Jeppesen Meteorology

Altimetry Chapter 3

The second diagram shows what happens with an aerodrome below sea level. In this case, when
it is warmer than ISA, the QFF is greater than the QNH, and when it is colder the QFF is less than
the QNH.

SUMMARY

Warmer Aerodrome above Aerodrome below
than ISA mean sea level mean sea level

Colder QFF < QNH QFF > QNH
than ISA
QFF > QNH QFF < QNH

Meteorology 3-9

Chapter 3 Altimetry

MOUNTAIN FLYING

There is a tendency for air to collect on the windward side of a mountain range. This leads to an
increased pressure on the windward side. Conversely, a lower pressure is experienced on the
leeward side. This means that if you use the QNH from the windward side you may be exposing
yourself to danger as you will be flying into a region of lower pressure.

To ensure adequate terrain clearance you should always use the lowest QNH for the area.

As air tries to flow through a mountain range, the air is obstructed by the range. In order to pass
over the mountain the air must speed up. Bernoulli’s theorem states that if the velocity goes up,
the pressure goes down.

This leads to a lower pressure over the top of the mountain — a potentially dangerous situation.
The greater the wind speed over the mountain, the greater the height loss.

In order to ensure safe terrain clearance you should add an extra margin to your minimum safe
altitude, depending on the wind speed, as shown below:

< 30 kt no addition necessary
31 – 40 kt add 500 ft
41 – 50 kt add 1000 ft
51 – 60 kt add 1500 ft
> 60 kt add 2000 ft

In summary, when calculating the actual altitude above high terrain:

Use the correct reference pressure (the lowest QNH).
Correct the reading for temperature deviation as described above.
Allow an extra margin of safety for strong winds.

ALTIMETER SETTINGS

For take-off and landing, the QNH is normally used — never the standard pressure setting.

When climbing, the standard pressure setting is set when passing the Transition Altitude.

When descending, the QNH is set on passing the transition level.

Transition Altitude The altitude at or below which the vertical position of an aircraft is
referenced to altitude
Transition Layer
Transition Level The airspace between the transition altitude and the transition level

The lowest flight level available for use above the transition altitude

Note: Pilots may, at their discretion, use QFE for take-off and landing in which case the aircraft
should have two altimeters and the second one should be set to QNH. If the barometric
pressure is very low (e.g. below the altimeter sub-scale minimum setting) then the
standard pressure setting is used and the aircraft is landed with a false altitude indicated
on the altimeter (the QNE).

3-10 Meteorology

Altimetry Chapter 3

CALCULATION OF MINIMUM USABLE FLIGHT LEVEL

Minimum usable flight level is important to know in the event of a decompression over high
terrain. Sometimes it is defined by the authorities; if not, you can calculate it as follows:

1. Note the highest elevation within 5 nm of track, then:
a) In the case of the terrain being higher than 6000 ft (1800 m), add 2000 ft to this
value.
b) In the case of the terrain being lower than or equal to 6000 ft (1800 m), add 1000
ft to this value.

2. Correct for temperature deviation as described earlier in the chapter.
3. Correct for wind as described earlier.
4. Convert from an altitude to a pressure altitude.
5. Round up to the nearest higher flight level.

Meteorology 3-11

Chapter 3 Altimetry

3-12 Meteorology

INTRODUCTION

Temperature is one of the most important variables that affect the atmosphere. The temperature
changes that occur on the Earth’s surface initiate both vertical air movement (leading to cloud
development) and horizontal air movement (wind).

Temperature normally decreases with height. If there is an increase with height, this is called an
inversion.

If temperature stays the same with change in height, this is called an isothermal layer.

TEMPERATURE SCALES

There are three scales of measurement for temperature. These are:

FAHRENHEIT

In the Fahrenheit scale, the freezing point of water is 32°F and the boiling point of water is 212°F.
This scale is not used in meteorology.

CELSIUS

The Celsius scale is widely used. The freezing point of water is 0°C and the boiling point is
100°C.

KELVIN

The Kelvin scale does not have units, but intervals of the scale are equal to 1°C. The scale
relates to absolute zero (−273°C) which is defined as 0K. The freezing point of water is 273K and
the boiling point is 373K.

0K is called absolute zero and is the temperature at which all molecules stop moving completely.

CONVERSION FACTORS

To convert from Celsius to Fahrenheit:

°F = (°C x 9 ) + 32
5

Meteorology 4-1

Chapter 4 Temperature

To convert from Fahrenheit to Celsius:

°C = (°F – 32) X 5
9

To convert from Celsius to Kelvin:

K = °C + 273

To convert from Kelvin to Celsius:

°C = K – 273

MEASUREMENT OF TEMPERATURE

Surface temperatures are measured using mercury thermometers housed in a Stevenson screen.
This is a louvred wooden box that allows air to circulate around the thermometers but protects
them from draughts and direct sunlight. It is held 4 ft above the ground so the temperature won’t
be adversely affected by the ground temperature.

High level temperatures are measured using a Radio Sonde, a radio transmitter that is carried
high into the atmosphere (up to 150 000 ft) by a hydrogen balloon and sends back continuous
readings of pressure, temperature, and humidity to stations on the ground.
Temperature is measured to the nearest 0.1°C and reported to the nearest whole number. If the
temperature ends in 0.5, it is rounded to the nearest odd whole number.

4-2 Meteorology

Temperature Chapter 4

HEATING OF THE ATMOSPHERE

The atmosphere is heated by five different processes:

1. Solar radiation
2. Terrestrial radiation
3. Conduction
4. Convection
5. Latent heat of condensation

A sixth process, advection, is responsible for the horizontal transfer of heat.

We will look at each of these processes in turn.

SOLAR RADIATION

Radiation from the sun is of the short-wave type. Most of the radiation that reaches the Earth’s
surface is of wavelengths less than 2 microns.

Nearly all the radiation passes through the Earth’s atmosphere without heating it. Ultra-violet
radiation is absorbed by ozone in the stratosphere. Still more is reflected by cloud cover. But on a
clear day, about 85% of the sun’s radiation will reach the Earth’s surface.

The radiation does not heat the atmosphere directly but does heat the surface of the Earth. This
process is called insolation. The atmosphere then becomes heated by the other processes
described below.

The amount of insolation (heating of the surface) depends on the angular elevation of the sun.
This in turn depends on latitude, season, and time of day.

Latitude
As can be seen from the diagram below, as you move further from the equator, the
curvature of the Earth means that the same amount of solar radiation is spread over a
larger area of the Earth’s surface. So insolation is less at higher latitudes

Meteorology 4-3

Chapter 4 Temperature

Season
For the same reasons mentioned above, the sun heats the Earth more efficiently if it is
directly overhead. Where this occurs depends on the time of year.

At the equinoxes, the sun is overhead the equator; at Summer Solstice (21st June) it is
overhead the Tropic of Cancer (23.5°N); at Winter Solstice (21st December) it is
overhead the Tropic of Capricorn (23.5°S).

Time of day
The amount of insolation is greatest at noon when the sun is highest in the sky.

TERRESTRIAL RADIATION

The Earth’s surface absorbs large amounts of solar radiation at short wavelengths and re-
transmits it as smaller amounts of long-wave radiation, between 4 and 80 microns.

This is the main method by which the atmosphere is heated. Since the atmosphere is heated
from below, it gets colder as you move away from the surface of the Earth. This is the reason for
the temperature lapse rate.

CONDUCTION

Conduction occurs when two bodies are touching one another. Heat passes from the warmer
body to the colder body. For example, heat passes from a warm ground surface to the air.

At night, the ground cools quickly due to lack of insolation from the sun. The air in contact with the
ground loses heat by conduction. As air is not a very good conductor, air at a higher level remains
warm, which results in a temperature inversion.

4-4 Meteorology

Temperature Chapter 4

CONVECTION

As air is heated by conduction or radiation, it becomes less dense and tends to rise. Likewise,
cold air is more dense and subsides. This vertical movement of air is called convection. This
process helps heat the upper levels of the atmosphere.

LATENT HEAT OF CONDENSATION

When heat is used to alter temperature it is called sensible heat. Heat used to alter the state of a
substance is referred to as latent heat (latent meaning hidden), as no temperature change
occurs.

For example, when water turns from vapour to droplets in the atmosphere, it is turning from the
gaseous state to the liquid state. Heat is released when this occurs.

Likewise, when it turns from liquid to gas, it absorbs heat to effect the change, but the actual
temperature remains constant within the substance.

As air is lifted it cools and is no longer able to hold as much water vapour. This condenses out as
water droplets and latent heat is released, warming the atmosphere.

ADVECTION

Advection is the process by which air moves horizontally. The movement is caused by variations
in pressure, but the air takes with it its characteristics, including its temperature.

DIURNAL VARIATION OF TEMPERATURE

The maximum amount of insolation occurs at noon when the sun is high in the sky. As the earth
takes time to heat up, it does not immediately transfer the heat out to the atmosphere — there is
a slight lag. This means that the highest air temperature occurs at about 1500 local time. The
lowest temperature occurs about a half an hour after sunrise, again due to lag.

Meteorology 4-5

Chapter 4 Temperature

THE EFFECT OF CLOUD COVER ON DIURNAL VARIATION
During the day, clouds prevent some solar radiation from reaching the Earth, hence reducing the
maximum temperature that the air near the surface reaches during the day.

At night, clouds trap some of the heat between them and the ground, hence raising the minimum
temperature that the air drops to at night.

The overall effect is to reduce the diurnal variation.

THE EFFECT OF WIND ON DIURNAL VARIATION
During the day, wind causes surface air to be mixed with cooler air above. The amount of time
that any air is in contact with the warm ground is short, so the maximum temperature the air near
the surface reaches is lower compared to calm conditions.

During the night, terrestrial radiation leads to a reduction in air temperature close to the ground.
Any wind causes mixing of the cold surface air with warmer air above. Therefore, the minimum
temperature of the air above the surface at night is not as low as it would be in calm conditions.

The overall effect is to reduce diurnal variation.

THE EFFECT OF SURFACE ON DIURNAL VARIATION
How much a surface heats up when exposed to insolation depends on its specific heat. The
specific heat is the amount of heat required to raise the temperature of the surface by 1°C.

Some examples of surfaces listed in the order of increasing specific heat follows:

1. Bare rock/stone
2. Concrete
3. Dry soil
4. Wet soil
5. Oceans
6. Snow surfaces

Those surfaces that take a long time to heat up also lose their heat very slowly, so the diurnal
variation over the sea is minimal but is much greater over the land.

Not only does water have a much higher specific heat than land, but due to the movement of the
sea surface, the energy is spread to a depth of several metres, whereas solar radiation only heats
the top few inches of the land surface.

Topics found later in the course detail why the different properties of land and sea are important.

4-6 Meteorology

Temperature Chapter 4

SUMMARY
In summary, greatest diurnal variation can be found over the land, with clear skies and no wind.

Least diurnal variation can be found over the sea and over the ice caps, when skies are cloudy
and it is windy.

THE GREENHOUSE EFFECT

Water vapour and carbon dioxide are transparent to short wavelength radiation, but they are less
permeable to long wavelengths. This means they allow solar radiation to reach the surface, but
do not allow all of the terrestrial radiation to leave the atmosphere and go back into space.

This leads to an increase of temperature at ground level, a process called the greenhouse effect,
since the glass in a greenhouse works in a similar way.

Meteorology 4-7

Chapter 4 Temperature

4-8 Meteorology

INTRODUCTION

Most water in the atmosphere is in the form of water vapour, which is water in its gaseous state.
This water cannot be seen. In order for water to become visible in the form of clouds, mist, or fog
it must turn into water droplets or ice crystals.

WATER STATES AND LATENT HEAT

Water can exist in three basic states: solid (ice), liquid (water), and gas (water vapour). When
changing from one state to another, latent heat is either released or absorbed.

EVAPORATION

This is the change of state from a liquid to a gas. Gas is a higher energy state than liquid so latent
heat is “absorbed” during this process.

Evaporation can take place at any temperature above absolute zero, but the rate of evaporation
is greater at higher temperatures.

MELTING

This is the change of state from a solid to a liquid. Liquid is a higher energy state than solid so
latent heat is “absorbed” during this process.

Meteorology 5-1

Chapter 5 Water in the Atmosphere

SUBLIMATION

Sometimes a substance can turn directly from a solid to a gas or from a gas to a solid without
passing through the intermediate liquid state. The term sublimation can be used to describe this
process in both directions. The change from gas to solid, however, can also be referred to as
deposition.

Latent heat is “absorbed” when a solid turns to a gas.

Latent heat is “released” when a gas turns to a solid. This process is important in the formation of
frost, hail, and some airframe icing.

CONDENSATION

This is the change of state from a gas to a liquid. Liquid is a lower energy state so latent heat is
“released”.

Condensation nuclei must be present in order for condensation to occur in the atmosphere.
Condensation nuclei are tiny particles of hygroscopic (water attracting) material, such as dust and
pollution.

FREEZING

This is the change of state from a liquid to a solid. Solid is a lower energy state so latent heat is
“released”.

For this to occur, freezing nuclei are required, similar to those for condensation. Without them,
the water droplets in the atmosphere become supercooled, which means they remain as a liquid
state despite being lower than freezing temperature.

Supercooled droplets are a major cause of airframe icing. They are discussed again later in the
course.

SATURATION

As water evaporates into the air, there comes a point in which the air can no longer accept any
more water vapour. The amount of vapour that air can hold is dependent on its temperature and
pressure.

The higher the temperature, the more water vapour the air can hold.

When the air contains the maximum amount of water vapour it can hold, it is described as being
saturated.

The air can become saturated in two ways: extra water vapour can be added, or the air can be
cooled, since cooler air holds less water vapour.

HUMIDITY

Humidity refers to the amount of water vapour in the air. It is often expressed as a percentage
and is known as relative humidity.

5-2 Meteorology

Water in the Atmosphere Chapter 5

ABSOLUTE HUMIDITY

Absolute humidity is the actual mass of water in a given volume of air and is generally expressed
in g/m3.

SATURATION CONTENT

Saturation content is the mass of water a given volume of air can hold, not that which it is actually
holding, again expressed as g/m3.

RELATIVE HUMIDITY

Relative humidity is an expression of how much water vapour is in the air, expressed as a
percentage of the maximum amount the air could hold at that temperature and pressure. Hence:

RELATIVE HUMIDITY (RH) = AMOUNT OF WATER VAPOUR IN THE AIR %

AMOUNT OF WATER VAPOUR THE AIR CAN HOLD

= ABSOLUTE HUMIDITY %

SATURATION CONTENT

Example: If the absolute humidity is 12 g/m3 and the saturation content is 26 g/m3,
what is the relative humidity?

Relative Humidity = Absolute Humidity / Saturation Content
= (12 ÷ 26) = 0.462 = 46.2%

Please attempt the following simple RH calculations. The answers can be found at the end of the
chapter:

Exercise 1:

Absolute Humidity Saturation Content Relative Humidity
(g/m3) (g/m3) (%)
6 20
34 45
14 30

HUMIDITY MIXING RATIO

Humidity mixing ratio (HMR) is similar to absolute humidity but is the mass of water in a certain
mass of air. The unit for this is therefore g/kg rather than g/m3.

Typically, the HMR is between 5 and 50 g/kg in temperate latitudes.

Meteorology 5-3

Chapter 5 Water in the Atmosphere

HMR FOR SATURATION CONTENT / SATURATION MIXING RATIO

The saturation mixing ratio (SMR) is the HMR when the parcel of air is saturated. Hence relative
humidity can also be expressed as:

RELATIVE HUMIDITY (RH) = HMR %

HMR FOR SATURATION CONTENT

SUPER-SATURATION

As mentioned earlier, condensation only occurs if there are condensation nuclei present. If no
nuclei are present, then the water remains as vapour and the air is described as super-saturated.
This means there can conceivably be a relative humidity greater than 100%.

SATURATION AND DEWPOINT

The graph below shows the HMR for saturation plotted against the temperature in °C. The higher
the temperature, the larger the amount of water the air can hold. However, the relationship is not
linear, it is logarithmic.

30

25

HMR for Saturation in g/kg 20

15

10

5

0

-30 -20 -10 0 10 20 30

Temperature in degrees C

5-4 Meteorology

Water in the Atmosphere Chapter 5

It follows that if a parcel of air contains a certain amount of water vapour and is cooled, it will be
able to hold less water vapour. If it continues to cool, it eventually reaches a point where the
amount of vapour it can hold is equal to the amount it is actually holding. The air is said to be
saturated.

The temperature at which this occurs is called the dewpoint. A parcel of air at 20°C with a HMR of
7 g/kg (as seen on the graph) is not saturated. Air at 20°C can hold up to 14 g/kg.

What happens if air is cooled to 10°C? Based on the graph, the HMR for saturation is 7 g/kg.
Therefore, the air is saturated — the relative humidity is 100%. So the dewpoint for air containing
7 g/kg is 10°C.

Cooling the air beyond this point results in water vapour condensing to become droplets, which
causes clouds, fog/mist, or dew.

Relative humidity also has an effect on the rate of evaporation. Evaporation does not occur if the
air is saturated. Warmer air can take more vapour so is less likely to be saturated. However,
evaporation can still occur if the air above the liquid is cold, especially if there is a breeze to take
away the saturated air and replace it with dry air.

Note: The term dry air is used to describe any air that is not saturated. So, even air with
a RH of 99% is still dry. Completely dry air, that is air with an RH of 0%, does not
occur in the atmosphere.

Using the graph above answer the following questions:

Exercise 2:

The HMR is 4 g/kg. The temperature is 20°C. What is the RH?

Exercise 3:

The HMR is 15 g/kg. What is the dewpoint?

Exercise 4:

The dewpoint is 18°C. The RH is 40%. What is the HMR?

CONDENSATION LEVEL

When unsaturated air is cooled, it eventually reaches its dewpoint and water vapour condenses
out as water droplets.

One way in which a pocket of air may cool is if it is lifted. As the air rises it cools. Once it reaches
a level where the RH becomes 100%, any further lifting leads to condensation. This level is
referred to as the condensation level.

As air rises it is said to cool adiabatically. Likewise, as air descends it is said to warm
adiabatically. This process of adiabatics and how it relates to dewpoint and cloud formation is
discussed more fully in the chapter on Stability.

Meteorology 5-5

Chapter 5 Water in the Atmosphere

DIURNAL VARIATION OF HUMIDITY

Assuming the absolute humidity of the air remains constant, the relative humidity varies as the
temperature varies. Cold air can hold less water, so just after dawn, when temperature is at its
lowest, RH is at its highest. This is why mist and fog are most likely to form around dawn.

Throughout the day as the temperature increases with increased insolation, the relative humidity
decreases, dropping to its lowest value at about 1500 LMT when the air temperature is at its
greatest.

After this, the temperature starts to drop again, so the RH starts to rise.

WATER VAPOUR PRESSURE

This is the part of the atmospheric pressure that is exerted by the water vapour present. When
the air is saturated, the water vapour pressure is called Saturation Vapour Pressure. The
dewpoint depends on the vapour pressure. The lower the vapour pressure, the lower the
dewpoint.

As air rises, it expands and cools. Its overall pressure goes down so the pressure exerted by the
water vapour also goes down. This leads to the dewpoint decreasing as well. The dewpoint
decreases by about .5°C per 1000 ft gain in height.

Yet another formula for dewpoint arises from the relationship between water vapour pressure and
saturation vapour pressure:

RELATIVE HUMIDITY (RH) = VAPOUR PRESSURE (hPa) %

CORRESPONDING VAPOUR PRESSURE FOR SATURATION

5-6 Meteorology

Water in the Atmosphere Chapter 5

SATURATION VAPOUR PRESSURE CURVE

12

10

Vapour Pressure in hPa 8
6 Ice

Water
4

2

0

-25 -20 -15 -10 -5 0 5 10

Temperature in degrees C

Saturation vapour pressure depends on a number of factors. The graph above shows that the
saturation vapour pressure is higher over ice than over water.

Other factors affecting saturation vapour pressure are:

1. Higher above a curved surface than a flat surface
2. Higher over clean water than a salt solution
3. Higher around a supercooled droplet than an ice crystal

Meteorology 5-7

Chapter 5 Water in the Atmosphere

MEASUREMENT OF HUMIDITY

PSYCHROMETER

DRY BULB WET BULB

MUSLIN
CLOTH

DISTILLED
WATER

To calculate humidity and dewpoint, a Psychrometer or Wet and Dry Bulb Hygrometer is used.
This apparatus consists of two mercury thermometers. One, the dry bulb thermometer, is an
ordinary thermometer that measures the air temperature. The other, the wet bulb thermometer,
has a piece of muslin cloth wrapped around the bulb. The other end of this cloth is dipped in a
container of distilled water.
As the water evaporates from the cloth, latent heat is drawn from the immediate surroundings.
This causes the wet bulb temperature to be lower than the dry bulb temperature. The wet bulb
temperature is the lowest temperature to which the air can cool by evaporation.
Note that if the air is already saturated, no evaporation occurs and the two readings are the
same. In this case the temperature displayed will also be the dewpoint.
The two figures obtained can be used to look up the dewpoint, RH, and HMR from tables.
An approximation of the dewpoint can be made using the following method:

1. Subtract the wet bulb temperature from the dry bulb temperature.
2. Subtract this figure (the wet bulb depression) from the wet bulb temperature.

5-8 Meteorology

Water in the Atmosphere Chapter 5

Example: Dry Bulb Temperature 20°C
Wet Bulb Temperature 15°C

Wet Bulb Depression = Dry Bulb Temperature – Wet Bulb Temperature = 5°C

Dewpoint Temperature = Wet Bulb Temperature – Depression = 15° - 5° = 10°C

Please complete the following dewpoint calculations:

Exercise 5:

Dry Bulb Temperature (°C) Wet Bulb Temperature (°C) Dewpoint (°C)
22 15
18 10
12 3

HUMIDITY METHOD

Another method of approximating the dewpoint is from the RH and air temperature. The formula
is:

DIFFERENCE BETWEEN TEMPERATURE AND DEWPOINT = (100 – RH)
5

Example: The temperature is 23°C and the relative humidity is 80%. What is the
dewpoint?

Difference = (100 – 80) ÷ 5 = 4°C
Dewpoint = 23°C - 4°C = 19°C

Test your understanding of the formula by completing the following table:

Exercise 6:

Air Temperature (°C) Relative Humidity (%) Dewpoint (°C)
18 70
12 4
85 2

Meteorology 5-9

Chapter 5 Water in the Atmosphere

ANSWERS TO EXERCISES

Exercise 1:

Absolute Humidity Saturation Content Relative Humidity
(g/m3) (g/m3) (%)
6 20
15.3 34 30
14 46.7
45

30

Exercise 2:
28.6%
Exercise 3:
21°C
Exercise 4:
4.8 g/kg
Exercise 5:

Dry Bulb Temperature (°C) Wet Bulb Temperature (°C) Dewpoint (°C)
22 15 8
18 14 10
21 12 3

Exercise 6:

Air Temperature (°C) Relative Humidity (%) Dewpoint (°C)
18 70 12
12 60 4
5 85 2

5-10 Meteorology

INTRODUCTION

The density of a substance is its mass per unit volume. Density in the atmosphere is usually
expressed as grams per cubic metre (g/m3). It may also be expressed as a percentage of the
standard surface density. This is called relative density.

Example 1: As chapter one detailed, the standard surface density is 1225 g/m3.
Hence if the actual density is 900 g/m3, the relative density would be:

Example 2: 900 × 100 = 73.47%
1225

If the actual density is 1500 g/m3 what is the relative density?

1500 × 100 = 122.45%
1225

A third way in which density may be expressed is as density altitude. This is described later in
the chapter.

THE IDEAL GAS LAWS

An ideal gas is one that is incompressible and without viscosity. The atmosphere is assumed to
be an ideal gas. There are several gas laws that apply.

In the next few formulae, the following key applies:

P = Pressure
V = Volume
T = Temperature
ρ = Density

Meteorology 6-1

Chapter 6 Density

BOYLE’S LAW

At constant temperature, as the pressure of gas increases, its volume must decrease. Therefore
the pressure is inversely proportional to volume:

1
Pα V

To remove the proportional sign, use:

Constant
P=

V
so:

PV = Constant
or:

P1V1 = P2V2

CHARLES’S LAW

At constant pressure, if the temperature of a gas increases, the gas expands. In other words, its
volume increases. The temperature is proportional to volume:

TαV

To remove the proportional sign, use:

V = Constant x T
so:

V
= Constant

T
or:

V1 = V2
T1 T2

THE GAS EQUATION

Combining Boyle’s and Charles’s laws, the gas equation becomes (where R is the gas constant):

PV = RT

6-2 Meteorology

Density Chapter 6

Density can also be a part of the equation. In an ideal gas, as volume increases, density
decreases. This is due to the same mass of air being contained in a larger volume.

So:

1
ρα V

Substituting this into the ideal gas equation:

P
= RT

ρ

Re-arranging to make density the subject of the equation:

P
ρ=

RT

So, maintaining a constant temperature: if pressure goes up, density goes up. Maintaining a
constant pressure: if temperature goes up, density goes down.

EFFECT OF WATER VAPOUR ON AIR DENSITY

Water vapour is less dense than air: approximately 5/8 of the density of dry air. Therefore, all other
things being equal, the density is lower in more humid atmospheres. This difference is usually
insignificant and can be ignored for aviation purposes. In the tropics, however, where it can be
very humid, it can make a large difference.

VARIATION OF SURFACE AIR DENSITY WITH LATITUDE

Air density is lowest with low pressure and high temperature. So in the equatorial regions, density
at the surface is low.

High pressure and low temperature equates to high density. Examples of this can be found at the
poles or at the centre of a large land mass in winter, (e.g. Siberia).

So, in general, density increases with increasing latitude.

The lowest density can be found at an aerodrome that is not only hot and high, but humid. An
example is Nairobi, which is very close to the equator, so experiences high temperatures and
humid conditions. It is also at an elevation of about 5500 ft, so has all the attributes that contribute
to low density.

VARIATION OF AIR DENSITY WITH HEIGHT

As height increases, both the temperature and pressure decrease. Based on the gas laws, a
decrease in temperature leads to an increase in density and a decrease in pressure leads to a
decrease in density.

Meteorology 6-3

Chapter 6 Density

So, with one law trying to increase the density and one trying to decrease it, will it therefore stay
constant?

The answer is no. Since pressure near the surface decreases by about 10 hPa per 300 ft, this
would produce a reduction in density of about 1%.

A similar height increase would cause a drop in temperature of less than 1°C. This would lead to
an increase in density of about 0.3%.

The change in pressure has more of an effect, therefore, density decreases with height.

This leads to the following observations:

20 000 ft Density is 50% of the surface value.

40 000 ft Density is 25% of the surface value.

60 000 ft Density is 10% of the surface value.

VARIATION OF AIR DENSITY WITH LATITUDE AND HEIGHT

As already mentioned, the air density at the surface tends to increase with increased latitude and
density decreases with increased height. Now it’s time to bring those two factors together.

Consider two columns of air of equal heights. Both columns have the same pressure at the base,
but one column of air is cold and the other warm.

LOW HIGH
PRESSURE PRESSURE

1013 hPa

The cold air has a higher density, so as height increases there is a greater reduction in mass and
the change in pressure is greater. Conversely, the warm air is less dense, so there is a small
reduction of mass above as height increases. The change in pressure is less, so pressure at the
top of the cold column is lower than at the top of the warm column.

6-4 Meteorology

Density Chapter 6

This is important when considering global patterns in density. At the equator, the air temperature
is high, so density at the surface is relatively low, as is pressure. At the Poles, the air temperature
is low, so density at the surface is relatively high, as is pressure.

However, as height increases over the equator, pressure, and therefore density, decreases
relatively slowly, like in the warm column of air described above.

As height increases over the Poles, pressure, and therefore density, decreases relatively quickly,
like in the cold column of air described above.

At the equator there is a relatively low density at the surface, compared to the Poles, but a
relatively high density at height, as the density decreases only slowly.

At the Poles there is a relatively high density at the surface, but a relatively low density at height,
as the density decreases quickly.

At approximately 26 000 ft the density is constant at all latitudes.

DIURNAL VARIATION OF DENSITY

Density is highest when temperatures are lowest, that is, just after dawn. It is at its lowest at
about 1500 LMT when temperatures are highest.

DENSITY ALTITUDE

The density altitude at which you are flying is the pressure altitude in the International Standard
Atmosphere at which that density would occur.

Meteorology 6-5

Chapter 6 Density

Logically, if it is warmer than ISA, your density altitude is higher than your pressure altitude and
vice versa for colder than ISA conditions.

The diagram below shows two columns of air: one is at ISA and the other is warmer than ISA.

WARMER ISA
THAN ISA 1000 g/m3

10 000 ft

1000 g/m3 1225 g/m3 0 ft

At surface level in ISA the density is 1225 g/m3. At 10 000 ft, the density is 1000 g/m3. The
warmer column of air has been heated such that the air density at the surface has decreased to
1000 g/m3, the same as that at 10 000 ft in ISA conditions.

Hence the density altitude at the surface is 10 000 ft.

CALCULATING DENSITY ALTITUDE

120 ft

ISA + 120 ft
1°C
ISA ISA -
1°C

Density altitude differs from pressure altitude by 118.8 ft per 1°C deviation from ISA. In the JAR
exams it is sufficient to use 120 ft per 1°C deviation from ISA. Add the difference to the pressure
altitude if warmer than ISA, subtract if colder.

6-6 Meteorology

Density Chapter 6

Example: The pressure altitude is 20 000 ft. The ISA deviation is 4°C. What is the
density altitude?

It is warmer than ISA so:
Density altitude = Pressure altitude + (120 × ISA deviation)
= 20 000 + 480 = 20 480 ft

Exercise 1:

The pressure altitude is 15 000 ft. The ISA deviation is -5°C. What is the density altitude?

Exercise 2:

The pressure altitude is 8000 ft. The ambient temperature is 9°C. What is the density altitude (use
a lapse rate of 2°C/1000 ft)?

Exercise 3:

The density altitude is 26 000 ft. The ISA deviation is +8°C. What is the pressure altitude?

EFFECT OF DENSITY ON AIRCRAFT PERFORMANCE

Low density reduces the performance of engines and aerofoils.

Engines work by accelerating air backward in order to produce thrust. Less dense air has lower
mass. The lower the mass, the less thrust the engine produces.

The production of lift by aerofoils such as the wings also depends on the density. The formula for
lift is shown below:

LIFT = CL ½ ρV2S

Where:

CL = COEFFICIENT OF LIFT
ρ = DENSITY
V = TRUE AIRSPEED
S = SURFACE AREA OF AEROFOIL

The amount of lift produced is directly proportional to the density. So if density is low the aircraft
will not produce as much lift, all other factors being equal.

This is very important on take-off and landing. In order to generate enough lift, the aircraft either
has to fly at a lower weight or a higher TAS. If a higher speed is chosen, then the aircraft requires
a longer take-off and landing run.

At an airport such as Nairobi, aircraft often have to operate with reduced weight at the hottest
time of the day.

Meteorology 6-7

Chapter 6 Density

ANSWERS TO EXERCISES

Exercise 1:

Density altitude = 15 000 - (120 × 5) = 14 400 ft

Exercise 2:

ISA temp for 8000 ft = 15 – (2 × 8) = -1°C. ISA deviation is therefore +10°C.
Density altitude = 8000 + (120 × 10) = 9200 ft

Exercise 3:

Density altitude = Pressure altitude + (ISA deviation × 120)
Hence:
Pressure altitude = Density altitude – (ISA deviation × 120)
= 26 000 – (8 × 120) = 25 040 ft

6-8 Meteorology

INTRODUCTION

The processes leading to cloud formation and precipitation depend greatly on the stability of the
atmosphere. In order to understand the concepts of stability and instability, you must understand
the concept of adiabatics.

ADIABATIC PROCESSES

As a bubble of air rises, the pressure in the surrounding atmosphere goes down and the bubble
expands. This leads to the temperature within the bubble decreasing. This is called adiabatic
cooling.

Conversely, if a bubble of air descends, it compresses and the temperature increases. This is
called adiabatic warming.

Air is not a very good conductor, so there is very little exchange of heat with the surrounding
environment.

Hence, an adiabatic process is one in which the temperature changes within the system but there
is no exchange of energy with the surroundings.

THE DRY ADIABATIC LAPSE RATE

When dry air (unsaturated air) is forced to rise, it cools at what is called the Dry Adiabatic Lapse
Rate (DALR). This has been found to be 3°C/1000 ft. This is the same regardless of how close to
saturation the air is. It is also independent of pressure and temperature.

THE SATURATED ADIABATIC LAPSE RATE

Once the air reaches saturation, water vapour starts to condense if the air is cooled any further.
This process of condensation releases latent heat, as discussed in earlier chapters.

This means that the temperature does not decrease as much as if it were dry, due to this extra
heat being added into the system. The rate is referred to as the Saturated Adiabatic Lapse Rate
(SALR).

The actual amount of heat released as latent heat depends on the amount of condensation that
occurs. In cold temperatures, even when the air is saturated, the actual amount of water vapour
present is low, so very little latent heat is released. In this situation, the SALR is nearly as high as
the DALR.

Meteorology 7-1

Chapter 7 Stability

In hot temperatures, saturated air contains a large amount of water vapour and condensation
releases large amounts of latent heat. The SALR, therefore, is considerably lower than the DALR.

The average SALR is taken to be 1.5°C/1000 ft.

THE ENVIRONMENTAL LAPSE RATE

This is the lapse rate of the air in the environment, that is, the air surrounding the adiabatic
system, not within the system itself. This air is not in vertical motion.

The ELR is variable. As discussed in Chapter 1, the average ELR is 1.98°C/1000 ft.

SUMMARY OF ADIABATICS

The following diagram shows the DALR and the SALR. The DALR is constant at 3°C/1000 ft, but
the SALR is not constant. As the height increases, the SALR approaches the DALR.

Height SALR

DALR

Temperature

Where the ELR falls in this picture is discussed later in this chapter.

STABILITY OF THE AIR

Air that is warmer than its surrounding environment is less dense and rises. This is called
instability.

Air that is colder than its surrounding environment is more dense and sinks. This is called
stability.

Air that is the same temperature as its surrounding environment neither rises nor sinks. It is
neutral.

The stability of the atmosphere depends on the relationship between the ELR and the DALR and
SALR.

7-2 Meteorology

Stability Chapter 7

ABSOLUTE STABILITY

Consider the following example. The ELR is 1°C/1000 ft. The diagram demonstrates what
happens when air is forced to rise. One bubble of air is dry, one is saturated.

4000 ft DALR SALR ELR 1°C/1000 ft
3°C 9°C 11°C

3000 ft 6°C 10.5°C 12°C
2000 ft 9°C 12°C 13°C
1000 ft 14°C
12°C 13.5°C 15°C

15°C

Dry air Saturated air

The surface temperature is 15°C. The dry air cools at 3°C, faster than the surrounding
environment is lapsing. This means that at each level the dry bubble of air is colder than the
surrounding environment, and therefore more dense, so it wants to sink.

The saturated air cools at 1.5°C, again faster than the lapse rate of the environment. So at each
level, the saturated bubble is colder and it too wants to sink.

This situation is known as absolute stability since, regardless of whether the air is saturated or
not, the air is stable.

ABSOLUTE INSTABILITY

Now, consider the diagram below. The ELR is 5°C/1000 ft, greater than both the DALR and the
ELR.

DALR SALR

ELR 5°C/1000 ft

4000 ft 8°C 14°C 0°C

3000 ft 11°C 15.5°C 5°C
2000 ft 10°C
1000 ft 14°C 17°C 15°C
20°C
17°C 18.5°C

20°C

Dry air Saturated air

Meteorology 7-3

Chapter 7 Stability

The unsaturated air cools at 3°C/1000 ft and at each level is warmer than the surrounding
environment. Thus, it’s less dense and, therefore, tends to keep rising.

The saturated air cools at 1.5°C/1000 ft. At each level it is warmer than the surrounding
environment, hence less dense. It too tends to keep rising.

We call this situation absolute instability.

CONDITIONAL INSTABILITY

Now consider a situation in which the ELR is between the SALR and the DALR, as in the diagram
below.

DALR SALR

4000 ft 8°C 14°C ELR 2°C/1000 ft
12°C

3000 ft 11°C 15.5°C 14°C
2000 ft 16°C
1000 ft 14°C 17°C 18°C
20°C
17°C 18.5°C

20°C

Dry air Saturated air

The environmental temperature is lapsing at 2°C/1000 ft. The unsaturated air is cooling at
3°C/1000 ft and at each level it is cooler than the surrounding environment, so it wants to sink.

The saturated air, however, is cooling at 1.5°C/1000 ft, so at each level it is warmer than the
surrounding environment and it tends to rise.

This situation is called conditional instability. The air is stable when unsaturated, but unstable
when saturated.

7-4 Meteorology

Stability Chapter 7

SUMMARY OF STABILITY SALR

DALR

Conditional
instability

Height Absolute
stability

Absolute
instability

Temperature

The diagram shows the stability of the air when the ELR falls in different areas of the graph.

Absolute Stability ELR < SALR < DALR

Absolute Instability ELR > DALR > SALR

Conditional Instability DALR > ELR > SALR

Note that in all the above cases, an initial trigger action is required to start the air rising. There are
several forms that this trigger can take, which are discussed thoroughly in the chapter on Cloud
Formation.

NEUTRAL STABILITY

There is one more type of stability not yet mentioned. If the air is unsaturated and the ELR is
exactly 3°C/1000 ft, then the rising air is cooling at the same rate that the environment is lapsing.
So the air is neutrally stable.

If the air was saturated, the ELR would have to be identical to the SALR for the air to be neutral.

Meteorology 7-5

Chapter 7 Stability

CONVECTIVE OR POTENTIAL INSTABILITY

Potentially unstable air occurs when horizontal air motion is present at the same time air is being
lifted, such as in a low pressure centre or along a frontal surface.

The air in the lower layers must be saturated and the air in the upper layers must be dry, as
demonstrated in the following diagram.

25 – 24
= 1°C

ELR 3.4°C/1000 ft 5000 ft
– unstable

Cools 30 – 12
=18°C
at the
DALR

8000 ft

Unsaturated air Cools
25°C at the
SALR

5000 ft ELR 1°C/1000 ft
– initially stable

30°C
Saturated air

The diagram above shows that before lifting occurs, the ELR is lower than the SALR, therefore,
the layer is stable. The lower air cools at the SALR as it is lifted because it is saturated. Since the
air above is dry, it cools at the DALR. When the air reaches the top of the obstruction, the
temperature difference between the bottom of the 5000 ft layer and the top has increased, hence
the ELR has increased. It is now greater than the DALR, so the layer is unstable.

In the next diagram, the lower air is dry and the upper air is saturated, so the opposite occurs.
Initially the ELR is high, but as the air cools, the temperature difference decreases, lowering the
ELR to below the SALR and making the layer stable.

7-6 Meteorology

Stability Chapter 7

14 – 12=
2°C

ELR 0.8°C/1000 ft 5000 ft
– stable

Cools 30 – 24
=6°C
at the
SALR

8000 ft

Saturated air Cools
14°C at the
DALR

5000 ft ELR 3.2°C/1000 ft
– initially unstable

30°C
Unsaturated air

In summary, the following processes increase stability:

1. Advection of cold air or other cooling at low level
2. Advection of warm air or other heating of upper air
3. Decreased humidity at low levels or infusion of dry air at high levels
4. Descending air motions such as subsidence created behind mountains in high

pressure centres or through divergence at low level

Factors that lead to increased instability are

1. Advection of warm air or heating of the air at low level
2. Advection of cold air or other cooling of the upper air such as night time radiation

from the top of clouds
3. Increased humidity at low level
4. Enforced lifting which may lead to conditional instability (over mountains, on shore

winds at coasts etc)
5. General lifting, as in low pressure centres and in the case of convergence

INVERSIONS

Inversions are extremely stable, as the ELR is in fact negative.

The most common inversion forms at low level during clear nights, when radiation and cooling at
ground level is at its maximum. This is known as a ground inversion.

When the surface is snow covered, the cooling can be intense, and surface temperature is often
10°C lower than at the level of the Stevenson Screen (1 – 2 m). From the screen upward, the air
temperature rises 10 – 20°C in extreme cases.

Meteorology 7-7

Chapter 7 Stability

In broken terrain, the cooled air at the surface drains into the lowest area of the ground, creating
what is called a katabatic wind. This can lead to fog formation. This is discussed in more detail in
later chapters.

When winds are light and the ground is covered with snow, the inversion may be at 4000 ft to
7000 ft and dominates the weather situation.

Inversions can form in the troposphere, when warm air moves over a colder layer of air, for
example, with a warm front. In many cases, clouds form in the inversion but these do not have
strong vertical air currents.

CLOUD FORMATION

As discussed at the beginning of the chapter, you must understand the concepts of stability and
adiabatics in order to understand the processes of cloud formation.

THE DRY THERMAL

Consider the following hypothetical situation. The surface temperature is 10°C and the
environmental temperature lapses normally until height ‘X’, then there is an inversion. The
surface is heated at a particular location, which causes the temperature to rise to 20°C. The air in
this region becomes less dense and starts to rise.

ELR

Height X

DALR

0 5 10 15 20
Temperature in °C

This is what is known as a “thermal.” In this case, the air is unsaturated so it is called a dry
thermal.

Because the air is unsaturated, it cools at the DALR — faster than the environment and hence
eventually the two lines will intersect. In the hypothetical example, the two lines intersect at X, the
height at which the inversion starts.

If the thermal were to continue to rise it would follow the dotted line, so it would be cooler than its
environment. Therefore, it will be more dense and no longer has the tendency to rise.

If you were to fly below height X, you would experience turbulence due to the updrafts in the
thermal. Above height X, the conditions would be smooth.

7-8 Meteorology

Stability Chapter 7

FORMATION OF A CLOUD

Dewpoint was not taken into account in the previous example, which assumed the air never
reaches saturation. What would happen if, at some point in the rise of the air, it became
saturated? The following diagram represents this situation.

ELR

Height X SALR
DALR
LCL

DP

0 5 10 15 20
Temperature in °C

As before, the trigger action is surface insolation, which leads to the formation of a thermal that
starts to rise. However, now there is a line representing the dewpoint, which has a lapse rate of
0.5°C/1000 ft.

In the diagram, the DALR line intersects the dewpoint line before it intersects the ELR line. Hence
the thermal has reached saturation before it has stopped rising. At this point, water vapour starts
to condense to form cloud.

The thermal is still warmer than the environment so it continues to rise. However, its temperature
now falls at the SALR. It eventually intersects the ELR and stops rising.

So, the base of the cloud is the point in which the DALR intersects the dewpoint line, known as
the lifting condensation level. The top of the cloud is where the SALR intersects the ELR.

Once the thermal reached saturation, the lapse decreased to the SALR. The lower the SALR the
longer it will take for this line to intersect the ELR. Warm air has a higher moisture content when
saturated so it has a lower SALR due to the large amounts of latent heat released.

If the air is cold, the SALR is close to the DALR and the line intersects the ELR quickly. Hence,
warmer air leads to a thicker cloud forming than those formed in colder air. The diagram on the
next page demonstrates this scenario.

Meteorology 7-9

Chapter 7 Stability

ELR

Height X Cloud top for warm air
DALR Cloud top for cold air
LCL
DP
Warm air SALR
Cold air SALR

0 5 10 15 20
Temperature in °C

CALCULATING CLOUD BASE

If the dewpoint was constant, we could quite easily calculate the height that the cloud base would
form. It would simply be:

(T – Td) ÷ 3 × 1000
Where:

T = surface temperature
Td = dewpoint

This would give an answer in feet.

However, since the dewpoint is also lapsing, it is not quite as simple as this. The temperature to
which the thermal must fall must be the same as the temperature to which the dewpoint must fall.
This is referred to ‘t’:

t = T – (3H ÷ 1000)

but also:

t = Td – (0.5H ÷ 1000)

Where ‘H’ is the height of the cloud base in feet.

Hence:

T – 3H ÷ 1000 = Td – 0.5H ÷ 1000

7-10 Meteorology

Stability Chapter 7

Rearranging the formula to make H the subject:

H = (T – Td)400

Using the same process, the following formula is derived:

h = (T – Td)125

Where ‘h’ is the cloud base in metres.

You must memorise both formulae. The derivation, however, is for your information only. Note
that the above formulae are only valid for convective clouds, that is, those formed by thermals.

FORECASTING CLOUD FORMATION

When forecasters determine whether or not convective clouds are likely to form, they must initially
select a representative environmental lapse rate curve for the air mass in question. The dewpoint
at the ground is checked and then assumptions are made of the development of the air
temperature near the ground (amount of cloud, insolation, estimated maximum temperature, etc.).
The condensation level can be calculated based on the forecast temperature and current
dewpoint.

To forecast convection a comparison is made between the lifting (path) curve with the actual
lapse rate curve. When such a comparison is made, four main types can be distinguished.

The following key applies:

DALR
SALR
ELR
Dewpoint

Meteorology 7-11

Chapter 7 Stability

1. The condensation level is on the cold side of the lapse rate.
No clouds form, dry thermals only.
Rate of ascent of the thermals 0.5 – 2 m/s.
Over hot (dry) surfaces, the dry thermals may be much stronger.

Height

Temperature

2. The condensation level is on the warm side of the lapse rate.

The moist adiabatic lapse rate intersects the environmental lapse rate curve rather early.
Small convective clouds form.
Rate of ascent 1 – 4 m/s below clouds, 5 – 10 m/s inside the clouds.

Height
Temperature

7-12 Meteorology

Stability Chapter 7

3. The condensation level is on the warm side of the lapse rate curve.

The moist adiabatic air does not intersect the lapse rate curve until high level. Large
convective clouds form.
Hail and electrical discharges may occur.
Rate of ascent at tens of metres/sec in the cloud subjects the aircraft to heavy
turbulence.

Height

Temperature

4. The condensation level is on the cold side of the lapse rate and no clouds form.

If the air is forced to rise, e.g. over an obstruction, temperature is forced to cool to the
condensation temperature and the thermals begin to rise by themselves. This condition is
called Latent Instability.

Height 7-13

Temperature

Meteorology

Chapter 7 Stability

7-14 Meteorology

ACKNOWLEDGEMENTS

Thank you to Ashley Gibbs for the use of his photographs.

INTRODUCTION

Clouds are collections of water droplets, ice crystals, or a mixture of both. They provide
indications of:

1. possible turbulence
2. poor visibility
3. precipitation
4. icing

The average lifetime of a cloud is 15 – 20 minutes, but cumulonimbus clouds can last 2 – 3 hours.

There are several different types of cloud, all with different characteristics regarding the weather
factors above.

Cloud formation is discussed in detail in the next chapter. This chapter focuses on defining the
different cloud types and their features, with a basic mention of formation processes.

CLOUD TERMS

Cirrus High clouds with a feathery appearance
Cumulus Clouds with a flat base and a top like a cauliflower
Stratus Widespread clouds of great horizontal but little vertical extension
Alto Medium level clouds
Nimbus Clouds with moderate precipitation
Lenticularis Clouds with a lens like appearance
Castellanus Clouds with a turret like appearance
Mamma Clouds with a base that has a pendulous or pouch like appearance
Fractus Clouds with a broken or ragged appearance

Meteorology 8-1

Chapter 8 Clouds

CLOUD CLASSIFICATION

The initial subdivision of clouds is into two main types: layer clouds and clouds of great vertical
extension (or heap clouds).

LAYER CLOUDS

These form in stable air and can be further subdivided into categories according to the height
bands in which they are found. Hence there are three further subcategories as follows:

High level clouds (16 500 ft to 45 000 ft)

Cirrus CI

Cirrocumulus CC

Cirrostratus CS

Medium level clouds (6500 ft to 23 000 ft)

Altostratus AS

Altocumulus AC

Low level clouds (Surface to 6500 ft)

Nimbostratus NS

Stratocumulus SC

Stratus ST

Each cloud type has a two letter abbreviation.

Notice that the medium level and the high level bands overlap. This happens because in the
summer the medium level clouds can extend up to 23 000 ft, and in winter the high level clouds
can come as low as 16 500 ft.

CLOUDS OF GREAT VERTICAL EXTENSION

These form in unstable air and air not restricted to a particular height band like the layer clouds.

Cumulus CU Surface to 25 000 ft

Cumulonimbus CB Surface to tropopause

Nimbostratus NS Surface to 15 000 ft

A nimbostratus cloud can be a low cloud or a cloud with vertical extension because when there is
strong lifting, nimbostratus can behave like a heap cloud and extend through several height
bands.

The next few sections look at each of the cloud types in turn and describe the characteristics of
each.

8-2 Meteorology

Clouds Chapter 8

LOW CLOUDS

STRATUS, ST

Stratus (ST) is a layer cloud with large horizontal extent but little vertical development. It generally
has a very low cloud base (below 1000 ft) and covers the whole sky. The typical depth is
1000 - 1500 ft. The base can be quite diffuse with veils hanging down beneath the cloud.

It is a turbulence cloud, often found in the warm sector of polar front depressions. It can also be
formed when low fog lifts.

ST consists of water droplets that are sub-zero in winter but are not very dense, so light to
moderate icing can be expected. Precipitation may occur as drizzle, freezing drizzle, or snow
grains.

Meteorology 8-3

Chapter 8 Clouds
STRATOCUMULUS, SC

A stratocumulus (SC) cloud is a stratiform cloud caused by turbulence. It can be found between
heights 1000 ft and 6500 ft. Because it is formed by turbulence, you might expect light to
moderate turbulence when flying in or below the cloud. Conditions are calm above the cloud.

Like stratus, this cloud consists of water droplets, so light to moderate icing, drizzle, freezing
drizzle, or snow grains can be expected. In addition, you can expect ice pellets and, from the
thicker stratocumulus, intermittent rain or snow. Heavy snowfall can be experienced in winter.

MEDIUM CLOUDS

ALTOSTRATUS, AS

8-4 Meteorology

Clouds Chapter 8

Altostratus is similar to nimbostratus but is less deep and less dense. This type of cloud can
cover the whole or a major part of the sky and is an indication of the approach of a warm front.

Altostratus contains water droplets and ice crystals, therefore, it can cause light to moderate
icing. Light to moderate turbulence can also be expected. Precipitation can take the form of
continuous or intermittent rain or snow.

ALTOCUMULUS CASTELLANUS, ACC
Altocumulus castellanus gets its name for the cloud’s appearance, which is similar to castle
turrets extending from the top. It develops from altocumulus when there is mid-level instability. It
can therefore indicate the possibility of CBs forming. It tends to be denser than altocumulus so
icing and turbulence can be moderate to severe.

ALTOCUMULUS LENTICULARIS, ACL
Altocumulus lenticularis is a lenticular cloud, which means it is lens-like in appearance. It is
formed orographically in association with mountain waves.

Icing in this cloud can be severe due to the constant replenishment of moisture by updraughts in
the wave.

HIGH CLOUDS

All high clouds fall within the 16 500 − 23 000 ft band. They use the prefix ‘cirr(o)’.

CIRRUS, CI

Cirrus is a thin wispy cloud. It is associated with the approach of a warm front. It can also indicate
the line of a jet stream.

It consists of ice crystals and does not produce icing or precipitation. Likewise, there is no
turbulence.

Meteorology 8-5

Chapter 8 Clouds
CIRRO-STRATUS, CS

Cirro-stratus is a sheet-like cloud, sometimes with a wispy veil underneath. It causes a bright ring
around the sun and the moon, known as the halo phenomenon. It is associated with warm
fronts.

Like cirrus, it consists of ice crystals and does not produce icing, precipitation, or turbulence.

CIRRO-CUMULUS, CC

8-6 Meteorology