airmet region and sometimes with time.
Any changes are highlighted by way of brief descriptive text.
Below the text about the freezing level is a description of the weather conditions forecast for the airmet region.
This region is often broken up into distinct zones. In our example we have two zones.
The boundaries of these zones are highlighted by explanatory text.
Within each zone is a description of the different weather elements.
These weather elements include forecasts of visibility, weather and cloud.
The different weather elements are written in the same order as they appear on the UK Low Level Significant Weather
Chart Form 215.
The last section of information at the bottom of each zone specifies any warnings which are applicable to the zone
area.
These often cover aviation hazards such as cloud on hills, moderate or severe icing or turbulence, sub-zero layers and
mountain wave activity.
Right at the end of the report is a line or two of text which gives the outlook for the
next six hours. This forecast is kept short and concise.
The format of airmets shown so far is that from the UK but this does change considerably from country to
country.
Therefore when flying outside UK airspace it is advisable to fully familiarise yourself with the different airmet formats from other
MET services.
The airmet itself is an excellent briefing tool.
The concise abbreviated text makes it a simple but effective means of rapidly determining the general forecast conditions.
Be careful though, the airmet is only a general forecast and useful for en route planning and for appreciating the
general weather outlook.
However for more accurate forecasts for particular airfields you should brief yourself using the TAF or METAR.
In the weather briefing section we examined methods of obtaining weather information prior to flight.
However there are means of obtaining weather information in flight by tuning into and listening to appropriate frequencies on your
radio.
One of these means is a volmet.
Volmet broadcasts are ground to air HF or VHF transmissions of meteorological reports and forecasts.
These transmissions are broadcast in plain language giving the latest weather reports such as METARs.
Recently however more information has been added to the volmet transmissions which may now include SIGMETs and TAFs.
Shown here is the list of VHF volmet services for the United Kingdom and the near continent.
There are four volmet transmissions.
These are London Volmet Main, London Volmet North and South and lastly the Scottish Volmet.
Next to each of these is the frequency on which the transmission may be found, the operating hours and the
airfields covered by the broadcast.
Let's take a closer look at the London Volmet Main broadcast.
Note that the VHF frequency is 135.375 MHz and that the volmet is broadcast continuously over a 24 hour
period.
Within the London Volmet Main transmission you will find plain language reports of the weather pertaining to several major airfields.
The London Volmet South contains reports for more local airfields within southern England.
These plain language reports are actually the current METARs for the selected airfields and are updated every half an hour.
Shown here are the weather elements that will be included in the report which is just like the format of
an actual METAR.
This example volmet is a text version of the London Volmet Main broadcast.
As you can see, each of the major airfields is listed with its associated weather information.
Let us listen to a typical broadcast.
Rain showers.
Cloud, few, 2,000 feet.
Few cumminimbus, 2,500 feet.
Broken, 4,000 feet.
Temperature 1.4, dew point, 9.
QNH 1.004, becoming, visibility, 1.0 km or more, nil significant weather.
Brussels at 1.120, wind 1.90°14 knots, maximum 2.4 knots.
Visibility 1.0 km or more, light rain showers.
Cloud, scattered 2,300 feet, scattered 5,000 feet, broken 10,000 feet.
Temperature 1.3, dew point, 1.0.
QNH 1.006, no significant change.
Dublin at 1.130, wind 2.60°6 knots, variable between 2.40 and 3.00°.
Visibility 10 km or more.
Cloud, scattered 2,400 feet, scattered 20,000 feet.
Temperature 1.3, dew point, 6.
QNH 9.97, no significant change.
Glasgow at 1.120, wind 0.70° 5 knots, variable between 0.30 and 1.10°.
Visibility 1.0 km or more.
Cloud, few 1,800 feet, scattered 4,500 feet.
Temperature 1.4, dew point, 8.
QNH 9.97.
London Gatwick at 1.120, wind 1.90° 1.0 knots, variable between 1.50 and 2.20°.
Visibility 1.0 km or more, showers in the vicinity.
Cloud, few 2,400 feet, scattered 4,000 feet.
Temperature 1.1°, dew point 9°.
QNH 999.
London Heathrow at 1.120, wind 2.20° 12 knots, variable between 1.90 and 2.50°.
Visibility 1.0 km or more, light rain showers.
Cloud, few 2,000 feet, few QNH 2,500 feet.
Broken, 1.1,000 feet.
Temperature 11, dew point, 8.
QNH 9.97.
Tempo, visibility 4,500 meters, rain showers.
London Stansted at 1.120, wind 2.00° 16 knots.
Visibility 9 km, light rain showers.
Cloud, few 1,600 feet, scattered QNH 2,200 feet.
Broken, 2,800 feet.
Temperature 10, dew point 7.
QNH 9.97.
Recent rain.
The examples we have seen so far are broadcast on VHF and are associated with mainland United Kingdom and the
near continent.
However, bolmets are broadcast all over the world and cover different areas.
These larger scale transmissions are broadcast on HF frequencies typically between 3 to 20 MHz.
Shown here, highlighted in green, are examples of such bolmets.
The Shannon volemet is illustrated as it is a vital source of weather information for the North Atlantic flight routes.
These types of bolmets contain the same information as the ones we have already seen
although they are more likely to have additional weather forecast details such as sigmets or en route weather.
As we have seen, volmet transmissions are designed to be simple and easily understood
so that fast efficient weather briefing can take place whilst in the air.
Before you fly, note down the volmet frequencies for the areas you are flying in
so that en route you can get hold of the latest weather for your destination and alternate airfields.
This is useful for many reasons.
It allows you to confirm that the weather conditions at your destination airfield are still favourable
and if a diversion is necessary, you can rapidly determine the current suitability of your planned diversion airfield.
The Automatic Terminal Information Services or ATIS is a transmission of current aerodrome information
in a continuous broadcast during operating hours.
It is usually transmitted on a discrete VHF frequency.
Sometimes there will be an ATIS for departure and another for arrival as shown here at Oxford Airport.
However, in order to free up air traffic VHF frequencies,
some aerodromes transmit the ATIS information on the voice channel of the VOR as shown here at Keflavik.
If the current weather or information changes, the ATIS is immediately updated and re-recorded to reflect this.
This new updated ATIS broadcast is then given a sequential alphabetical code which supersedes the previous recording.
For example, ATIS broadcast Bravo will have replaced the previous ATIS broadcast Alpha.
When speaking to air traffic control, the pilot is required to state the alphabetic code of the ATIS information he
has received
in order for ATC to verify that the pilot has the latest information.
Like the Volmet, the ATIS will be broadcast in plain language and must contain some or all of the following
information if applicable.
Aerodrome name.
ATIS sequence designator or information code.
Time of observation.
Runway in use and status.
Surface wind in knots and reference to magnetic north.
Visibility and RVR.
Present weather.
Significant cloud.
Temperature and dew point.
Altimeter setting.
Transition level.
Type of approach expected.
And finally any warnings pertinent to flight operations.
Having looked at the ATIS, let's now hear the transmission as a pilot would when he has tuned into the
relevant frequency.
This is Brice Norton information code X-ray timed at 0900 Zulu.
Runway 26.
Surface wind 18004 knots.
Visibility 3000 meters in drizzle.
Cloud scattered 300 feet.
Broken at 500 feet.
Outside air temperature 9 degrees Celsius.
Two point 9 degrees Celsius.
QNH 1013 millibars.
29.92 inches.
Transition level 35.
IFR.
Serviceability full.
Brice Norton information code X-ray out.
The ATIS is usually obtained prior to engine start up.
When you make the initial contact with air traffic control you must mention the information code of the ATIS you
have obtained in order to ensure air traffic that you have the latest airfield information.
ATIS is also used by pilots before arriving at an airfield.
Though on initial contact with the destination aerodrome mention the information code of the ATIS broadcast you have obtained.
The ATIS prepares you very well for the conditions at the airfield and the runway in use so that planning
can be done prior to your takeoff or landing ensuring expeditious joining and departing.
Obtaining the ATIS will also ensure that radio transmissions between air traffic control and the pilot are kept to the
minimum.
This is especially important in busy airspace where radio transmission must be kept short to allow for effective communication with
all the aircraft in the vicinity.