Jeppesen Meteorology

Clouds Chapter 8

Cirro-cumulus is divided into smaller cloud elements that look like the scales of a mackerel. It is
formed when there is turbulence within cirrus or cirrostratus.

Cirro-cumulus consists of ice crystals and occasionally freezing water droplets. There is no icing
or precipitation. There may be light turbulence.

CLOUDS WITH GREAT VERTICAL DEVELOPMENT

CUMULUS

This photo features heap clouds, which are clouds that generally have greater vertical than
horizontal extent. They are formed convectively and the base can be found between 3000 and
7000 ft in the summer and 700 and 4000 ft in the winter. The tops can extend to 25 000 ft.

Cumulus clouds consist of water droplets, which are supercooled above the freezing level.

Precipitation can be present when the cloud has a vertical extent greater than 10 000 ft. It can
take the form of rain or snow showers.

When the cloud becomes towering without being ‘iced’ (cirrus forming) at the top, it is called
towering cumulus, TCU.

Strong vertical currents can be present and larger CU should be avoided. Moderate to severe
icing conditions can be encountered, but because the time taken to traverse the cloud is usually
short, any ice build up tends to be small.

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Chapter 8 Clouds
CUMULONIMBUS

Cumulonimbus is a towering cumulus cloud with a top that has turned into cirrus. This is called
the anvil and extends in the direction of the wind. The anvil is fibrous and diffuse in appearance.
This cloud is very hazardous to aircraft. It is very dense and consists of water droplets of varying
sizes, so moderate to severe icing may be expected. Moderate to severe turbulence is also likely.
CB can give precipitation in the form of rain or snow showers and hail.
Due to the severe weather conditions associated with this cloud, it is discussed in detail in a
separate chapter on thunderstorms.

CLOUD AMOUNTS

Not only is the type of cloud important, but also the amount of cloud. If half or less than half of the
sky is covered with clouds, there should be little if any problem in avoiding them. If more than half
the sky is covered, avoidance becomes difficult.
In aviation meteorology, the sky is divided into eight equal parts called oktas. You can describe
the amount of cloud as a number of oktas, for example 4 oktas. This would mean that 4/8ths, one
half, of the sky is covered.

8-8 Meteorology

Clouds Chapter 8

In meteorological messages, use three letter abbreviations. These correspond to a number of
oktas as specified below:

SKC Sky clear 0 oktas
FEW Few 1 – 2 oktas
SCT Scattered 3 – 4 oktas
BKN Broken 5 – 7 oktas
OVC Overcast 8 oktas

Aerodrome reports use an observation area with a radius of 5 km around the airport plus the area
in the direction of approach. The exception is CBs, which are reported if they are within a 15 km
radius of the airport.

For airfields equipped with instrument landing systems, cloud base reports are referenced to the
site of the middle marker beacon.

You may also see or hear the term CAVOK in meteorological messages. This means ceiling and
visibility OK. In the following conditions you can replace the visibility, weather, and cloud group in
a meteorological report with the word CAVOK.

1. Visibility > 10 km.

2. No clouds occur below 5000 ft or the highest Minimum Sector Altitude, whichever is the
greater.

3. No CB in the vicinity (> 15 km).

4. No precipitation (except ice crystals), thunderstorms, low snowdrift, shallow fog, low
drifting dust or sand, or sand or dust storms.

CLOUD BASE

In addition to the amount and type of cloud, the cloud base is also reported based on the distance
from the ground to the cloud.

The cloud base is the lowest zone in which the type of obscuration perceptibly changes from that
corresponding to clear haze to that corresponding to water droplets or ice crystals.

A METAR or MET REPORT uses 100 ft intervals for clouds up to 10 000 ft, and 1000 ft intervals
for those above 10 000 ft.

For example, you may receive the following in a report:

FEW003 SCT010 BKN040

The numbers after the descriptive abbreviations give the cloud base in hundreds of feet, so
FEW003 means 1 – 2 oktas with a cloud base of 300 ft.

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Chapter 8 Clouds

CLOUD CEILING

The cloud ceiling is the height above aerodrome level of the lowest layer of cloud of more than
4 oktas.

MEASURING CLOUD BASE

AIREPS

There are several ways to measure the cloud base. The cheapest and easiest way is to use
AIREPs (reports from the pilots of aircraft). This may not always be possible on approach and
departure routes that aren’t used frequently.

HUMAN OBSERVATION

The general weather service uses an imperfect method in which the observer estimates the cloud
base. Estimated cloud bases can have large errors and have to be supplemented by the direct
measurements to be used in aviation meteorology.

BALLOONS

If the cloud base is low, as is the case of ST/SC clouds, balloons that rise at a known rate can be
used to determine the cloud base. The time taken for the balloon to disappear into the cloud is
measured, and the measurement is converted into a distance.

CEILOMETER

Most ceilometers use a light-pulse that is reflected by the cloud. The laser reaches the higher
levels without any significant scattering. The reflected light-pulse is received by a light-sensitive
cell and half the time of transport gives the measurement of the cloud base. One problem of this
type of ceilometer is that precipitation can also give reflecting light-pulses, which leads to the
cloud base measurement being too low.

ALIDADE

The Alidade is used at night. The alidade is positioned a known distance from a searchlight. The
searchlight is shone on the cloud and the alidade measures the angle above the horizontal of the
searchlight glow on the base of the cloud. The cloud base is calculated by trigonometry.

VERTICAL VISIBILITY

If fog is thick or snowfall is heavy, the cloud base loses its importance and vertical visibility is
reported. Vertical visibility indicates at what height above the ground the pilot of an aircraft should
have visual contact with the ground vertically down below.

An important difference between cloud base and vertical visibility is that the cloud base mostly
indicates a height in which the pilot can see forward, while vertical visibility only indicates at what
height the pilot can see vertically down.

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Clouds Chapter 8

SUMMARY OF CLOUD TYPE AND CHARACTERISTICS

Cloud Type Height Composition Turbulence Icing Visibility Significance

Cirrus CI 16 500 ft to Ice crystals Nil Nil 1000 m + Found 400 to 600
45 000 ft nm ahead of a warm
front

Cirrostratus CS 16 500 ft to Ice crystals Nil Nil 1000 m + Found 400 to 600
45 000 ft nm ahead of a warm
front

Cirrocumulus CC 16 500 ft to Ice crystals Light Nil 1000 m + Found 400 to 600
45 000 ft nm ahead of a warm
front when
turbulence exists

Altocumulus AC 6500 ft to Water Light to Light to 20 to Turbulence cloud
23 000 ft droplets and moderate moderate 1000 m
ice crystals

Altostratus AS 6500 ft to Water Light to Light to 20 to Warm front 200 nm
23 000 ft droplets and moderate moderate 1000 m ahead. Merges with
ice crystals NS as the front is
approached

Nimbostratus NS Ground level to Water Moderate Moderate 10 to 20 m Warm front very

6500 ft. Can be droplets but to severe to severe close

10 000 ft to can be ice

15 000 ft crystals at

merging into AS medium

at higher levels levels

Stratocumulus SC 1000 ft to 6500 Water Light to Light to 10 to 30 m Turbulence cloud
ft droplets moderate moderate

Stratus ST Ground level to Water Nil to light Occasion 10 to 30 m Turbulence cloud
ally light
6500 ft droplets to
moderate

Cumulus CU 1000 ft to Water Moderate Moderate Less than Instability cloud.
25 000 ft droplets and
ice crystals to severe to severe 20 m Large CU may

develop into CB

Cumulonimbus CB 1000 ft to Water Moderate Moderate 10 to 20 m Instability cloud
45 000 ft droplets and to severe to severe
ice crystals

Altocumulus AC 6500 ft to Water Moderate Moderate - An indication of
castellanus C 23 000 ft droplets and to severe to severe unstable air at mid
ice crystals levels; can indicate
approaching CB

Altocumulus AC 6500 ft to Water Moderate Moderate - Associated with
Lenticularis L 23 000 ft droplets and to severe to severe mountain waves
ice crystals

Meteorology 8-11

Chapter 8 Clouds

8-12 Meteorology

INTRODUCTION

This chapter covers the formation of clouds in more depth than previous chapters.

Clouds form when air rises and cools adiabatically. If rising air cools to its dewpoint, the water
vapour will condense out as water droplets.

The height at which this occurs is the condensation level. This is also the level the cloud base
occurs.

There are several different lifting processes that can lead to cloud formation. They are as follows:

1. Turbulence
2. Convection
3. Orographic uplift
4. Frontal uplift
5. Convergence

TURBULENCE

CONDITIONS

Turbulence clouds can form whenever there is a stable layer. Such a stable layer may occur if
there is an inversion or isothermal layer above it, preventing lifting.

If the wind speed is greater than about 10 kt, turbulence within the layer can lead to a steepening
of the lapse rate.

Note: Although a wind speed of greater than 10 kt is necessary for turbulence clouds to
form, once formed it can persist at lower speeds.

If this steepening is such that the saturation layer occurs within the turbulent layer, then
turbulence clouds form.

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9-1

Chapter 9 Cloud Formation

MECHANISM

The diagrams below show what happens when there is a stable layer of 3000 ft thickness and
turbulent mixing occurs within the layer.

Isothermal layer

3000 ft - 12°C

2000 ft - 13°C Stable layer –
1000 ft - 14°C ELR of

1°C/1000 ft

0 ft - 15°C

The above diagram shows the layer before turbulence commences. The layer is stable, the ELR
being only 1°C/1000 ft. Surface temperature is 15°C, making the top of the layer 12°C. Above
3000 ft is an isothermal layer, where the temperature remains 12°C (although this could equally
be an inversion layer).

3000 ft Isothermal layer
6°C 12°C

2000 ft 9°C 15°C

1000 ft 12°C 18°C

0 ft 15°C 21°C

The above diagram shows the situation during turbulence. Pockets of air are circulated within the
layer. Due to the nature of air as a bad conductor, the pockets cool or warm adiabatically.

9-2 Meteorology

Cloud Formation Chapter 9

As you can see from the diagram, this means bubbles of air ascending to the top of the layer is
6°C, colder than the environmental temperature. Descending bubbles of air are 21°C when they
reach the bottom of the layer, warmer than the environment.

3000 ft Isothermal layer
9°C

2000 ft 12°C

1000 ft 15°C

0 ft 18°C

The final diagram shows the situation after turbulence. The temperature at any one level
becomes the average of the temperatures of the bubbles that have ascended and those that have
descended.

The surface temperature has increased and the temperature at the top of the layer has
decreased. Overall the ELR has increased. It is now 3°C/1000 ft. This may result in the dewpoint
being reached below the top of the layer.

For example, assume that the surface dewpoint is 12°C. The dewpoint lapses at 0.5°C so it
would not be reached before the top of the layer in the pre-turbulence case. However, after
turbulence the dewpoint would fall within the layer, hence saturation would occur and clouds
would form, as shown in the next diagram.

Meteorology 9-3

Chapter 9 Cloud Formation

3000 ft Isothermal layer DEWPOINT
9°C 10.5°C

2000 ft 12°C 11°C

1000 ft 11.5°C

15°C

0 ft 18°C 12°C

By comparing the new environmental temperature with the dewpoint at various levels, you find
that the cloud base is at 2400 ft.

CLOUD TYPES

The following cloud types are formed by turbulence:

1. Stratus
2. Stratocumulus
3. Altocumulus
4. Cirrocumulus

CONVECTION

Convective processes were introduced in the chapter on Stability, but the processes are
recapped below.

CONDITIONS

Convective clouds form when the surface is heated. This heat energy passes to the air above the
surface by conduction. This air is now warmer than the surrounding environment so it starts to
rise, that is, convection occurs. If the rising air reaches its dewpoint before it reaches the same
temperature as the environment, condensation occurs.

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Cloud Formation Chapter 9

MECHANISM

The following key applies to the next few diagrams:

DALR
SALR
ELR
Dewpoint

Height

Temperature

In the diagram above, the surface is heated, which starts a vertical motion of air. Initially, the air
cools at the DALR until it reaches the dewpoint. Water vapour then starts to condense out as
droplets and a cloud starts to form. The level at which this occurs is the condensation level and
is coincident with the cloud base.

The air now cools at the SALR. Lifting, and hence cloud formation, ceases when the rising air
reaches the same temperature as the surrounding environment.

The temperature to which the surface must be heated in order for air to be lifted to its
condensation level is the critical temperature.

In the diagram, the DALR intersects the dewpoint curve when the dewpoint temperature is quite
close to the environmental temperature at a low height. Only a small amount of lifting occurs after
this point, so the cloud form is quite small.

Such small clouds are not large enough vertically to produce precipitation. They are usually
isolated (forming over hot spots on the surface) and the sky is otherwise clear. They are,
therefore, referred to as fair weather cumulus/cumuli.

This is common on warm summer days. As temperatures fall in the evening, they tend to
disappear.

If fair weather cumulus form in the morning it may mean there will be large Cu or Cb later on in
the day when insolation increases, for example in the next diagram.

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Chapter 9 Cloud Formation

Height

Temperature

The relative positions of the ELR and the dewpoint curves are the same. The only difference is
that the surface is heated to a much higher temperature. The DALR intersects the dewpoint curve
at a greater height. After this point, there is much more lifting before the SALR intersects the ELR.

So, with greater surface heating there is a much bigger cloud, but one with a higher cloud base. If
this cloud exceeds 10 000 ft in height, it may produce precipitation.

Another factor is the stability of the atmosphere. The steeper the environmental lapse rate, the
longer it takes for the temperature of the rising air to reach the same temperature as the
environment, so the larger the cloud that forms.

ADVECTION

Another way for convective clouds to form is with advection. Advection is the horizontal
movement of air. If cold air passes over a warm surface it becomes heated from below, starting
the process of convection. Typical convective clouds such as cumulus and cumulonimbus can
form.

An example of this is cold air passing over a warmer sea surface such as polar air moving south
over the North Atlantic.

CLOUD TYPES

The following types of clouds are formed convectively:

1. Cumulus
2. Towering cumulus
3. Cumulonimbus

OROGRAPHIC UPLIFT

CONDITIONS

Orographic clouds form when air is forced to rise over an obstruction, such as high ground. This
may occur in a stable or an unstable environment. The type of cloud that forms depends on the
stability and moisture content of the atmosphere.
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Cloud Formation Chapter 9

MECHANISM

In stable conditions, air is forced to rise over the obstruction. Initially, it cools at the DALR. Once it
reaches its dewpoint, cloud starts to form. This formation is a stratiform cloud. The air cools at the
SALR.

As it passes over the crest of the ridge, the lifting force no longer is present so the air flows down
the other side. It initially warms at the SALR. Since much of its moisture has condensed out as
cloud, it becomes unsaturated again at a lower temperature than the original dewpoint.

Hence the base of the cloud is higher on the leeward side than the windward side.

The air then warms at the DALR. The diagram below shows the temperature at ground level on
the lee side is higher than that on the windward side. This warming wind is known as the Foehn
Wind.

6°C 4000 ft 6°C

7.5°C 3000 ft 7.5°C

9°C 2000 ft 10.5°C

12°C 1000 ft 13.5°C

15°C 0 ft 16.5°C

In drier conditions, the cloud base may be above the top of the ridge. If this happens, the clouds
that form are altocumulus lenticularis.

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Chapter 9 Cloud Formation
Altocumulus Lenticularis (Lenticular Cloud)

These clouds get their name from their lens shape and, generally, indicate the presence of
mountain waves, which are discussed in detail in the chapter on Windshear and Turbulence.
These types of clouds can cause severe turbulence.
The cloud is being continuously replenished with moist air. It, therefore, contains a high
concentration of supercooled droplets. Icing, therefore, can also be severe.
If the conditions are unstable, the obstruction provides the initial lifting force. After the crest is
reached, the air continues lifting due to the unstable nature of the air. The cloud that forms is a
cumuliform rather than a stratiform.
The bulk of the cloud forms on the windward side of the obstruction. Most of the precipitation falls
here as well. The lee side is said to be in rain shadow.

9-8 Meteorology

Cloud Formation Chapter 9
Cap Cloud

Another situation that causes orographic uplift is when the atmosphere is initially stable then
becomes unstable. Initially, stratiform clouds form. If this is at a medium level, it becomes
altocumulus. If the atmosphere then becomes unstable, this can develop into altocumulus
castellanus. Stratocumulus can develop into stratocumulus castellanus but this is rare.

Meteorology 9-9

Chapter 9 Cloud Formation

CLOUD TYPES

The following clouds can be formed orographically:

IN UNSTABLE CONDITIONS
Cumulus
Cumulonimbus

IN STABLE CONDITIONS
Stratus
Stratocumulus
Altocumulus
Altocumulus lenticularis

WHEN ATMOSPHERE IS INITIALLY STABLE AND LATER BECOMES UNSTABLE
Altocumulus castellanus

FRONTAL UPLIFT

CONDITIONS

A front is the boundary between two air masses, generally in motion, with different properties.
Usually the comparison is made between the relative temperatures of the air masses. There are
two main types of front: the warm front and the cold front.

A warm front is found when warm air is replacing cold air. A cold front is found when cold air is
replacing warm air. In both cases the warm air, being less dense, rises up over the cold air.

Looking at it from the point of the warm front, the warm air slides up over the cold air it is
replacing. From the point of view of the cold front, the cold air undercuts the warm air it is
replacing.

The fronts have different properties and hence the cloud types that form along them differ.

MECHANISM

THE WARM FRONT
The warm air rises over the cold air, forming a sloping front with a gradient of only about 1 to 150,
so the lifting is very gentle and a stratiform cloud forms. From the ground up, the types of cloud
that forms will be stratus, nimbostratus, altostratus, cirrostratus, and cirrus.

Note that when flying toward a warm front from the cold side, you will encounter a progressively
lowering cloudbase.

The gradient is such that the first cloud, the high cloud cirrus, can be encountered up to 600 nm
ahead of the surface position of the front.

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Cloud Formation Chapter 9

WARM FRONT

WARM AIR CI
CS
NS AS
ST
COLD AIR

THE COLD FRONT
Cold air pushes underneath the warm air it is replacing. The slope of the cold front is very
different from that of the warm front. It averages a slope of 1 to 50, and close to the ground it can
be almost vertical, sometimes forming a protruding area that looks like a nose, as shown in the
next diagram.

The air may be unstable, but if it is not, it can be made so by the large amount of enforced lifting.
Hence the type of cloud which forms on this kind of front is generally cumuliform in type, although
there can be shallow bands of stability where NS and CI can form.

Since the front is steeper, the associated cloud ceases no more than about 200 nm after the
passage of the surface front.

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Chapter 9 Cloud Formation

CI COLD FRONT

CU/CB

WARM AIR

COLD AIR
NS

CLOUD TYPES

COLD FRONTS ONLY
Cumulus
Cumulonimbus

WARM FRONTS ONLY
Stratus
Altostratus
Cirrostratus

MAINLY WARM FRONTS, OCCASIONALLY COLD FRONTS
Nimbostratus
Cirrus

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Cloud Formation Chapter 9

CONVERGENCE

CONDITIONS

Wherever there is convergence, air is forced to rise. Such convergence occurs in depressions
and non-frontal troughs.

MECHANISM

As air converges into the low pressure area, the air at the centre of the low, or the centre line of
the trough, is forced to rise. This leads to instability and saturation, hence the formation of clouds.

CLOUD TYPES

The cloud types that form are those that are associated with instability. These are cumulus,
cumulonimbus, and towering cumulus.

Meteorology 9-13

Chapter 9 Cloud Formation

9-14 Meteorology

INTRODUCTION

Clouds can consist of a combination of water droplets, supercooled water droplets, and ice
crystals. Individual water droplets and ice crystals are very small and light, and due to upcurrents
in the clouds, they do not fall as precipitation on their own.

If they combine with other water droplets or ice crystals they become progressively heavier. If the
upcurrents in the cloud are not strong enough to support their weight they fall as precipitation.

It follows that the stronger the upcurrents are, the heavier the droplet or crystal has to be in order
for precipitation to occur. So the largest droplets fall from convective clouds such as cumulus and
cumulonimbus.

PRECIPITATION PROCESSES

There are two theories concerning the formation of precipitation. These processes are not
mutually exclusive and, given the right conditions, may both occur within the same cloud.

BERGERON THEORY (THE ICE CRYSTAL EFFECT)

Where sub-zero conditions occur, both ice crystals and water droplets may be present. Water
vapour may sublimate onto the ice crystals. Collision with supercooled droplets allows the crystal
to grow in size.

Once the crystal reaches a sufficient size, it falls as precipitation. The type of precipitation
depends on the temperature of the air through which it falls. If sufficiently warm, the crystal melts
and falls as a rain droplet. If not, it might fall as snow.

The difference in saturation vapour pressure between ice and water is greatest at approximately
-12°C, so clouds reaching this temperature produce precipitation. Snow has a relatively low rate
of fall, so a cloud thickness of 1500 to 3000 ft is sufficient if the temperature at the cloud top is
approximately -8°C to -12°C.

If supercooled water droplets fall through colder air they might freeze and form freezing rain. This
is common with nimbostratus clouds on a warm front. The droplets fall through the front into
colder air.

In dense clouds such as cumulonimbus, there may be a sufficient concentration of supercooled
water droplets for them to freeze onto ice crystals to form a snow pellet.

Meteorology

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Chapter 10 Precipitation

COALESCENCE THEORY (CAPTURE EFFECT)

The Bergeron Theory requires part of the cloud to be below 0°C, so ice crystals are present. In
many clouds in lower latitudes, no part of the cloud is below 0°C yet precipitation still falls. The
Coalescence Theory covers this scenario.

In the cloud there are water droplets of varying sizes. The larger, heavier droplets fall faster and
collide with smaller droplets on their way down. When the droplets become sufficiently heavy,
they fall as precipitation.

INTENSITY OF PRECIPITATION

Precipitation is described by the following terms:

Rainfall Rate (mm per hour) Snow Accumulation
(cm per hour)
Rain Rain/Hail Showers
< 0.5
Slight < 0.5 <2 0.5 to 4
Moderate
0.5 to 4 2 to 10 >4
Heavy
Violent > 4 10 to 50

> 50

CONTINUITY OF PRECIPITATION

Continuity of precipitation is described using the three terms described below.

Showers
Showers are of short duration and are associated only with convective clouds, that is,
cumulus and cumulonimbus.

Intermittent
Intermittent is associated with layer clouds. Precipitation falls from time to time with short
breaks.

Continuous
Continuous precipitation is that which falls for periods of an hour or longer without breaks.
Continuous precipitation is also associated with layer clouds.

10-2 Meteorology

Precipitation Chapter 10

PRECIPITATION TYPES

The following table describes the different types of precipitation and the clouds they fall from.

Precipitation Type Cloud Type Comments
ST or SC
Drizzle Diameter: 0.2 to 0.5 mm
Freezing Drizzle Visibility: 500 to 3000 m
Snow Grains
Imperceptible impact.

Drizzle does not make a splash on the ground.

Rain (continuous) Thick AS and NS Diameter: 0.5 to 5.5 mm
Visibility: 3000 to 5.5 km

1000 m in heavy rain

Perceptible impact:

Drops have to be large to overcome the up-
currents in the cloud in order to fall.

Larger drops break up into smaller drops as the
rain falls.

Snow (continuous) Thick AS and NS Grains/Needles: < 1 mm diameter

Pellets: 2 to 5 mm diameter

Flakes:

A collection of crystals greater than 4 mm in
diameter. The lower the temperature the smaller
the flake size.

Surface temperature must be < 4°C for snow to
reach the ground before melting.

Hail CB Diameter: 5 to 50 mm

Weight: up to 1 kg

Height: up to 48 000 ft

Rain (intermittent) Thick AS and SC A mixture of rain and snow or snow that has
Snow (intermittent) Heavy CU and partially melted in the descent.
Rain Showers CB
Snow Showers Sleet falls when the temperature is between + 5°C
Sleet CB to + 6°C
SC
Soft Hail, or Small rounded pellets of less than 5 mm diameter
Graupel
Ice Pellets Can be the early stage of hail growth

Diameter: < 5 mm

Transparent pellets either spherical or rounded.

Meteorology 10-3

Chapter 10 Precipitation

HAIL

Hail forms by the ice crystal effect when there are updraughts stronger than 10 m/s.

Hail can cause serious damage to an airframe, especially with larger hailstones. The table below
summarises the strength of updraughts required to produce stones of various sizes and masses.

Vertical Speed Type of Hail Diameter Weight
10 m/s Small Hail (Graupel) < 5 mm 1g
20 m/s 9g
30 m/s Hail (Grêle) 2 cm 80 g
40 m/s 6 cm
70 m/s 10 cm 370 g
14 cm 1 kg

In the UK and Northern Europe, the updraughts in thunderstorms are rarely strong enough to
allow the hailstones to grow to any appreciable size. Large hailstones are more likely to be
encountered in heat air mass thunderstorms in tropical locations.

10-4 Meteorology

INTRODUCTION

It is estimated that every day there are about 44 000 thunderstorms across the planet.

Thunderstorms develop from well-developed cumulonimbus clouds. Not all cumulonimbus clouds
develop into thunderstorms, however. The features described in this chapter apply to very active
CBs as well as actual thunderstorms.

CONDITIONS

Thunderstorms are most likely to occur with the following combination of conditions:

1. An environmental lapse rate greater than the SALR through a depth of at least 10 000 ft
and extending to above the freezing level.

2. Sufficient water vapour to provide early saturation and to form and maintain the cloud.
3. A trigger action to start the lifting process. This can take several forms.

TRIGGER ACTIONS

There are five different possible trigger actions:

1. Convection
2. Orographic uplift
3. Advection
4. Convergence
5. Frontal lifting (generally in association with cold fronts and occlusions)

THUNDERSTORM CLASSIFICATION

Thunderstorms are generally classified as one of two types:

1. Heat or airmass — in this case the trigger action is one of the first four above.
2. Frontal — the trigger action is the fifth in the list.

Meteorology

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Chapter 11 Thunderstorms

HEAT/AIRMASS THUNDERSTORMS

CONVECTION
Although heat/airmass thunderstorms can form with one of four triggers, convection is the most
likely one. Since surface heating is greater in the summer, statistically these thunderstorms are
more likely in the summer. They are also more likely during the day and over land and tend to be
isolated, especially if they have formed in a cold air mass. The cold air mass thunderstorms tend
to dissipate in the evening.

Thunderstorms that form in a warm air mass may form a multicell structure.

A multicell thunderstorm is a cluster of CBs where various cells at differing stages interact. The
downdraughts from dissipating and mature cells spread out as a pool of cold air along the ground
surface. This forces the updraught in the front of the system to ascend providing the uplift for the
formation of more CB clouds.

These can persist until late into the evening.

OROGRAPHIC UPLIFT
With orographic uplift, thunderstorms can occur at any time of the day or night, in summer and in
winter. If the uplift is over a range of hills they may occur in a line formation. Thunderstorms are
formed when the conditions are unstable or conditionally unstable.

Orographic processes may enhance an existing thunderstorm that moves over the obstruction.

ADVECTION
With advection, storms can occur in the day or at night, in summer or in winter. In summer, they
can be caused by maritime air from a cold sea passing over the warm land and being heated
from below. However, the more common case is in winter, when cold, moist air moves over a
progressively warmer sea. A prime example of this would be polar maritime air moving south. The
process then becomes similar to the convective case above.

CONVERGENCE
The fourth type of trigger is convergence. This can be in association with low pressures or non-
frontal troughs. Time of day and year depends on the type of low. The different types of lows are
discussed in a later chapter.

When associated with a trough, thunderstorms can form in a line along the centre line of the
trough and can cause difficulties for a pilot trying to avoid them.

FRONTAL THUNDERSTORMS

Frontal thunderstorms are more frequent in winter due to the increased frequency in the passage
of fronts. They can form over land or sea, by day or night, and are associated with both cold
fronts and occluded fronts.

Because they are associated with a front, these thunderstorms tend not to be isolated but to form
in a line. They can be embedded in other clouds and are difficult to identify, especially when
formed on an occlusion in which there are significant layer clouds present.

They are often accompanied by line squalls, which is a line of thunderstorms formed just ahead
of the front.

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IDENTIFICATION OF THUNDERSTORMS

A thunderstorm cloud, whether of the air mass or frontal type, usually consists of several self-
contained cells, each in a different state of development. New and growing cells can be
recognised by their cumuliform shape with clear-cut outline and cauliflower top. The tops of more
mature cells appear less clear-cut and are frequently surrounded by fibrous cloud.

Development of cells is not always seen since other clouds may obscure the view. In frontal or
orographic conditions, extensive layer cloud structures may obscure a view of the development of
cumulonimbus thunderstorm cells, or ACC.

STAGES OF DEVELOPMENT

There are three stages in the development of a thunderstorm, summarised in the diagram below.

40 000

Altitude (feet) 30 000 Updraught
20 000

10 000 Updraught Updraught Downdraught
5000 GROWTH STAGE DISSIPATING STAGE
0 Downdraught
MATURE STAGE

GROWTH STAGE

In this stage, several small cumulus clouds combine together to form a large cumulus of about
5 nm across. Strong updraughts are present, typically on the order of 1000 fpm, but can be as
great as 4000 fpm.

Air is drawn in from the sides and underneath the cloud, replacing the lifting air within the cloud.

This stage lasts approximately 15 to 20 minutes.

MATURE STAGE

The mature stage is characterised by the onset of precipitation. This precipitation is produced by
the combination of ice crystals and water droplets. The precipitation causes downdraughts of
approximately 2000 − 3000 fpm.

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The updraughts are still present, increasing to as much as 10 000 fpm, though 5000 fpm is a
more typical figure. Tops can reach the tropopause, which can be in excess of 50 000 ft in low
latitudes.

Cloud tops can rise by as much as 5000 fpm. The tops of the clouds are affected by a stronger
upper wind which causes it to tilt in the direction of the wind.

This mixture of updraughts and downdraughts causes strong turbulence within and below the
cloud.

The downdraughts are colder than the surrounding air when they reach the base of the cloud,
due to some water droplets evaporating and latent heat being absorbed. Once clear of the base
of the cloud, they warm at the saturated adiabatic lapse rate and remain colder than the
surrounding air.

This combined with the absorption of latent heat intensifies the temperature difference between
the downdraughts and the environment and causes the downdraught to descend even more
rapidly.

This strong downdraught of cold air reacts with the ground and causes a gust front extending up
to 17 nm ahead of the storm. Also at this stage, there may be roll (rotor) clouds, which are
stratocumulus caused by turbulence.

Other hazards associated with this stage, such as microbursts and lightning, are discussed later
in this chapter.

The mature stage lasts approximately 20 – 30 minutes.

DISSIPATING STAGE

This stage commences when the local supply of moisture is no longer sufficient to support the
storm.

The stage is characterised by the appearance of an anvil. This occurs when the cloud top
reaches the tropopause and is spread out by the strong upper winds to form a flat-topped anvil
shape. This anvil is part of a cirrus cloud.

The cloud at this stage can be referred to as Cumulonimbus capillatus.

Updraughts cease and the cloud starts to dissipate as the downdraughts remove the moisture
from the cloud. The precipitation diminishes and the downdraughts are too strong to support roll
clouds. Lightning might still occur.

The dissipating stage lasts about 30 minutes but the cloud can persist for 2 to 3 hours.

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SUPERCELL THUNDERSTORMS

Supercell thunderstorms are severe local storms that form when there is:

¾ Great depth of instability
¾ Strong vertical windshear
¾ A stable layer between the warm lower air and cold upper air

In the mature stage of these storms there are severe updraughts and downdraughts, which can
give rise to very violent weather such as torrential rain, large hail, strong winds, and even
tornadoes. The mature stage can last for several hours.

MOVEMENT OF THUNDERSTORMS

Thunderstorms formed in a col or slack pressure gradient tend to move erratically, but generally
thunderstorms move with the wind at the 700 hPa level, which is equivalent to approximately
10 000 ft.

Supercell thunderstorms in the Northern Hemisphere tend to move 20° to the right of the 500 hPa
(18 000 ft) wind.

SQUALL LINES

Squall lines are usually formed in the warm air mass ahead of a cold front. Squall phenomena are
most frequent during the evening and early night. They are not very common in Western Europe.
Squall lines are more common over large continental areas such as Eastern Europe or, more
frequently, North America. A squall line with thunderstorms also contains hail, and tornadoes can
occur.

Although the CB along the squall can seem very small and insignificant compared to the frontal
clouds behind, in reality the most intense weather phenomena are caused by squalls.

HAZARDS

TURBULENCE AND WINDSHEAR

Turbulence is moderate to severe in thunderstorms, caused by updraughts and downdraughts
within the cloud. Gusts associated with thunderstorms can cause vertical displacements of up to
5000 ft. The effects can be felt up to 40 miles away.

Severe turbulence can be encountered several thousand feet above the cloud tops, as well as
within and below the cloud. Flying within a few thousand feet of the tops of CBs should be
avoided.

Windshear is a more sustained change in windspeed or direction. It is, therefore, likely to be more
dangerous, especially on the approach where the effect on an aircraft’s airspeed can have
serious consequences. In the most extreme cases changes of as much as 80 kt in speed and 90°
in direction can be experienced within a layer of only a few hundred feet.

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GUST FRONT

Some thunderstorms may have a well defined area of cold air flowing out from a downdraught in
all directions, but tending to lead the storm along its line of movement. A gust front might extend
out 24 to 32 km from the storm centre and can be felt from the surface to about 6000 ft. The cold
air undercuts warm air and windshear may be associated with it.

This gust front can be quite distant from the cloud and without precipitation it does not show up
on weather radar and can therefore be quite unexpected. Occasionally there may be roll cloud
associated with it.

Storm Movement

Possible Roll
Cloud Formation

Warm Air Inflow Turbulence

Outflow

Downdraughts Gust Front

MICROBURSTS

Microbursts are strong downdraughts of air that descend from the centre of CB clouds with
speeds up to 60 kt down to levels as low as 300 ft. They are typically less than 5 km across and
last from 1 to 5 minutes.

As the downdraughts approach the ground, the air splays out in all directions. The following
diagram shows an aircraft approaching the CB. It initially experiences a strong headwind (A), then
a downdraught (B), followed by a tailwind (C).

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Microbursts are the most extreme example of windshear and can result in large airspeed changes
that can result in the loss of large aircraft.

There are two types of microburst: wet and dry. The wet type has large amounts of precipitation
associated with it so shows up well on weather radar. In the dry type any precipitation has
evaporated before reaching the ground, so is less easy to identify. Some virga may show up on
radar.

Dry microbursts are generally the more severe type and tend to be associated with heat airmass
thunderstorms over dry near-desert regions. The evaporation of the precipitation absorbs latent
heat and enhances the downdraughts.

HAIL

Hail can be encountered in the cloud, below the cloud, and beneath the anvil. Since it is not
possible to tell whether or not a given storm produces hail, for avoidance purposes it is safer to
assume that it will. The stronger the lifting and the greater the moisture content, the greater the
chance of hail.

Hail can be up to 14 cm in diameter and can be encountered up to 45 000 ft, producing severe
skin damage with even a short exposure.

ICING

Any flight in cloud or precipitation can result in icing when the temperatures are below zero. Icing
can occur down to temperatures as low as -40°C. Icing is more severe near the base of the cloud
where the droplets are larger. This is discussed more thoroughly in the chapter on Icing.
Carburettor icing is also a risk and can occur in the temperature range -10°C to +30°C.

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Chapter 11 Thunderstorms

LIGHTNING

Various processes can lead to different charges separating within a CB cloud.

In a CB cloud, hail can collide with water droplets and ice crystals in the cloud. This results in a
net transfer of positive ions from the warmer hail to the colder supercooled water droplet or ice
crystal. This results in the positively charged ice crystal/water droplet moving upward in
updraughts and the negatively charged hail falling downward with gravity.

As a water droplet falls within a cloud it gathers speed. Once it reaches about 9 m/s it starts to
split. Larger parts of the split droplet become positive and smaller parts become negative. The
small negative parts are lifted higher up the cloud than the larger positive parts.

Supercooled water droplets might also freeze onto hail. Tiny splinters of ice break off, become
negatively charged and ascend within the cloud.

These processes result in a net charge difference within the cloud. Once this reaches a potential
difference of about 3 million volts per metre over a distance of about 50 metres, a discharge of
current, lightning, takes place.

Most lightning occurs within 10°C (approximately 5000 ft) of the freezing level.

Hazards associated with lightning are temporary blindness caused by the flash, interference with
compasses and other instruments, and possible airframe damage.

STATIC

Static causes interference on LF, MF, HF, and VHF radio equipment. In severe cases a visible
discharge may occur, called St. Elmo’s Fire, which is a purple light around windscreen edges,
wing tips, propellers, and engine nacelles. Although not dangerous in itself it is an indication that
the air is highly charged and lightning is likely.

WATER INGESTION

Turbine engines have a limit to the amount of water they can ingest. If the updraught velocity in
the thunderstorm approaches or exceeds the terminal velocity of the falling raindrops, very high
concentrations of water may occur. It is possible that these concentrations can be in excess of
the quantity of water turbine engines are designed to ingest, which could result in flame-out
and/or structural failure of one or more engines.

To eliminate the risk of engine damage or flame-out, it is essential to avoid severe storms. During
an unavoidable encounter with extreme precipitation, the recommendation is to follow the severe
turbulence penetration procedure contained in the approved aircraft flight manual, with special
emphasis on avoiding thrust changes unless excessive airspeed variations occur. Water can exist
in large quantities at high altitudes even where the ambient temperature is as low as -30° C.

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Thunderstorms Chapter 11

TORNADOES

Tornadoes are associated with severe thunderstorms. They form with massive convergence in a
trough with sharply inclined isobars. Differing wind directions give a rotating twist and the lifted air
becomes a spiral.

They are very localised — less than 300 metres across — and the lifting can be so strong that it
can pick up water from a sea surface or dust from the land. Wind speeds in the vortex can reach
200 kt.

If the funnel does not touch the ground it is called a funnel cloud; if it does touch, it is called a
tornado.

Tornadoes are common in the United States but rare in the UK and Europe.

PRESSURE VARIATIONS

Pressure variations can cause the given QNH/QFE to be in error, sometimes by as much as 1000
ft. Local gusts exacerbate the problem and VSIs are also subject to errors. Aircraft should be
flown for attitude rather than altitude.

WEATHER RADAR

Weather radar is provided to enable pilots to avoid thunderstorms and is designed to detect areas
of heavy precipitation.

The strength of the echo is not necessarily an indication of the strength of the associated
turbulence. Radar return intensities may be misleading because of attenuation resulting from
intervening heavy rain. This may lead to serious underestimation of the severity of the rainfall in a
large storm, and an incorrect assumption of where the heaviest rainfall is likely to be
encountered.

The echo from that part of an area of rain furthest from the radar is relatively weaker and the
actual position of the maximum rainfall at the far edge of the storm area is further away than
indicated on the radar display, sometimes by distances up to several miles. Additionally, a storm
cell beyond may be completely masked.

The high rate of growth of thunderstorms and the danger of flying over or near to the tops both of
the main storm and the small convective cells close to it must be considered when using weather
radar for storm avoidance.

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Chapter 11 Thunderstorms

AVOIDANCE CRITERIA

When using weather radar the following avoidance criteria should be used:

Echo Characteristics

Flight Altitude Shape Intensity Gradient of Rate of Change
0 to 20 000 ft Intensity
Avoid by 10 nm
20 to 25 000 ft Avoid by 10 nm Avoid by 5 nm Avoid by 5 nm echoes showing
25 to 30 000 ft echoes with echoes with echoes with rapid change of
Above 30 000 ft hooks, fingers, sharp edges or strong gradients shape, height or
scalloped edges strong intensity of intensity intensity
or other
protrusions

Avoid all echoes by 10 nm

Avoid all echoes by 15 nm

Avoid all echoes by 20 nm

General rules:

¾ If a storm cloud has to be overflown, maintain at least 5000 ft vertical separation from the
cloud tops.

¾ If the aircraft has no weather radar, avoid any storm cloud by 10 nm that is tall, growing
rapidly, or has an anvil top.

¾ Avoid flying under a CB overhang.

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INTRODUCTION

Visibility is a measurement of atmospheric clarity. Reduction in visibility can be caused by:

¾ Water droplets, such as cloud, fog, or rain.
¾ Solid particles, such as sand, dust, or smoke.
¾ Ice, such as crystals, hail, or snow.

Poor visibility is more common in stable conditions, for example, beneath an inversion. Visibility is
generally better upwind of towns and industrial areas, away from the atmospheric pollutants.

TYPES OF VISIBILITY REDUCTION

There are several types of visibility reduction. These are:

Mist Caused by very small water droplets in a RH of more than 95%. The visibility is between
1000 and 5000 metres.
Fog
Haze Also water droplets. Visibility is less than 1000 metres and RH is very close to 100%.

Caused by solid particles such as sand, dust, or smoke. There is no lower or upper limit
to visibility but haze is not reported above 5000 m visibility.

TYPES OF VISIBILITY

METEOROLOGICAL VISIBILITY

Meteorological visibility is also known as Meteorological Optical Range (MOR) and is the
furthest horizontal distance on the ground that an observer with normal eyesight can recognise a
dark-coloured object. At night, lights of known power are used. Readings are taken at a person’s
eye level.

RUNWAY VISUAL RANGE

Runway Visual Range (RVR) is the maximum distance in the direction of take-off or landing at
which a pilot in the threshold area at 15 ft above ground can see marker boards by day, or
runway lights by night. It is only used when the meteorological visibility is less than 1500 metres
or when fog is reported or forecast.

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Chapter 12 Visibility

OBLIQUE VISIBILITY

When flying at altitude, slant visibility is the maximum distance a pilot can see to a point on the
ground. The oblique visibility is the distance measured along the ground from the point directly
beneath the aircraft to the furthest point the pilot can see.

The distinction is made in the diagram below.

DOWNWARD SLANT
VISIBILITY VISIBILITY

OBLIQUE
VISIBILITY

MEASUREMENT OF VISIBILITY

BY DAY

Measurement by day is made by reference to suitable landmarks at known distances from the
observing position.

BY NIGHT

Measurement by night is done by using a suitable arrangement of lights of known power as a
substitute for landmarks.

If this is not possible, a Gold’s Visibility Meter can be used. A variable filter in the viewing
mechanism adjusts until light is no longer seen and the reading off the meter gives an equivalent
daylight visibility.

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Visibility Chapter 12

MEASUREMENT OF RUNWAY VISUAL RANGE

HUMAN OBSERVER

When an observation of runway visual range is taken by a human observer, the observer is
positioned 76 metres from the centreline of the runway in the touchdown area. The observer
sights the number of marker boards or lights in the appropriate direction. Then, the number of
observed boards or lights is converted into a distance and reported. Human reporting is
inaccurate at the maximum and minimum reporting ranges and visibilities < 100 m and > 1200 m
are unlikely to be reported.

INSTRUMENT REPORTING

Instrument reporting is done with an instrument called a transmissometer, which consists of a
projector and a receiver.

The receiver contains photoelectric cells which measure the opacity of the air and give an
equivalent daytime visibility.

RVR REPORTING

Three transmissometers are positioned alongside the runway giving three readings, one for
touchdown, one from the mid-point, and one for the stop-end of the runway.

RVR is reported in increments of 25 m up to 200 m, 50 m up to 800 m, and 100 m over 800 m.

Sometimes not all three readings are transmitted. The touchdown reading is always reported but
the mid-point and stop-end values may be omitted if certain conditions are met. If one reading is
omitted, the second figure in the group must be specified as the mid-point or stop-end value.

The conditions for the omission of midpoint and stop-end RVR values are that:

a. They have equal to or greater values than the touchdown value, and.
b. They are above 400 metres.

E.g. 300/500/600 would be reported as R 300.
300/350/500 would be reported as R 300 mid-point 350.

OR
c. They are 800 metres or greater.

E.g. 900/850/950 would be reported as R 900.
Meteorology 900/850/750 would be reported as R 900 stop-end 750.

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Chapter 12 Visibility

VISIBILITY WHILE FLYING

EFFECT OF SUN AND MOON

Visibility is reduced looking into the sun due to the harsh glare of the strong rays. Conversely,
looking into the moon may improve visibility at night as it casts a gentle light on water surfaces
and other ground based features.

WITH A DEEP HAZE LAYER

When flying within the layer at different heights the slant visibility stays the same. When flying
higher, the vertical component of the slant visibility increases, so the horizontal component, that is
oblique visibility, decreases.

Conversely, while flying above the layer flying higher increases oblique visibility.
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WITH A SHALLOW FOG LAYER

If the fog is shallow the pilot may be able to see the airfield quite clearly from directly above it.
Once the pilot descends and turns onto final, visibility may be much poorer looking through the
horizontal extent of the fog instead of the depth. It is important, therefore, to heed the visibility
readings given by the tower even if your own observations are different.

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Chapter 12 Visibility

TYPES OF FOG

RADIATION FOG

At night, the ground loses its heat by radiation. The ground becomes cold and cools the air in
contact with it. If this lowers the air temperature below the dewpoint, water vapour condenses out
as droplets, resulting in fog if there is a light wind, or dew/frost if there are calm conditions.

Conditions necessary for radiation fog to form are:

¾ Clear sky which increases the rate of terrestrial radiation (fog can still form in light, high
cloud cover such as scattered cirrus).

¾ High relative humidity so that only a little cooling will be required for the air to reach
saturation.

¾ A light wind of 2 to 8 kt which mixes the air bringing warmer air from above to the surface
to be cooled and thickening the fog.

Radiation fog is most common in autumn and winter when there is a long night giving the land
time to cool. It occurs at night and early morning after a prolonged period of cooling. It doesn’t
occur over the sea as the sea has insufficient diurnal variation. It forms first in the valleys due to
katabatic effect and is common in anticyclones, ridges, and cols where the air remains in contact
with the ground for a prolonged period.

Dispersal of the fog can occur by:

¾ The increase of insolation during the course of the morning, raising the temperature
above the dewpoint and evaporating the fog away from the base.

¾ The increase of thermal turbulence during the morning which lifts the fog to form low
stratus.

¾ An increase of cloud cover preventing the loss of radiation from the lower atmosphere
and raising the temperature of the air above the dewpoint.

¾ Replacement of the air mass with a drier air mass by advection.

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Visibility Chapter 12

ADVECTION FOG

Advection fog forms when warm moist air flows over a cold surface. It can occur over land or sea.
Conditions necessary for it to form are:

¾ A wind of up to 15 kt (20 kt over the sea).
¾ A high relative humidity so little cooling is required to bring the air to saturation.
¾ The cold surface over which the air moves must have a temperature lower than the

dewpoint of the warm moist moving air.

Advection fog is common over land areas in winter and early spring when the land is colder than
the sea and over sea areas in late spring and early summer when the land becomes warmer than
the sea.

This type of fog is much more persistent than radiation fog and can last several weeks. Examples
are the coast of Newfoundland and the Kamchatka peninsula where the temperature difference
between land and sea is extreme.

Dispersal comes when there is a change of airmass or an increase in windspeed beyond that
described in the conditions above.

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Chapter 12 Visibility

Some types of advection fog experienced in and around the UK are listed below:

Thaw Fog These fogs occur over land surfaces in winter and spring when severe frost or
snowfall gives way to milder Atlantic air from the southwest.

Haar Frequent in the spring and early summer off the Northeast coast of the UK. The
sea is at its coldest having been cooled gradually through the winter months.
Warm air from the continent passes over the colder sea.

Sea Fog Common in the approaches to the English Channel during the spring and early
summer when the sea is still cool. If the wind speed is over 25 kt then the fog will
lift into ST.

STEAMING FOG (ARCTIC SEA SMOKE)

Steaming fog occurs at very high latitudes over sea areas such as around Iceland, Greenland,
and Norway. It is similar to advection fog in that the airmass is moving but in this case it is a cold
moist air mass passing over a warmer sea.

Normally this would lead to convection and the formation of cumuliform cloud. However, in this
case the air is too cold and stable for sufficient lifting to occur. Instead, the small amount of lifting
and evaporation from the sea leads to saturation and fog formation.
At such high latitudes the water content is likely to be ice crystals giving the fog a white
appearance which is the reason for its nickname of Arctic Sea Smoke.

FRONTAL FOG

Frontal fog is associated with warm fronts and warm occlusions. Precipitation from NS cloud
above the front falls into the colder air beneath the front, saturating the colder air. Additionally, the
precipitation wets the ground and the moisture then evaporates into the air just ahead of the front
aiding saturation.

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Visibility Chapter 12

This produces a band of fog up to 200 nm wide that travels just ahead of the front as shown in the
diagram.

HILL FOG

Hill fog is really stratiform cloud that forms when there is orographic lifting in stable conditions.
The cloud stays next to the surface obscuring the tops of the hill or mountain.

A nice example is the tablecloth effect on Table Mountain in Cape Town, South Africa.

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OTHER VISIBILITY REDUCERS

SMOKE FOG (SMOG)

Smoke fog is a combination of ordinary water droplet fog and solid particles. It occurs in industrial
cities when there is an inversion layer preventing air from lifting and removing the pollutants.

In addition to being visibility reducers themselves, the solid particles are hygroscopic nuclei and
enhance the severity of the fog.

DUST AND SAND

Dust is a solid particle less than 0.08 mm in diameter. Sand is between 0.08 mm and 0.3 mm in
diameter. Winds can carry these particles aloft causing dust or sand storms.

In dust storms, the wind is upwards of 15 kt and the dust can rise to up to 15 000 ft agl. In sand
storms, the winds are upwards of 20 kt but these remain within a few feet of the surface due to
the weight of the particles.

Both types tend to be daytime phenomena as wind strengths are usually insufficient at night.

Visibility in dust or sand storms is generally less than 1000 m.

PRECIPITATION

Precipitation also causes reduction in visibility.

Drizzle reduces visibility more than rain, as drizzle consists of large numbers of small water
droplets. Drizzle can lower the visibility to 500 m.

The worst type of precipitation is snow. Heavy snow can lower the visibility to 50 m and possibly
even less if it is blowing or drifting.

For more information, see the chapter on Precipitation.

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Visibility Chapter 12

VISUAL ILLUSIONS

SHALLOW FOG

If the pilot enters a shallow fog layer on descent it can give the illusion that the aircraft has
pitched up. If the pilot believes this illusion and pitches the nose down, a very dangerous situation
can arise, especially if this happens on the approach to land.

RAIN SHOWERS

A rain storm moving toward the aircraft can give the illusion of the horizon moving lower, causing
the pilot to reduce power or lower the nose unnecessarily.

LAYER CLOUD

In the absence of a well-defined horizon, the pilot may orientate himself with respect to layer
clouds. If the layer clouds are not parallel to the ground, the orientation to a false horizon will
cause banking.

RAIN EFFECTS

Rain can have two opposing effects:

1. Rain falling between the aircraft and visual landmarks such as the runway lights will
diffuse the light and make the objects or runway lights appear further away than they
really are. The pilot might perceive this as being low on approach.

2. Rain on the windscreen can make runway lights bloom, making the runway appear closer
than it really is. The pilot might perceive this as being high on approach and may make
adjustments to the aircraft’s power and/or attitude which will result in undershooting the
runway.

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12-12 Meteorology

INTRODUCTION

Ice accretion can have serious implications for performance and handling of aircraft. Modern
aircraft are equipped with efficient anti-icing and de-icing equipment. However, these systems
may become inoperative or icing conditions may be so severe that these systems become unable
to cope.

Even if these systems operate perfectly there is quite a significant fuel cost in running the
systems. The preferred approach would be to avoid the conditions in which severe icing may
occur. It is necessary for the pilot to understand the conditions and the risks associated with icing.

CONDITIONS

Ice forms on an airframe if the following three conditions are present:

1. Water is present in a liquid state.
2. The ambient air temperature is below 0°C.
3. The airframe temperature is below 0°C.

EFFECTS OF ICING

The detrimental effects of icing can include the following:

AERODYNAMIC

Ice forms mostly on the leading edges of the airframe and aerofoils. This spoils the aerodynamic
shape of the airframe and leads to:

¾ Reduced lift (up to 30%)
¾ Increased drag (up to 40%)
¾ Increased weight

The increased weight coupled with loss of lift leads to an increased stalling speed. The added
weight and increased drag results in greater fuel consumption.

In addition, ice accumulation may lead to control surfaces becoming jammed, especially where
ice has broken off in chunks from other surfaces and become lodged.

WEIGHT

The rate of accumulation of ice is rarely constant across the airframe. This inconsistency may
lead to a shifting centre of gravity which causes instability and difficulty controlling the aircraft.

Uneven ice build-up on propellers can lead to severe engine vibration and possible engine
damage.

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Chapter 13 Icing

INSTRUMENTS

Ice may block the pitot and static inlets leading to gross instrument errors in the altimeter,
airspeed indicator, vertical speed indicator and Machmeter.

The safety implications of this are far-reaching.

OTHER EFFECTS

Other miscellaneous effects include:

¾ Skin damage from chunks of ice breaking off propellers
¾ Obscuration of windscreens
¾ Increased skin friction and associated performance effects
¾ Radio interference due to ice build-up on aerials
¾ Landing gear deployment/retraction problems if ice forms in gear wells or freezes gear

doors closed

ICING DEFINITIONS

Any pilot encountering unforecast icing should report the time, location, level, intensity, icing type,
and aircraft type to the ATS unit they are in contact with. The following definitions are the
reporting definitions for levels of icing:

TRACE
Ice becomes perceptible; rate of accumulation slightly greater than the rate of sublimation. It is
not hazardous. De-icing/anti-icing equipment is not used unless ice is encountered for more than
one hour.

LIGHT
The rate of accumulation might create a problem if flight in this environment exceeds one hour.
Occasional use of de-icing/anti-icing equipment removes/prevents accumulation. It does not
present a problem if anti-icing equipment is used.

Note: The ICAO definition of light icing is: “Change of heading or altitude not considered
necessary.”

MODERATE
The rate of accumulation is such that even short encounters become potentially hazardous and
the use of de-icing/anti-icing equipment, or diversion, is necessary.

Note: The ICAO definition of moderate icing is: “Change of heading or altitude considered
desirable.”

SEVERE
The rate of accumulation is such that de-icing/anti-icing equipment fails to reduce or control the
hazard. Immediate diversion is necessary.

Note: The ICAO definition of severe icing is: “Immediate change of heading and/or altitude
necessary.”

13-2 Meteorology

Icing Chapter 13

SUPERCOOLED WATER DROPLETS

In order for a droplet of water to freeze, it not only must be below freezing point, but there must
be a freezing nucleus present. This could take the form of salt, dust, pollen, or smoke particles.
There are less freezing nuclei than condensation nuclei. Hence it is a frequent occurrence that a
droplet cools to a temperature below zero but there is no freezing nucleus available. When this
occurs, the droplet stays in liquid form even though it is below zero. It is then referred to as a
supercooled water droplet. These droplets can exist in temperatures as low as -40°C.

Most icing is caused by aircraft colliding with these droplets while in cloud or fog. As the droplet
touches the airframe its surface tension breaks down and it starts to freeze.

SIZE OF SUPERCOOLED WATER DROPLETS

There are two factors dictating the size of the supercooled water droplets in a cloud.

First, consider the type of cloud. Layer clouds only have small water droplets, so when these
become supercooled they remain small. Cumuliform clouds can have small and large water
droplets, so the size of the droplets when supercooled varies.

The second factor is temperature. Once the temperature drops below -20°C, large supercooled
droplets freeze, regardless of the lack of a freezing nucleus. So even in cumuliform cloud, if the
temperature drops below -20°C, only small supercooled droplets will be present.

LARGE SUPERCOOLED WATER DROPLETS
In summary, large supercooled water droplets occur:

1. In CU and CB from 0°C to -20°C.
2. In NS at temperatures from 0°C to -10°C.
3. If the NS has been enhanced by orographic uplift, between 0°C and -20°C.

SMALL SUPERCOOLED WATER DROPLETS
In summary, small supercooled water droplets occur:

1. In CU and CB from -20°C to -40°C.
2. In NS at temperatures from -10°C to -40°C.
3. If the NS has been enhanced by orographic uplift, between -20°C and -40°C.
4. In ST, SC, AS, AC from 0 to -40°C.

Note: Supercooled water droplets do not occur in the cirriform clouds. These consist of ice
crystals.

Meteorology 13-3

Chapter 13 Icing

FREEZING PROCESS

When a supercooled water droplet impacts an airframe, not all of it freezes instantly. The fraction
that freezes instantly depends on the temperature of the droplet.

For every degree below zero, 1/80 of the droplet will freeze on impact. So if the temperature is
-20°C, 1/4 will freeze on impact; if the droplet is -40°C, 1/2 will freeze on impact.

So with a warmer droplet, the freezing process is slower. As a fraction of the droplet freezes,
latent heat is released which delays the freezing of the remainder of the droplet. This allows the
liquid part to flow over the airframe (called flowback) and freeze more gradually.

Also, the size of the droplet is important. Large droplets tend to retain latent heat better, so
freezing is delayed even more, allowing a greater spread of the droplet.

The importance of these differences is discussed below.

TYPES OF ICING

CLEAR ICE (GLAZE ICE)

Clear ice, or glaze ice, forms when large supercooled droplets impact with an airframe. When the
droplet impacts the airframe it does not freeze instantly. It starts to freeze and as a result some
latent heat is released. This raises the temperature slightly, allowing the water to flow over the
airframe before subsequently freezing. This results in a clear coating of ice which adheres
strongly to the surface of the aircraft.

Clear ice is a very serious form of icing which is heavy and difficult to remove. Uneven formation
on propellers can lead to vibration and chunks breaking off and causing skin damage.

The weight addition, which can be uneven, leads to stability and control problems and the aerofoil
shape is spoiled. Because of this, clear ice is usually described as moderate to severe.

Since large droplets only occur in CU, CB, and NS, this type of ice is only found in those clouds,
and only in the temperature range 0°C to -20°C.

RIME ICE

This forms from impact with small supercooled droplets. When the droplet impacts, most of the
droplet freezes instantly with little or no flowback.

Air becomes trapped between the droplets causing the ice to be opaque or cloudy. It is a granular
coating which is generally easy to remove. It can cause some loss of the aerofoil shape and an
increase in surface friction. It can also cause blockage of air intakes.

Usually rime icing is classed as light to moderate as build up is generally light enough for anti-
icing measures to cope.

This type of icing can occur in any cloud where there are small supercooled droplets. Hence it will
occur in layer clouds at any temperature below zero (except cirriform clouds which consist of ice
crystals). It will also occur in cumuliform clouds where temperatures are below -20°C.

It may also occur in freezing fog. Meteorology
13-4