Jeppesen Meteorology

Icing Chapter 13

MIXED ICE

This is a combination of clear ice and rime ice and occurs where both types of water droplets are
present. This applies to clouds where the temperature is close to the transition between small and
large supercooled droplets. This will be within a few degrees of:

1. -20°C for CU and CB.
2. -10°C for NS.
3. -20°C for NS enhanced by orographic uplift.

RAIN ICE

This type of icing is very severe and very similar to clear ice. It is common beneath a warm front
or an occlusion, when precipitation falls from NS cloud above the front. The warm rain falls into
colder air and becomes supercooled. If the aircraft is above the freezing level, the airframe is
below zero and the droplets strike the airframe and form ice in the same way as described above
in the section on clear ice.

The colder the air is below the front, the more common this type of icing becomes. Hence, it is a
common occurrence over large land masses such as North America and Central Europe, but is
much rarer over the UK where the temperatures are milder.

HOAR FROST

This type of icing occurs when air is cooled to the temperature at which saturation occurs and the
airframe is below 0°C. The frost forms by sublimation, that is, water vapour turns directly to ice
without passing through the liquid state.

Note that the temperature to which the air must be cooled for saturation to occur is called the
frost point in this situation, rather than the dewpoint.

It is a white crystalline deposit of the kind you find on your car on a cold morning.

It can occur on the ground when the aircraft is parked, or during flight.

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Chapter 13 Icing

The correct conditions for hoar frost formation occur when an aircraft takes off from an aerodrome
at a sub-zero temperature and climbs through an inversion into warm moist air. Likewise, if an
aircraft descends from a very cold region into a warm moist layer, the same conditions will be
present.

This causes similar problems to those caused by rime ice.

FACTORS AFFECTING THE SEVERITY OF ICING

There are several factors which affect icing severity. These are detailed below.

SIZE OF SUPERCOOLED WATER DROPLETS

As discussed above, larger supercooled droplets cause more severe icing of the clear type, and
small supercooled droplets cause rime ice, which is less serious. The size of the droplets
depends on the type of cloud and the ambient temperature. This was discussed above and is
summarised below:

Type Severity Conditions
Clear / Glaze Ice Moderate to severe
Caused by large supercooled droplets,
Rime Ice Light to moderate hence only found in cumuliform clouds such
Nil as CU and CB, and also in NS and ACC
Light which have heap-type characteristics.
N/A
Caused by small supercooled droplets. In
layer clouds from 0°C to -10°C. In
cumuliform clouds from -20°C to -40°C.

Caused by small supercooled droplets. In
layer clouds from -10°C to -40°C.

In CI, CS and, CC (only ice crystals are
present).

CONCENTRATION OF SUPERCOOLED WATER DROPLETS

The higher the concentration of supercooled droplets, the more serious the icing risk. Upcurrents
are stronger in the convective clouds, hence able to support a higher concentration of droplets.
This increases the risk in these clouds.

There is a higher concentration of droplets at the base of the cloud. This is for two reasons. First,
gravity tends to increase the density lower down. Second, the base is where condensation
commences, where the temperature is higher so the water content of the moist air is greater.

OROGRAPHIC UPLIFT

Where clouds have formed orographically, or existing clouds have been enhanced by lifting
against a hill or mountain, uplift is stronger, so the cloud can support a higher concentration of
water droplets, and also a greater size of droplet. For both these reasons, icing tends to be more
severe.

CLOUD BASE TEMPERATURE

The higher the temperature, the greater the amount of water vapour the air can hold. If the cloud
starts to form at a high temperature, the moisture content will be greater making the concentration
of droplets greater. Upcurrents result in the concentration of water droplets at all levels of the
cloud being greater so the icing will be more severe.

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Icing Chapter 13

AEROFOIL SHAPE

Air flowing around thin, low-drag aerofoils tends to follow the shape quite closely, whereas air
flowing around thick, high-drag aerofoils tends to be deflected away from the surface more.
Hence, supercooled water droplets are more likely to adhere to the thin aerofoil shape.

Aircraft with low-drag aerofoils tend to fly at a higher speed, and so they impact with more
droplets in a given amount of time.

This may be offset by kinetic heating effect, more details of which are given below. If the skin
temperature is raised to above zero, no icing will occur.

KINETIC HEATING

As an aircraft travels through the air it experiences kinetic heating of its surface which is related to
its true airspeed. The formula is as follows:

( )Temperature Rise (°C) = TAS 2
100

So if the true airspeed is 300 kt, the temperature rise will be 9°C.

If this raises the temperature to above zero, no ice will form. However, it also has the potential to
worsen the effect of icing. If the temperature were a low sub-zero temperature and was heated to
a temperature which was still below zero, this may lead to increased flowback and a greater
likelihood of clear ice.

Hence it is important not to assume that kinetic heating will always improve the situation.

ENGINE ICING

Icing can occur in both piston and turbine engines. The types of icing and conditions for formation
differ between the engine types. Icing can occur to a much higher temperature in piston engines
than in turbine engines. The processes involved are described below.

PISTON ENGINE ICING

Several different types of icing can occur.

IMPACT ICING

Impact icing occurs in the intake area of the engine. It forms by direct impact of supercooled
water droplets with the surface, in much the same way as airframe icing. Temperatures need to
be sub-zero for this to occur.

FUEL ICING

Fuel icing is caused by water in the fuel freezing in the pipes and reducing or preventing fuel flow
to the engine. Again, the temperature needs to be below zero.

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Chapter 13 Icing
CARBURETTOR ICING

ICE

FUEL

INTAKE AIR

This is the only form of icing where the ambient temperature can be above zero. It is caused by
two things:

1. Latent heat being absorbed from the surroundings as fuel evaporates.
2. As air passes through the venturi its speed increases, but its pressure, and therefore its

temperature, go down.

The temperature reduction can be in excess of 30°C. So even at quite high temperatures the air
may be cooled to a temperature below zero. If the air has sufficient moisture, content icing
occurs.

The effects can be more severe if a low throttle setting is used with the carburettor butterfly only
partially open. A total blockage may occur.

Carburettor icing is common on warm, humid days as the moisture content of the induction air is
high.

Indications that the conditions for carburettor ice formation may be present include wet ground or
dew, reduced visibility from mist or fog, proximity to clouds, or precipitation.

JET ENGINE ICING

As for piston engines, the problem of fuel icing in the supply pipes exists.

Impact icing may accumulate in the intakes of a jet engine. If this breaks off, it can cause blade
damage.

In the early intake stages, there is a pressure reduction which can lead to adiabatic cooling on the
order of 5°C. This is a particular problem if the aircraft is at high revs, such as on approach or
climb-out.

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Icing Chapter 13

In potential icing conditions, use engine igniters to help prevent failures. If there is precipitation or
the outside air temperature is less than 10°C, engine anti-icing systems should be switched on.

ICE PROTECTION

ANTI-ICING

Anti-icing measures are designed to prevent the formation of ice. They include:

¾ Kill-frost paste applied to the leading edges.
¾ Heated windscreen and pressure head.
¾ Hot air system on leading edges and tailplane.
¾ Hot air system on engine cowling lips and spinner.
¾ Anti-icing fluids.

DE-ICING

De-icing measures are designed only to remove icing after it has formed, not to prevent its
formation. Examples are:

¾ De-icing fluids.
¾ Pulsating rubber boots.
¾ Hot air systems.
¾ Electrical heating systems.

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Chapter 13 Icing

13-10 Meteorology

INTRODUCTION

Wind is the horizontal movement of air over the surface of the Earth due to forces acting upon it.
It is expressed as a wind velocity, which is a combination of direction and speed. The direction
given is always that from which the wind is blowing.

Calm 20 kt, further additions up to 45 kt

1 to 2 kt 50 kt

5 kt 60 kt

10 kt 65 kt, further additions as
necessary

15 kt

The wind is depicted as a straight line coming from the periphery of a circle. The examples above
show a wind direction of 090°.

The wind speed is normally given in knots. Other units used are kilometres per hour and metres
per second.

Direction is usually given in °T. Exceptions to this are in an ATIS or verbally from the control
tower, where wind direction is given in °M. This is because runway direction is magnetic, enabling
the pilot to calculate the wind components if the wind speed is also given in magnetic.

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Chapter 14 Wind

TERMS ASSOCIATED WITH WIND

Veer is a change of direction in a clockwise direction.
Back is a change of direction in an anti-clockwise direction.

Gust is a sudden increase in wind speed lasting a few seconds.

Squall is a wind speed increase of at least 16 kt to a uniform speed of at least 22 kt lasting for at
least one minute. Squalls are often associated with CBs.

Lull is a decrease in wind speed lasting from a few seconds to a few minutes.

Gale is a mean surface wind of 34 kt or more, or gusting to 43 kt or more.

Hurricane is a wind with a mean surface value of 63 kt or more.

Wind Gradient is the gradual change in wind velocity between the surface and the top of the
friction layer.

Gust factor is calculated by the following formula:

GUST FACTOR % = (MAXIMUM GUST SPEED – MINIMUM LULL SPEED) * 100%

MEAN WIND SPEED

For example: A wind averaging 35 kt with gusts to 50 kt and lulls of 20 kt would
have a gust factor of:

(50 – 20) × 100= 85.7%
35

FORCES ACTING UPON THE AIR

There are two main forces acting upon the air. These are:

1. The Pressure Gradient Force.
2. Geostrophic Force.

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Wind Chapter 14

There is a third force, friction, that acts close to the surface. The thickness of the friction layer
varies.

THE PRESSURE GRADIENT FORCE

The Pressure Gradient Force (PGF) is the force that initiates movement of air. If there is a region
of high pressure adjacent to a region of low pressure, the air flows from the high pressure to the
low pressure. If there were no other forces acting, this would continue until the two pressures
were equal, resulting in no more pressure gradient.

PGF

HL

1004 1002 1000 998

The diagram shows the pressure in mb or hPa. As seen in the diagram, the PGF acts at right
angles to the isobars.

Calculate it using the following equation:

PGF = dp
ρdn

where:

dp = the pressure difference between two points
dn = the horizontal distance between the two points
ρ= air density

THE GEOSTROPHIC FORCE

This is also referred to as the Coriolis Force.

Geostrophic force is due to the rotation of the Earth and the law of inertia. The Earth rotates at a
fixed speed. At the Equator, the line of latitude with the largest circumference, objects on the
Earth move faster than those at higher latitudes, because they have to travel a longer distance in
the same amount of time.

In the diagram below, the thick horizontal arrows show how a position on the Earth moves in a
given time at the equator and at two temperate latitudes, one in the Northern Hemisphere, one in
the Southern Hemisphere.

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Chapter 14 Wind

Four different situations are shown. A and B show movement away from the Equator in the
Northern and Southern Hemispheres respectively.
C and D show movement towards the Equator in the Northern and Southern Hemispheres
respectively.

Take, for example, situation A.

A parcel of air leaves the point represented by the start of the thick horizontal arrow at the
Equator and travels due north. As it travels, the point on the ground from which it left and the
point on the ground for which it is aiming move due to the Earth’s rotation.

You would expect the parcel of air to end up at the point of the arrow at the higher latitude, that is,
the initial aiming point after following a path represented by the dashed line.

However, due to inertia the parcel of air moves at the speed of objects at the Equator, so travels
further east than expected, following a path represented by the thick diagonal arrow.

Hence, the parcel of air appears to have turned right in the Northern Hemisphere.

Now look at the Southern Hemisphere, situation B. You can see that the parcel of air appears to
turn left.

Now look at situations C and D. You will find that the same rule applies for movement toward the
Equator.

AC
BD

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Wind Chapter 14

In summary, due to Coriolis effect, objects appear to turn right in the Northern Hemisphere, and
left in the Southern Hemisphere.

Geostrophic Force (GF) can be calculated using the following equation:

GF = 2 Ω ρ V SIN θ

Where:

Ω = THE ANGULAR ROTATION OF THE EARTH
ρ = AIR DENSITY
V = WINDSPEED
θ = LATITUDE

Note that the Pressure Gradient Force must initiate movement of a parcel of air before
Geostrophic Force can come into play. Geostrophic Force has no effect on a stationary parcel of
air.

As the Geostrophic Force is proportional to SIN θ, it is zero at the Equator and a maximum at the
poles. Within 15° of the Equator Geostrophic Force is negligible.

THE GEOSTROPHIC WIND

As already discussed, movement of air is initiated by the Pressure Gradient Force. The air is then
affected by the Geostrophic Force. The Geostrophic Force initially acts at right angles to the
pressure gradient force, so produces a resultant wind that is at an angle between the two.

However Geostrophic Force is now no longer at right angles to the wind, and so acts from the
resultant wind (see diagram below). This process continues until the PGF and the GF are acting
in opposite directions and are in balance, as shown in the diagram.

PGF

GW

1004

GF

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Chapter 14 Wind

The resultant wind is now at right angles to the PGF. In the Northern Hemisphere it will be 90° to
the right of the PGF, in the Southern Hemisphere 90° to the left.

This resultant wind is called the Geostrophic Wind and flows parallel to the straight isobars as
shown in the diagram. It gives rise to Buys Ballot’s law, which states:

“In the Northern Hemisphere with your back to the wind, the low pressure is on your left.”

Note that the opposite is true for the Southern Hemisphere.

This wind does not take the third force, friction, into account and is taken to be the wind just
above the friction layer.

The equation for geostrophic force applies:

GF = 2 Ω ρ V SIN θ

Since PGF and GF are now in equilibrium, the following is also true:

PGF = 2 Ω ρ V SIN θ

The formula can be re-arranged to make V the object as follows:

V = PGF
2 Ω ρ SIN θ

Hence the windspeed (V) is proportional to the PGF and inversely proportional to the latitude.
Therefore as latitude decreases, the windspeed increases. This continues until about 15° of the
equator, where the equation breaks down due to the negligible geostrophic force.

If the windspeed at a certain latitude is known, the windspeed at another latitude, assuming the
same isobar spacing, can be calculated using the relationship described above. The derived
formula is as follows:

VLAT A SIN LAT A = V LAT B SIN LAT B

For example: If the geostrophic wind speed is 40 kt at 30°N, calculate the
geostrophic wind speed at 60°N

40 SIN 30 = V SIN 60
V = 23 KT

In summary, the conditions for the geostrophic wind are:

¾ Above the friction layer. Meteorology
¾ Greater than 15°N/S.
¾ A pressure system that is not changing rapidly.
¾ Straight and parallel isobars.

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Wind Chapter 14

THE GEOSTROPHIC WIND SCALE

Consider the formula for windspeed again: PGF
V=

2 Ω ρ SIN θ

Weather charts are usually for a limited latitude range and altitude. The angular rotation of the
Earth is constant so the denominators of the equation can be replaced by a constant:

V = PGF

K

This simple relationship means that windspeed can be determined from the pressure gradient
force, which in turn comes from the isobar spacing. A scale called the Geostrophic Wind Scale is
printed on the chart. An example is shown:

100 50 30 20 15 10 5 kt

As you can see, the relationship is not linear but logarithmic.

To find the windspeed for a given point, measure the distance between successive isobars
passing through that point, and compare this to the scale. Align your measured distance with the
left end of the scale but read the speed off from the right. In the example above the isobars are
well spaced, giving a speed of about 18 kt. The closer the isobars, the stronger the wind.

THE GRADIENT WIND

The Geostrophic Wind only applies to straight parallel isobars. When dealing with curved isobars
the situation becomes slightly more complicated.

Consider circular pressure systems. In the Northern Hemisphere the pressure gradient force and
geostrophic force act opposite to each other, and the resultant wind is 90° to the right of the PGF.

However, the wind follows the curved isobars so the air starts to rotate around the centre of the
system.

This rotation brings an additional force into play, called centrifugal force. This is a force acting
outwards from the centre of the system.

In the next two diagrams, the following key applies:

CF — Centrifugal Force
PGF — Pressure Gradient Force
GF — Geostrophic Force
GW — Gradient Wind

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Chapter 14 Wind

GW

CF GF

L

PGF

In the case of a low pressure system, centrifugal force opposes the pressure gradient force,
hence the resultant wind speed is lower than the geostrophic wind for the same isobar spacing.
This is termed sub-geostrophic. If a geostrophic wind scale is used it will over-read.

The resultant wind is called the Gradient Wind, and blows anti-clockwise around a low pressure
system in the Northern Hemisphere.

H PGF
CF
GF

GW

In the case of a high pressure system, centrifugal force supports the pressure gradient force,
hence the resultant wind speed is higher than the geostrophic wind for the same isobar spacing.
This is termed super-geostrophic. If a geostrophic wind scale is used it will under-read.
The resultant Gradient Wind blows clockwise around a high pressure system in the Northern
Hemisphere.

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Wind Chapter 14

WINDS NEAR THE EQUATOR

At latitudes less than 15° the formula for geostrophic wind breaks down due to the low value of
the geostrophic force. With straight isobars the wind tends to flow across the isobars from high to
low pressure.

However, with curved isobars the situation is different. In some situations the centrifugal force
becomes so large that it balances the pressure gradient force. When this happens, the wind is
said to be cyclostrophic. Examples are in a tropical revolving storm or a tornado.

THE SURFACE WIND

Both the Geostrophic and the Gradient wind act above the friction layer. The third force, friction,
must be taken into account in this layer.

The strength of the frictional force depends on the following factors:

¾ The roughness of the landscape – the rougher the landscape, the greater the friction;
¾ Stability of the air – an unstable air mass creates thermal turbulence. This causes the

slow surface wind to interact with faster higher winds, resulting in increased wind speed
at the surface;
¾ Season – in summer the turbulence layer is thicker over land due to surface heating. The
same effect will be seen as above;
¾ Type of system – the layer is thicker in low pressure than in high;
¾ Windspeed – the higher the windspeed, the greater the resulting frictional effect.

Friction between the moving air and the surface slows the air down. Therefore V, the windspeed,
decreases. Any decrease in V leads to a decrease in geostrophic force, according to the
geostrophic wind equation discussed above.

PGF PGF

SW
GW FF

GF GF

2000 ft wind Surface wind

In the above diagram GW is Geostrophic Wind, SW is Surface Wind, and FF is Friction Force.

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Chapter 14 Wind

If the geostrophic force reduces then PGF and GF will no longer be in balance. PGF dominates
so the surface wind deflects toward the PGF, that is, deflected toward the low pressure. As seen
in the diagram, this will be a back in the Northern Hemisphere. In the Southern Hemisphere it will
be a veer. In both cases the surface wind will be slower than the wind above the friction layer.

Note: The above process applies equally to the wind around curved isobars.

The number of degrees of deflection and the reduction in windspeed for different situations are
shown in the table.

Over the Sea Deflection of Surface Speed of Surface Wind
Over the Land by Day Wind from 2000 ft wind as a % of the 2000 ft
Over the Land by Night wind
15°
30° 75%
45° 50%

25%

DIURNAL VARIATION OF THE SURFACE WIND

The following paragraphs describe the diurnal variation of winds at different heights. Note that
these are for the Northern Hemisphere. In the Southern Hemisphere the speed changes are the
same but changes of direction are opposite.

SURFACE WIND
During the day, surface heating causes turbulent mixing and an increase in wind speed at the
surface. During night the air cools down, turbulence ceases, and the friction has full effect.

Night to day Veer and increase
Day to night Back and decrease

Over land from night to day the surface wind approximately doubles and veers by about 15°.
Windspeeds are highest at around 1500 hours as this is when there is greatest surface heating.
Windspeeds are lowest at around 0600 hours when temperatures are lowest.

1500 FT WIND
By day 1500 ft lies within the friction layer, hence is affected by friction. By night it lies above the
layer so is not affected.

Night to day Back and decrease
Day to night Veer and increase

2000 FT WIND

2000 ft is generally above the friction layer by day and by night, hence experiences little diurnal
variation.

Night to day Little variation
Day to night Little variation

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Wind Chapter 14

MEASUREMENT OF SURFACE WIND

At an airport, wind is measured by placing the sensors 10 metres above an even-ground surface.
This prevents false readings caused by surges due to ground obstacles or uneven ground. The
wind vane gives direction as shown in the simple version below.

Wind Vane

270°

360°

180°

90°

The most common wind velocity sensor is the cup anemometer, shown below. Pressure tube
anemometers may also be used. The cup anemometer tends to under-read the value of gusts
and over-read the average wind speed due to its inertia.

3-CUP
ANEMOMETER

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Chapter 14 Wind

ISALLOBARIC EFFECT

If the pressure gradient changes, the three forces of PGF, GF, and centrifugal force are
temporarily out of balance. The wind tends to flow across the isobars from high to low until
balance is restored. An Isallobar is a line joining places that have an equal rate of pressure
change, hence the term Isallobaric Effect.

When air blows toward an area of falling low pressure, this is called convergence. When air
flows outwards from an area of increasing high pressure this is called divergence.

14-12 Meteorology

INTRODUCTION

The previous chapter explored lower winds which come about as a result of pressure differences
on a large scale. In this chapter more localised wind effects will be explored.

These tend to become apparent when the pressure gradient is slack or when the same air mass
remains in contact with the ground for an extended period, such as in a stable high pressure
system.

LAND AND SEA BREEZES

These winds are common when there is an anticyclone with a light pressure gradient on a clear
sunny day.

SEA BREEZE

During the day, the land heats up more quickly than the sea. The air in contact with the land
heats up and rises by the process of convection which leads to a decrease in pressure at the
surface and an increase in pressure at approximately 1000 — 2000 ft agl.

This causes air at that height to move over the sea. Air then descends over the sea causing an
increased pressure at the surface of the sea. Air then flows from the slightly higher pressure over
the sea surface to the lower pressure over the land surface and creates the sea breeze.

The circulation is shown in the diagram below.

Return Flow

Warm Sea Breeze Cool

L H

Meteorology 15-1

Chapter 15 Local Winds

Sea breezes are typically 10 kt in temperate latitudes and extend to about 10 nm either side of
the coastline. In tropical areas they can be 15 kt and extend to 40 or 50 nm inland.

Initially the wind will be at right angles to the coastline but as insolation increases throughout the
day the wind will extend further from the coast and due to this longer fetch Coriolis effect comes
into play. This causes a veer in the Northern Hemisphere and a back in the Southern
Hemisphere.

LAND BREEZE

After sunset the land starts to cool down much more rapidly than the sea. This leads to a reversal
of the above situation. The sea surface experiences a lower pressure and the land a higher
pressure as shown in the diagram. The wind now blows from the land to the sea.

Return Flow

Cool Warm

H L

The temperature difference between land and sea is less at night so the land breeze is weaker
than the sea breeze – typically half the speed (5 kt in temperate latitudes) – and only extends to
about 5 nm out to sea.

OPERATIONAL IMPLICATIONS OF THE LAND AND SEA BREEZES

At coastal airfields, the landing and take-off direction is reversed from day to night if the runway is
at right angles to the coast. During the day landing/take-off will be towards the sea and at night
towards the land.

Coastal airfields with runways running parallel to the coast experience crosswinds when the sea
and land breeze are well-established.

Fog off the coast can be blown inland during the day reducing visibility at coastal airfields.

Lifting of air over land by the sea breeze can cause small cumulus clouds to form which assist
pilots in the identification of coastlines.

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Local Winds Chapter 15

KATABATIC AND ANABATIC WINDS

These winds occur on hillsides and valley sides and tend to form in slack pressure gradients.

KATABATIC WIND

During the night a hillside cools down rapidly. The air in contact with it is cooled by conduction
and becomes more dense than the free air next to it. It therefore flows down the hillside.

The katabatic wind is more apparent if the sky is clear as radiation is greater. If the slope is snow
covered this also assists.

The air remains in contact with the ground at all times and does not warm adiabatically. The
average speed is 10 kt.

If this wind occurs in a valley cold air collects at the bottom increasing the likelihood of fog or
frost.

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Chapter 15 Local Winds

ANABATIC WIND

Anabatic wind is the opposite of the Katabatic wind and occurs during the day on slopes which
are subject to direct sunlight. As insolation increases, the air in contact with the land warms up,
becomes less dense and flows up the slope.

The Anabatic wind is typically weaker than the Katabatic (about 5 kt) since it flows against the
force of gravity.

15-4 Meteorology

Local Winds Chapter 15

FOEHN WIND/EFFECT

This topic was already mentioned in the chapter on Cloud Formation.

The Foehn Wind was named for a warm dry wind that occurs in the Alps. There are several other
winds in other parts of the world which are caused by the same effect, such as the Chinook,
which flows down the east side of the Rocky Mountains.

The Foehn Wind occurs when air is forced to rise up a mountain side in stable conditions. It cools
initially at the DALR until it reaches saturation. At this point, cloud starts to form and the air
continues to rise, but now cools at the SALR.

Once it reaches the top of the mountain it starts to flow down the other side. Initially it warms at
the SALR but quickly becomes unsaturated as much of its moisture has already been lost. It then
warms at the DALR.

Since the cloud base is higher on the lee side, the air at the base on that side will be warmer than
on the windward side. The difference can be as much as 10°C (20°C with the Chinook).

8000 ft - 0°C 0°C
6000 ft - 3°C 3°C
4000 ft - 6°C 9°C
2000 ft - 9°C 15°C
21°C
0 ft - 15°C

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Chapter 15 Local Winds

VALLEY/RAVINE WIND

When wind blows against a mountain barrier it finds its progress impeded. If there is a gap or
valley it is forced to flow through this. The restriction acts like a venturi and the wind speeds up.
Wind speeds of 70 kt can be experienced.

The combination of high wind speeds and rough terrain can result in turbulence at low level. An
additional hazard results from the fact that small changes in the general direction of the wind can
lead to sudden reversals in direction of the ravine wind.

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Local Winds Chapter 15

HEADLAND EFFECT

Where the 2000 ft wind blows parallel to the coast around a headland or cape the isobars push
together causing an increase in pressure gradient and hence an increase in wind speed.

LOW-LEVEL JET

A Low Level Jet (LLJ) is defined as a narrow, horizontal band of relatively strong wind (usually
between 20 and 80 kt) located between 500 to 5000 feet AGL. They are often several hundred
miles long and a few hundred miles wide. There are four common types of LLJ.

NOCTURNAL JET

When the ground cools quickly, an inversion may build, and the wind quickly slows along the
surface by friction. However above the inversion, the wind is not affected by friction, and the cold
calm air along the ground serves as a gliding layer.

The result is a strong wind, just above the inversion. Maximum wind speed is usually attained
about 4 − 8 hours after sunset, the time depending on the latitude. The wind abates when
insolation and convection destroys the inversion layer.

VALLEY INVERSION

Often accentuated in mountainous regions where cold air drains into the bottom of a valley, valley
inversions create an elevated stable layer and surface inversion. Wind speeds of more than 50 kt
are sometimes reported above such inversions.

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Chapter 15 Local Winds

COASTAL JET

Water temperature differentials along many coasts around the world create elevated inversions or
shallow frontal zones where low level jet (LLJ) phenomena occur. These LLJ can persist both day
and night for as long as the temperature differentials last.

LOW LEVEL JET IN FRONT OF AN EXTRA-TROPICAL COLD FRONT

A large temperature contrast across a cold front can create a similar wind phenomenon as the
shallow coastal front and a pronounced LLJ forms ahead of a cold front inside the warm air mass.

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INTRODUCTION

Air masses are large volumes of air with properties of humidity and temperature which remain almost
constant in the horizontal.

This phenomenon of more or less constant properties arises from the fact that the air in air masses
remains stationary over its source for an extended period of time. This essentially means that air
masses originate only in high pressure areas, as low pressures tend to be temporary features.

ORIGIN AND CLASSIFICATION

Air masses are initially classified by the latitude from which they originate. This gives us three main
types:

¾ Tropical
¾ Polar
¾ Arctic

They are further subdivided depending on whether they originate over sea or land:

¾ Maritime
¾ Continental

This gives us five main air masses:

1. continental Tropical (cT)
2. maritime Tropical (mT)
3. continental Polar (cP)
4. maritime Polar (mP)
5. Arctic (not subdivided) (A)

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maritime
Arctic (mA)

COLD

continental
Polar (cP)

maritime
Polar (mP)

maritime continental
Tropical (mT) Tropical (cT)

WARM

Tropical air originates in the sub-tropical high pressure zones. An example of continental tropical air
would be the air mass which originates in North Africa.

Maritime tropical air originates in the permanent high pressures over the oceans. In the North Atlantic
this is the Azores high. There is an equivalent high pressure in the North Pacific.

Continental polar air originates in the high pressures over large land masses, hence this air mass is
mainly a winter phenomenon. Examples of sources are Siberia and North America.

Maritime polar air originates in the north of the North Atlantic and North Pacific.

Arctic air originates over the North Polar ice cap. Since the region is ice covered, arctic air is not
subdivided into continental and maritime. In the Southern Hemisphere there is an Antarctic air mass
originating over the South Polar ice cap.

MODIFICATION OF AIR MASSES

As the air masses pass over other regions as they travel away from their sources, their properties
alter. In general, the following rules apply:

¾ An air mass passing over a warmer area:
• Becomes warmer.
• Becomes more unstable.
• Experiences a reduction in relative humidity.

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¾ An air mass passing over a colder area:
• Becomes colder.
• Becomes more stable.
• Experiences an increase in relative humidity.

AIR MASSES AFFECTING EUROPE

We have introduced the various types of air masses. The next sections go into more detail about the
kind of weather conditions that these air masses bring to Europe.

ARCTIC

Originating over the North Polar ice cap, the arctic air mass is very cold and stable at the source. It
has a low absolute humidity and low relative humidity.

It is more common in the winter and moves south if there is a high pressure to the west of the UK and
a low pressure to the east.

HL

As an arctic air mass moves south toward Scotland, it becomes warmer and more unstable. It also
picks up moisture from the sea to the north of Scotland. Over land, large cumulus will form bringing
very cold weather, snow showers, and possible blizzards.

If it occurs in summer, there will be rain showers and the region will experience a marked drop in
temperature.

POLAR

MARITIME POLAR
A maritime polar air mass is cold and stable at its source, with a low absolute humidity but a high
relative humidity.

The air mass which comes to the UK originates in the far North Atlantic in the Greenland/Iceland
areas. As it moves south over the sea it becomes heated in the lower layers and becomes unstable. It
also picks up moisture.

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Once it reaches the UK it produces unstable weather with cumulus, cumulonimbus with heavy
showers, and sometimes thunderstorms and hail.

Visibility is generally good outside of cloud and showers.

At night in winter the clouds clear and radiation can lead to an inversion and radiation fog.

RETURNING MARITIME POLAR
This is maritime polar air that has reached the UK via an indirect route. It occurs when the air gets
deflected by a low pressure system in the North Atlantic.

This results in the air first travelling to the south of the North Atlantic before changing direction and
approaching the UK from the south-west.

The result is that the air becomes unstable as it travels south. Once it has turned north the lower
layers become stable, but the upper layers remain unstable.

In summer, convection can break through the lower stable layer resulting in Cu, Cb, and
thunderstorm activity, with hail and heavy showers.

CONTINENTAL POLAR
A continental polar air mass is mainly a winter phenomenon which originates in Siberia. It is very cold,
stable, and dry. It brings a cold easterly wind to the UK, with mainly good visibility except for some
occasional industrial smoke from Northern Europe.

If the air mass originates from further north it may pass over the North Sea on its way to the UK. In
this case it will become unstable and increasingly moist, resulting in cumulus clouds and heavy
showers on the east coast of England and Scotland.

The conditions are not as severe as those associated with maritime polar as the air mass has a much
shorter sea passage.

In the summer, the high pressure over Siberia replaces low pressure as the land mass heats up. Air
originating in this area is then generally referred to as continental tropical.

Occasionally there may be a high pressure over Scandinavia. This results in an air mass passing
over the North Sea. This sea will now be colder than the surrounding land areas, so the air mass will
become cooled and more stable. It will absorb moisture as it passes over the sea.

This results in what is referred to as Haar conditions on the coast of east Scotland and north-east
England. These conditions are very low stratus with drizzle, advection fog, and bad visibility. In the
northeast of England, these conditions are colloquially termed Sea Fret.

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TROPICAL

MARITIME TROPICAL

A maritime tropical air mass originates in the Azores high in the south of the North Atlantic. It is warm
and stable with a high absolute humidity and a moderate relative humidity.

As it moves northeast, it cools and becomes more stable with increased relative humidity.

On reaching the south-west coast of the UK it produces low stratus and stratocumulus with drizzle
and poor visibility. Advection fog occurs over the land areas in winter and early spring and sea areas
in late spring and early summer.

In summer the increased insolation and convection clears the low cloud resulting in clear skies and
good visibility, with occasional fair weather cumulus.

CONTINENTAL TROPICAL

A continental tropical air mass originates in North Africa and south-east Europe, plus Siberia in the
summer. It is a warm dry air mass which brings clear dry weather with generally good visibility.
Occasionally, some dust haze comes north from the Sahara region.

Occasionally the air mass picks up some moisture over the Mediterranean and becomes unstable but
this moisture is lost as showers over France.

AIR MASS SUMMARY

ARCTIC
Normal Winter Only

Source Region Conditions at Modifications Weather
Source
North Polar Ice Moves south and is Arrives over Europe as
Cap Temperature heated from below, extremely cold, moist and
becoming unstable unstable
Cold
Evaporation from sea CU or CB give heavy snow
Relative Humidity causes increased showers, possibly TS on
Low dewpoint and RH north and north-east facing
coasts
Absolute Humidity
Low Inland clear and cold

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MARITIME POLAR
Summer

Source Region Conditions at Modifications Weather
Source
Sea areas The air mass is heated as Widespread CU and CB
around Iceland Temperature it moves south-east activity overland with
and Greenland moderate to heavy
Cold It becomes unstable over showers of rain or hail
a great depth
Relative Humidity Moderate to severe icing
High Moisture evaporates from and turbulence in cloud
the ocean so RH
Absolute increases Visibility good outside
Humidity cloud

Low

Winter

Source Region Conditions at Modifications Weather
Source
Sea areas The air mass is heated as Day – as above but more
around Iceland Temperature it moves south-east to a severe. Strong gusts and
and Greenland greater extent than in squalls common
Cold summer
Night – skies clear with
Relative Humidity Becomes unstable over a possible radiation fog
High great depth

Absolute Moisture evaporates from
Humidity the ocean so RH
increases
Low

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RETURNING MARITIME POLAR
Summer

Source Region Conditions at Modifications Weather
Source
Sea areas As the air moves south it Day
around Iceland Temperature becomes unstable over a Warmer than average
and Greenland, great depth temperatures with a
with a low Cold relatively high RH
pressure to the Continuous evaporation
west of Ireland Relative Humidity raises the dew point and Insolation heats up the land
High the RH remains high surfaces

Absolute Humidity The depression west of As air moves over the
Low Ireland drags the air mass heated surfaces the lower
in an anti-clockwise layer becomes unstable
direction towards Europe leading to the development
of CU and CB
The movement into colder
regions stabilises the air CB produce widespread TS
mass in the lower layers and showers which are
and leaves the upper most marked in the
layers unstable afternoon

Near the surface the air Visibility moderate to good
mass has similar
characteristics to the mT Night
Convective activity dies out
as surface temperatures
fall

CU may spread into SC
Visibility moderate

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RETURNING MARITIME POLAR
Winter

Source Region Conditions at Modifications Weather
Source
Sea areas As the air moves south it As for the mT although
around Iceland Temperature becomes unstable over a medium level instability
and Greenland, great depth may be encountered
with a low Cold
pressure to the Continuous evaporation CB formation over
west of Ireland Relative Humidity raises the dew point and mountains
High the RH remains high
Medium level ACC may be
Absolute The depression west of apparent
Humidity Ireland drags the air mass
in an anti-clockwise
Low direction toward Europe

The movement into colder
regions stabilises the air
mass in the lower layers
and leaves the upper
layers unstable

Near the surface the air
mass has similar
characteristics to the mT

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CONTINENTAL POLAR
Normal Winter Only

Source Region Conditions at Modifications Weather
Source
Siberia, Moves over the cold If the airflow is from the
Northern Europe Temperature winter land of Europe and east via continental
and Scandinavia remains cold, dry, and Europe, the weather is very
Cold stable cold and very dry with no
precipitation
Relative Humidity If the air passes over the
Low relatively warm North Sea If the airflow is over the
the air is heated from North Sea, Then CU and
Absolute below and the absolute CB can give showers on
Humidity humidity increases the east coast of UK

Low

(In summer the air mass is
rare. With a high pressure
over Scandinavia in early
summer a North Easterly
flow occurs. The air is dry,
warm, and stable, leading
to Haar conditions)

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Chapter 16 Air Masses

MARITIME TROPICAL
Summer

Source Region Conditions at Modifications Weather
The Azores high Source
As it moves north-east Advection fog likely over
Temperature toward Europe, the air the sea
is cooled from below,
Warm increasing stability Warm moist conditions with
some ST or SC, visibility
Relative Humidity Continual evaporation moderate or poor
Mod gives a high dew point
and high RH
Absolute Humidity
High

Winter

Source Region Conditions at Modifications Weather
The Azores high Source
As it moves north-east Extensive low SC giving
Temperature toward Europe the air is continuous drizzle or light
cooled from below, rain
Warm increasing stability
Temperatures above the
Relative Humidity Continual evaporation seasonal average, with
Mod gives a high dew point moderate to poor visibility
and high RH
Absolute Humidity Advection fog forms if the
High air flows over a snow
covered surface. This flow
can also cause a general
thaw

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CONTINENTAL TROPICAL

All seasons, but more common in summer

Source Region Conditions at Modifications Weather
Source
North Africa and Moves north and is Hot, dry conditions
South East Temperature cooled from below
Europe becoming more stable Sometimes hazy with dust
Warm from the Sahara
Movement overland
Relative Humidity keeps the humidity low Some cloud and
Low precipitation over France if
the air mass picks up
Absolute Humidity moisture and becomes
Low unstable over the
Mediterranean

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16-12 Meteorology

INTRODUCTION

The previous chapter discussed air masses, where the properties of temperature and humidity
are relatively constant in the horizontal throughout the air mass.

Also discussed was how the properties of air masses differ from those of other air masses. The
boundary between two air masses with different properties is called a front.

Fronts can produce quite active weather. This chapter discusses the characteristics of various
types of front.

TYPES OF FRONT

Where two air masses meet, the warmer air is less dense and rises up over the colder air. This
gives a sloping frontal surface.

Initially this chapter explores the three main types of front.

WARM FRONT

Where warm air replaces cold air, as shown below, it is called a warm front. Also shown below is
the symbol used on synoptic charts to represent the warm front.

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COLD FRONT

Where cold air replaces warm air, as shown below, it is called a cold front. Also shown below is
the symbol used on synoptic charts to represent the cold front.

QUASI-STATIONARY FRONT

Where there is little frontal movement, and neither air mass can be said to be replacing the other,
it is termed a quasi-stationary front. A diagram representing this situation is shown below along
with the chart symbol for the quasi-stationary front.

PRESSURE SITUATION AT A FRONT

As an aircraft flies from a warm air mass into a cold air mass across a front, if it maintains the
same true altitude then the colder air means higher density and hence higher pressure.
Shown below is the view from above as an aircraft flies along an isobar towards the front. Once it
crosses the front, the pressure increase means that the isobars have changed orientation. They
bend towards the low pressure.
The greater the temperature change at the front, the greater the change in direction of the
isobars. As the isobars determine the direction of the wind, one would expect stronger windshear
when the temperature change is greater.

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SEMI-PERMANENT FRONTS OF THE WORLD

In both hemispheres there are several semi-permanent or quasi-stationary fronts.

ARCTIC FRONT

This is the boundary between arctic and polar air and is found at latitudes above 65°.

POLAR FRONT

A polar front is the boundary between polar and tropical air. It is found between latitudes 35° and
65° in the Northern Hemisphere and at around 50° in the Southern Hemisphere.

In winter the polar front stretches from Florida to south-west UK. In the summer it retreats north,
stretching from Newfoundland to the north of Scotland.

In this region, a phenomenon called the Polar Front Depression arises. This is the major factor
in the weather patterns found in the UK and Europe and will be discussed later in this chapter.

MEDITERRANEAN FRONT

This front only exists in winter when there is low pressure in the Mediterranean. It is the boundary
between polar continental or maritime air and tropical continental air.

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INTER-TROPICAL CONVERGENCE ZONE (ITCZ)

Originally called the inter-tropical front, the inter-tropical convergence zone was renamed since it
is not really a front. It is a boundary zone around 300 nm wide between tropical air masses on
either side of the heat equator. Since both masses are tropical, the word ‘front’ is misleading
hence the name change.

It is also sometimes referred to as the Equatorial Trough or just the Heat Equator.

The ITCZ is discussed in considerably more detail in the chapters on Climatology.

CHARACTERISTICS OF FRONTS

This section explores characteristics of the warm front and the cold front, including the likely kinds
of weather to expect.

WARM FRONT

A warm front occurs when warm air replaces cold air. It rides up over the cold air forming a
sloping frontal surface with an average gradient of about 1:150.

Since warm air is less dense, its progress is retarded by the cold dense air ahead of it. The front
therefore travels at about 2/3 of the geostrophic wind speed that would otherwise be expected
from the isobar interval along the front.

The gentle slope of the front means that lifting will not be strong enough to form cumuliform cloud.
Instead, layer cloud will form. Approaching the front from the cold air side layer clouds appear in
the following order: Ci, Cs, As, Ns.

A progressively lowering cloud base results. The cirrus cloud will be seen up to 600 nm in
advance of the surface position of the front.

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No precipitation will be experienced prior to reaching the altostratus. where you will see virga –
precipitation that doesn’t reach the ground. As you approach the nimbostratus the rain will
become continuous moderate or heavy.

As the front approaches, the pressure drops, but once it passes the fall will be arrested. However,
since the air behind the front is warmer, it settles to a lower value than that preceding the front.

The wind veers, but since the passage of the system is quite slow, this change tends to be
gradual and doesn’t usually result in problematic windshear.

COLD FRONT

A cold front occurs when cold air replaces warm air. The cold air undercuts the warm air because
it is more dense and its progress is not impeded by the warm air it replaces. It therefore moves at
the geostrophic wind speed.

The cold front is much steeper, averaging about 1:50. Sometimes it becomes vertical and even
bulges out into the warm air forming a nose-like protrudence.

Cold front lifting is much greater hence this front produces cumuliform cloud such as Cu and Cb
and possible thunderstorm activity. There may be shelves of nimbostratus or cirrus cloud
extending into the cold air when there is a stable layer.

Since the slope is much steeper than that of the warm front, the band of associated cloud only
spans up to about 200 nm.

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CI COLD FRONT

CU/CB

WARM AIR

COLD AIR
NS

As the front approaches, the pressure drops due to the rising air, but after its passage it rises
again and settles at a greater value than that preceding the front since the air is now colder.

Wind direction changes over a much shorter passage of time than that of the warm front. Hence
strong windshear tends to be associated with active cold fronts.

POLAR FRONT DEPRESSIONS

These form on the polar front – the boundary between polar and tropical air. At the front the
pressure is lower as the warm air rises up over the cold air. Moving away from the front on either
side the pressure increases.

Obeying Buys Ballots Law the wind flows along the isobars with the low pressure to the left. As
the diagram below illustrates the wind on either side of the front flows in opposite directions.

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The above situation causes friction which leads to the formation of waves or ripples along the
front. As the size of the ripples increases with increasing wind speed, the warm air bulges into the
cold air as shown below.

More warm air flows into the depression, causing the depression to deepen.

The result is a system shaped like a shark fin, with a warm front followed by a cold front. The tip
of the shark fin is a low pressure centre.

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Growth of a polar front depression takes about four days. The depression dies away as it fills
which typically takes ten days.

The system moves in an easterly direction under the influence of the westerly upper winds,
forming an overall picture like that shown on the synoptic chart below. This is known as a
westerly wave.

WEATHER ASSOCIATED WITH THE POLAR FRONT
DEPRESSION

INTRODUCTION

As a polar front depression passes over a point, the first weather experienced will be that
associated with a warm front before the cold front arrives. The weather in this sector will depend
on the stability of the air in this sector, as described below.

After the warm front comes the cold front, bringing with it the expected cold front weather. After
the cold front passes there will be a period of cold clear weather before the arrival of the next
polar front depression.

A typical picture is shown in the next figure:

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WARM FRONT

The weather associated with the passage of the warm front is summarised in the table below:

Warm Front

In Advance At the Passage In the Rear

Pressure Steady fall Fall arrested Little change or slow
Wind fall
Backing slightly and Veer and decrease
Temperature increasing Steady direction
Dewpoint Rise
Steady or slow rise Rise Little change
Relative Humidity Rise in precipitation May rise further if not Steady
Cloud Rise in precipitation already saturated
Low ST Little change, may be
Weather CI, CS, AS, NS in saturated
succession, Precipitation eases
Visibility or stops ST, SC may persist
increasing to 8 oktas perhaps some CI
Light continuous from Poor, often mist or
fog Dry or intermittent
AS becoming rain or snow
moderate continuous
Moderate or poor,
from NS mist or fog may
Good except in persist

precipitation

WARM SECTOR

The weather in the warm sector depends on the stability of the air. If the air is stable it is called a
kata front. The clouds will be mainly stratiform.

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Unstable air produces cumuliform cloud, with the possibility of embedded CBs.

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COLD FRONT

The weather associated with the passage of the cold front is summarised in the table below:

Cold Front

In Advance At the Passage In the Rear

Pressure Fall Sudden rise Rise continues more
Wind slowly
Backing and Sudden veer,
Temperature increasingly perhaps squall Further squalls before
Dewpoint becoming squally settling
Relative Humidity Steady, but falling in
pre-frontal rain Sudden fall Little change, variable
Cloud Little change in showers
Weather Rise in pre-frontal Sudden fall
precipitation Remains high in Little change
Visibility precipitation
ST or SC, AC, AS Rapid fall as
then CB CB, CU sometimes precipitation ceases,
Some rain, perhaps NS and CI variable in showers
thunder Heavy rain or
snow, perhaps hail Lifting rapidly
Moderate or poor, and thunder
perhaps fog Heavy rain or snow for
Good except in usually a short period,
showers sometimes more
persistent, then fine

Becomes excellent
well behind the front

OCCLUSIONS

Consider the polar front depression. The warm front is followed by a cold front. As previously
mentioned, the cold front moves at a speed equivalent to the geostrophic wind speed expected
by measuring the isobar spacing at the front.

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However, the warm front is moving at only 2/3 this speed. Hence as the polar front depression
travels east across the North Atlantic, the cold front gains on the warm front, progressively
narrowing the warm sector between the two fronts. Eventually it catches up with the warm front,
as shown in the diagram below.

A
B

This occurrence is called an occlusion. The two types of occlusions are warm and cold.

Which type of occlusion occurs depends on the relative temperatures of the air masses ahead of
the warm front (A) and behind the cold front (B). If the air at A is colder, it is termed a warm
occlusion; if the air at B is colder, it is a cold occlusion.

Both air masses are in fact part of the same air mass, the polar air. However, as an air mass
travels, its characteristics change according to the surface over which it passes.

In the UK in summer, the most common type of occlusion is the cold occlusion. This is because
the air ahead of the warm front has spent a greater length of time over the warmer land, but the
air behind the cold front has much more recently been over the cold sea.

Conversely, in winter, the sea is warmer than the land; hence the common occlusion type is the
warm occlusion.

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