Jeppesen Meteorology

Fronts and Occlusions Chapter 17

Occlusions are shown as below on the synoptic chart. As you can see, for a warm occlusion, it is
the warm front which continues along the same line. For a cold occlusion, it is the cold front that
continues.

WARM OCCLUSION

As shown above, in a warm occlusion the air behind the cold front is less cold than the air ahead
of the warm front. Hence it rides up over the air in front.

The warm front extends down to the surface, but the cold front doesn’t. The warm sector is never
in contact with the ground.

The expected cloud types are the same as with a warm front initially, with cumuliform cloud
coming at around the same time as the later warm front clouds.

Most of the weather will be experienced before the passage of the surface front.

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Chapter 17 Fronts and Occlusions
COLD OCCLUSION

In the case of a cold occlusion, the air behind the cold front is colder, so it undercuts the air in
front of the warm front. The cold front extends to the surface but the warm front does not. Again,
the warm sector is no longer in contact with the ground.

Expect the same pattern of clouds as for the warm occlusion, but more of the cloud occurs after
the passage of the surface front.

BACK BENT OCCLUSION
As the polar front depression travels, the occluded section can lag behind, in which case it may
bend back on itself. This can give a region of intense weather at the two occluded sections and
the low pressure between them.

17-14 Meteorology

INTRODUCTION

The chapter on Wind introduced the concepts of the geostrophic wind. The formula for
geostrophic wind speed is given again here:

PGF

V=
2 Ω ρ SIN θ

Just like lower winds, the upper winds are caused by the same forces: pressure gradient force,
geostrophic force, and cyclostrophic force.

This means that the geostrophic wind formula above also applies to upper winds. Since the wind
speed is inversely proportional to air density, wind speed would be expected to increase as height
increases and density decreases.

For example, the density at 20 000 ft is approximately half that at the surface, thus doubling the
wind speed.

THERMAL WIND COMPONENT

INTRODUCTION

The following diagram shows two columns of air, one cold, and one warm. The surface pressure
is the same in both cases – in the example we have used 1020 hPa.

Pressure falls more quickly over cold air and less quickly over warm air, so the air pressure over
the cold air would be expected to be lower than that at the same height over the warm air.

1010 hPa 1009 hPa

1020 hPa 1020 hPa

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Chapter 18 Upper Winds

The wind must obey Buys Ballot’s Law:

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

Hence, in this example, the wind must be blowing off the page. This gives a new law similar to
Buys Ballot’s Law:

“In the Northern Hemisphere with your back to the upper wind, the cold air is on
your left.”

In the Southern Hemisphere the cold air is on your right.

CALCULATING THE THERMAL WIND COMPONENT

To calculate the thermal wind component use the following formula:

THERMAL WIND SPEED = TEMP GRADIENT PER 100 NM X ALTITUDE DIFF (FT)
1000

8000 ft

24°C 18°C

200 nm

For example, to calculate the thermal wind component for the above picture case, determine that
the temperature gradient is 3°C/100 nm and the thickness of the layer is 8000 ft. Hence the
thermal wind component is:

3X 8000 = 24 KT
1000

The direction of the wind depends on the relative positions of the cold and warm air masses.

Note: This formula is only valid for the 50° latitude. For other latitudes, multiply the answer by
sin 50 ÷ sin Latitude.

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Upper Winds Chapter 18

UPPER WIND

If the geostrophic wind was calm, the upper wind at any level would simply be the thermal wind
component over the layer between that level and 2000 ft.

Geostrophic Thermal wind
wind component

Upper wind

If there is a geostrophic wind, then the upper wind will be the vector sum of the geostrophic wind
and the thermal wind component. Resolve this graphically or by using the CRP-5.

Continuing on from the previous example, assume a geostrophic wind of 040/20 with cold air to
the north, in the Northern Hemisphere. The following steps show how to calculate the upper wind
for 10 000 ft using a CRP-5.

STEP 1 The cold temperature is to the north. Using Buys Ballot’s Law with the wind
behind, the low temperature is on the left. The wind direction must be from 270°.

STEP 2 Having already calculated the thermal wind speed at 24 kt, the thermal wind
component is 270/24 kt.

STEP 3 Set the 2000 ft wind velocity using the zero line.

Meteorology 18-3

Chapter 18 Upper Winds

STEP 4 Set the TWC. The origin of the TWC is the end of the geostrophic wind
component.

STEP 5 Move the end of the TWC component to the centre line and read off the upper
wind at 10 000 ft — 325/20.

Note: If the geostrophic and the thermal wind component are in opposite directions, the
wind first decreases in speed as height increases, becoming calm before
reversing in direction and increasing in speed.

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Upper Winds Chapter 18

GLOBAL UPPER WINDS

The diagram below shows the Earth with warm air over the equator and a decreasing
temperature as we move towards the poles. In the Northern Hemisphere the wind keeps the low
temperature to its left, in the Southern Hemisphere it keeps the low temperature to its right. In
both cases this gives a westerly wind.

Exceptions to the rule occur in the tropics and over the poles, where the upper winds are easterly.

JET STREAMS

INTRODUCTION

A jet stream is a wind greater than 60 kt in speed, which manifests itself as a long corridor of wind
with typical dimensions of 1500 nm in length, 200 nm in width and 12 000 ft in depth.

They are caused by large temperature differences in the horizontal.

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Chapter 18 Upper Winds

The wind speed is fastest at the core and decreases with movement away from the core.

60 kt 80 kt

100 kt
120 kt

Speeds in excess of 100 kt are quite common, but it is rare for jet streams to be faster than
200 kt. However, jets of 300 kt have been reported on occasion. These extreme examples tend to
occur in the east Asia/Japan area.

COMMON JET STREAMS

The table below shows the common global jet streams.

Polar front jet stream Latitude Pressure Level
Sub-tropical jet stream 45° to 65° N/S 300 hPa – 30 000 ft
Equatorial jet stream 20° to 40° N/S 200 hPa – 45 000 ft
Polar jet stream 10° to 15° N/S 100 hPa – 55 000 ft
70° to 80° N/S
50 hPa – 75 000 ft

Details of the equatorial and the polar jet stream are not required for the course, but this chapter
goes into more detail about the Sub-tropical and the Polar Front jet streams.

SUB-TROPICAL JET STREAM

These occur above the sub-tropical anti-cyclones and are caused by the circulation of the Hadley
cells. The Hadley cells are a circulation which starts with lifting over the heat equator due to
surface heating. When the air reaches the tropopause it flows away from the equator to higher
latitudes.

At approximately 30° latitude, the air is cooled such that it starts to descend. Where it reaches the
surface it forms the sub-tropical anti-cyclones. It then flows into the low pressure at the heat
equator.

The following diagram shows the circulation of air on the Earth.

18-6 Meteorology

Upper Winds Chapter 18

Polar
cell

Ferrel
cell

Hadley
cell

The sub-tropical jet stream forms when air from the Hadley cells meet air from higher latitudes.
Due to the large amount of air, not all of it descends; some of it is forced to flow horizontally. In
both the Northern and Southern Hemispheres geostrophic force turns it to the right.

In both cases this results in a westerly jet.

PGF GF PLAN VIEW

PGF NH
GF
Heat equator

Heat
equator

SH

The sub-tropical jet streams exist all year round but move as the heat equator moves. In winter
they exist in the latitude band 25° — 40° and in the summer are found in the latitude band 40° to
45°.

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Chapter 18 Upper Winds

POLAR-FRONT JET STREAM

Like the name suggests, a polar-front jet stream occurs on the Polar Front. The following diagram
shows the position of the jet stream in cross section and in plan view.

The diagram shows that the jet stream forms in the warm (tropical air) just below the warm air
tropopause. In the plan view the jet stream appears to be in cold sector. However, it is the surface
position of the fronts that is shown. The fronts slope so in fact the jet is in the warm air.
Unlike the sub-tropical jet stream, the polar front jet stream is not in a constant westerly direction.
It follows the patterns of the polar front depressions and forms a zig-zag shape which is westerly
on average.
They are less permanent than the sub-tropical jets, tending to die out a bit in summer. Average
speeds in summer are 60 kt; in winter, 80 kt.
Like the sub-tropical jet stream, the polar front jet streams change position with the movement of
the heat equator. Approximate positions are between 40°N and 65°N and at around 50°S.

18-8 Meteorology

Upper Winds Chapter 18

WINDS AROUND A POLAR FRONT DEPRESSION

This chapter has explored the polar front depression and the pattern of isobars around it. The
2000 ft wind follows the isobars in an anti-clockwise direction around the low pressure centre, as
shown in the diagram.

Super-imposed onto this is the polar front jet stream, which obeys the rule of always keeping the
cold air to its left. As a result, the 2000 ft wind and the upper wind often come from different
directions. This is summarised below:

Position 2000 ft wind Upper wind Trend
Ahead of the warm front South-westerly North-westerly Veer and increase

In the warm sector Westerly Westerly Increase
Behind the cold front North-westerly South-westerly Back and increase

CLEAR AIR TURBULENCE

The windshear within and around jet streams leads to friction within the atmosphere. This causes
turbulence known as clear air turbulence (CAT), due to the fact that it is not caused by clouds or
by proximity to the ground.

The most severe CAT is found level with the core of the jet on the cold air side. A secondary area
of severe CAT is found above the core, above the warm air tropopause.

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Chapter 18 Upper Winds

If CAT associated with a polar front jet stream in the Northern Hemisphere is experienced,
descend and turn to the south. This brings the aircraft into the warm air and away from the
strongest turbulence.

IDENTIFICATION OF JET STREAMS

It is usually impossible to identify a jet stream visually. However, if the air is moist, there may be a
trail of cirrus cloud associated with the jet stream, as shown below.

This cirrus is caused by a lowering of pressure and temperature around the jet stream, due to the
high velocity of the air. This cools the air to its dewpoint causing some water vapour to sublimate
out as ice crystals.

Another way to identify a jet stream is by looking at meteorological charts, like those discussed
below. Other important charts are discussed in the chapter on Upper Air Charts.

CONTOUR CHARTS

For lower winds, use synoptic charts. These show isobars (lines of constant pressure) and from
this the direction of the wind is predictable.

For upper winds a different system is used. Rather than using a chart for a given height above
mean sea level and showing the different pressures on the chart, charts with constant pressure
are used and the lines drawn join places of constant height above mean sea level at which that
pressure occurs.

This is useful for high altitude flying as flights are conducted at flight levels/pressure altitudes, that
is, the aircraft flies along a line of constant pressure.

Common charts in use are as follows:

Pressure (hPa) Equivalent Pressure
Altitude (feet)
700
500 10 000
300 18 000
250 30 000
200 34 000
150 39 000
53 000

The lines joining places of equal height are called contour lines and the heights are expressed in
one of two ways. The number may represent the height in 100s of feet or the height in
decametres (10s of metres).

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Upper Winds Chapter 18

A line with a low value means that the pressure for which the chart is produced is found at a lower
height, whereas a high value means the pressure is found at a greater height. As can be seen
from the following diagram, this means that areas of low contour heights are areas of low
pressure.

31 000 ft

30 000 ft < 300 hPa > 300 hPa
29 000 ft
300 hPa

Since the wind follows Buys Ballot’s law, it flows with the low contour lines to its left in the
Northern Hemisphere. As for a synoptic chart, the closer the contour lines, the stronger wind.

THICKNESS CHARTS

Another chart used to discern wind direction is the thickness chart which shows the thickness of
the layer between two given pressure values.

As shown in the diagram below, a low thickness value is associated with cold air and a high value
is associated with warm air. Lines of constant thickness are called isopleths.

500 hPa

Cold air – low Warm air – high
thickness value thickness value

Meteorology 1000 hPa

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Chapter 18 Upper Winds

In the Northern Hemisphere the thermal wind keeps the cold air to its left, hence it travels parallel
to the isopleths keeping the low value isopleths to its left. In the Southern Hemisphere the low
value isopleths are to the right.

If the isopleths are closely spaced, this indicates a steep temperature gradient and hence
stronger winds.

18-12 Meteorology

WINDSHEAR

The following meteorological factors can cause windshear:

1. Inversions
2. Mountain waves and rotors
3. Katabatic winds (fall winds)
4. Sea breeze fronts
5. Air mass fronts
6. CB cloud
7. Low level jet
8. Jet streams

DEFINITIONS AND THE METEOROLOGICAL BACKGROUND

In discussing windshear it is not easy to find a definition which satisfies both meteorologist and
pilot. At its simplest, windshear is a change in wind direction and/or speed in space, including
updraughts and downdraughts. Despite the emphasis on the windshear hazard in recent years,
there are still some who argue that aviators have lived with windshear since the dawn of aviation,
seeing it as an extreme form of wind gradient, which would itself fit this definition.

DEFINITION

Variations in vector wind along the aircraft flight path of a pattern, intensity, and duration so as to
displace an aircraft abruptly from its intended path requiring substantial control action.

LOW ALTITUDE WINDSHEAR

Low altitude windshear is windshear along the final approach path or along the runway and along
the takeoff and initial climb out flight path.

Further refinement offers:

¾ Vertical windshear as the change of horizontal wind vector with height, as might be
determined by two or more anemometers at different heights on a mast;

¾ Horizontal windshear as the change of horizontal wind vector with horizontal distance as
might be determined by two or more anemometers mounted at the same height at
different points along a runway;

¾ Updraught/downdraught shear as changes in the vertical component of wind with
horizontal distance.

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Chapter 19 Windshear and Turbulence

Setting aside the basic windshear definition above, the other definitions allow for changes of
vector wind from the relatively minor event upwards. The essence of the windshear with which
this chapter is concerned is spelt out by the basic definition with its emphasis on abrupt
displacement from the flight path and the need for substantial control action to counteract it.

A windshear encounter is a highly dynamic and potentially uncomfortable event; to think of
windshear as an aggravated form of wind gradient is unwise. Windshear can strike suddenly and
with devastating effect, sometimes beyond the recovery powers of experienced pilots flying
modern and powerful aircraft. An encounter may cause alarm, a damaged landing gear, or a total
catastrophe. The first and most vital defence is avoidance.

METEOROLOGICAL FEATURES

The most potent examples of windshear are associated with thunderstorms (cumulonimbus
clouds), but windshear can also be experienced in association with other meteorological features
such as the passage of a front, a marked temperature inversion, a low-level wind maximum, or a
turbulent boundary layer. Topography or buildings can exacerbate the situation; particularly in a
strong wind.

THUNDERSTORMS

The chapter on Meteorological Notes describes thunderstorm formation and how the wind flows
in and around the thunderstorm which causes the most severe windshears. Diagrams do no
justice to the violence of totally dynamic and unpredictable thunderstorms with turbulence, hail,
windshear, and lightning as separate or joint hazards. Shears and draughts may strike from all
angles and are certainly not limited to the horizontal or vertical; an assessment of the aircraft’s
actual angle of attack relative to some thunderstorm wind flows is difficult to make, which in turn
makes the risk of a stall harder to gauge. This is significant if a thunderstorm is encountered on
the approach or following take-off.

FRONTAL PASSAGE

Fronts, whether warm, cold, or occluded, vary in strength. It is only well developed active fronts,
with narrow surface frontal zones and with marked temperature differences between the two air
masses, which are likely to carry a risk of windshear.

Warning signs to look out for include sharp changes in wind direction indicated on the weather
charts by an acute angle of the isobars as they cross the front, a temperature difference of 5º C
or more across the frontal zone, and the speed of movement of the front, especially if 30 kt or
more.

It should be mentioned that windshear is possible in fronts which are slow moving, stationary or
even reversing direction. The passage of a vigorous cold front poses the greater risk though,
relative to a warm front, as the period of windshear probability is likely to be much shorter and
occurs just after the surface passage of the front. With a warm front, the effect precedes the
passage and is more prolonged.

To illustrate the potential severity of frontal windshear, there is the case of a twin jet aircraft
caught by the passage of a cold front while flaring to land. Within about ten seconds, the wind
shifted from 230/10 kt to 340/16 kt, so that a 10 kt crosswind from the left and slight tail wind
changed to an 8 kt crosswind from the right with 14 kt headwind. The pilot, finding directional
control for landing to be difficult, wisely carried out a missed approach from a very low level.

19-2 Meteorology

Windshear and Turbulence Chapter 19

This is a classic case of horizontal windshear. A sea-breeze front may occasionally present a
hazard; for example if it impinges on a thunderstorm it may significantly alter the outflow from the
storm; a catastrophic accident in the USA in 1975 involved such a feature.

INVERSIONS

Vertical windshear is nearly always present in the boundary layer, but this normally involves a
gradual change in the wind with which pilots are well familiar. A hazard exists, however, when an
unexpectedly strong vertical shear develops.

This can occur broadly in two situations:

¾ A low-level jet (more accurately referred to as a low-level wind maximum) can form just
below the top of, or sometimes within, a strong radiation inversion which may develop at
night under clear skies. Other low-level jets may develop in association with a surface
front, particularly ahead of cold fronts;

¾ On occasions, low-level inversions develop and decouple a relatively strong upper flow
from layers of stagnant or slow moving air near the surface. Windshear may be
pronounced across the interface.

TURBULENT BOUNDARY LAYER

Within the boundary layer, turbulence becomes a windshear hazard in two different situations:

¾ Strong surface winds are generally accompanied by large gusts and lulls (horizontal
windshear). Roughly speaking, the stronger the mean wind, the greater the gust or lull.

¾ Thermal turbulence (updraughts and downdraughts) is caused by intense solar heating of
the ground, which is more common in hot countries, but can occur anywhere on a hot
sunny day.

TOPOGRAPHICAL WINDSHEARS

Either natural or man-made features affect the steady state wind flow and cause windshears of
varying severity. The strength and direction of the wind relative to the obstacle are significant and
a change of direction of relatively few degrees may appreciably alter the residual effect. The flow
of wind across a mountain range is a simple large scale example, with waves and possibly a rotor
forming on the leeside.

Wind blowing between two hills or along a valley, or even between two large buildings may be
funnelled, thus changing direction and increasing in speed, or a strong flow may be heavily
damped. Either way, this creates the possibility for shear, with sudden changes of wind vector
becoming a hazard.

Usually local effects become well known and predictable, with warnings given on aerodrome
approach plates (e.g. Gibraltar). Large airport buildings adjacent to busy runways can create
hazardous local effects and typical windshear problems, such as loss of airspeed and abrupt
crosswind changes, causing upsets to airliner-size aircraft which have been near to major
accidents.

On smaller aerodromes, lines of trees can mask the wind and cause problems at a late stage in
the approach. These incidents usually contribute to a pilot’s experience, but damaged landing
gear can result from wind effects of greater significance than a steep wind gradient or low-level
turbulence alone.

Meteorology 19-3

Chapter 19 Windshear and Turbulence

THE EFFECTS OF WINDSHEAR ON AN AIRCRAFT IN FLIGHT

Windshear affects aircraft in many different ways and during an encounter the situation is
constantly changing, especially during the more dynamic thunderstorm windshears. Particular
types of aircraft vary in their reaction to a given shear; a light high-wing piston-engined aircraft
may react in a totally different way to a swept-wing four-engine jet. It is not easy to describe the
effects in general terms, as they do not apply universally. The notes which follow only attempt to
describe stylised windshears and their progressive effects. Windshear can, of course, be
encountered at any height and the effects will be similar. The windshear encounter at low level
which is a great hazard; it is this which must be borne in mind when the effects are described.

An understanding of windshear is difficult, unless the relationship of an aeroplane in a moving air
mass to its two reference points is appreciated. One reference is the air mass itself, the other is
the ground. In a windshear encounter it is not only the magnitude of the change of wind vector
that counts but the rate at which it happens.

For example, an aeroplane at 1000 ft agl may have a headwind component of 30 kt, but the
surface wind report shows that the headwind is only 10 kt on the runway. That 20 kt difference
may taper off evenly with the effect of a reasonable wind gradient. However, it may be noticed
that the 20 kt differential still exists at 300 ft and the change, when it comes, will clearly be far
more sudden and its effects more marked. Shear implies a narrow borderline and the 20 kt of
wind speed may well be lost over a vertical distance of 100 ft as the aircraft descends from 300 to
200 ft.

If the pilot wanted a stabilised approach speed of 130 kt, the power would be set according to
conditions, providing the required airspeed and rate of descent.

On passing through the shear line, the loss of airspeed is sudden, but the inertia of the aircraft at
first keeps it at its original groundspeed of 100 kt and power is needed to accelerate the aircraft
back to its original airspeed. This takes time; meanwhile the aircraft having lost 20 kt of airspeed,
sinks faster as a substantial amount of lift has also been lost.

19-4 Meteorology

Windshear and Turbulence Chapter 19

The headwind was a form of energy and when it dropped 20 kt, an equivalent amount of energy
loss occurred. One source available to balance that loss is engine power; this arrests the
increased rate of descent and starts the process of accelerating back to the approach reference
speed.

The opposite effect can be illustrated using similar conditions, but seen from the point of view of
an aeroplane taking off. Initially take-off along the runway and into the second segment of the
climb, with a 10 kt headwind, the wind becomes a 30 kt headwind after encountering the shear
between 200 and 300 ft. Assuming a target climbing speed of 120 kt, the effect of a sudden
transition through the shear line into a 20 kt increase of headwind, increases the lAS by the same
amount until the momentum of the groundspeed is lost.

This is a case of temporary energy gain, with lift added so the aircraft climbs more rapidly. This
example shows the windshear as being positively beneficial and it is true to say that a rapid
increase in headwind (or loss of tailwind), because they are “energy gains,” temporarily enhances
performance.

It may help with understanding windshear to see it in terms of energy changes, when it is readily
apparent that the windshear which causes temporary loss of energy (sudden drop of headwind or
increase in tailwind, and downdraughts) is the main danger at low altitude.

The effect of a downdraught is not always easy to visualise, as we normally think of the aeroplane
in relation to airflow along the flight path even when climbing or descending. It is now necessary
to envisage flying suddenly from a horizontal flow into air with a vertical component.

In turbulent conditions, air in motion may strike the aeroplane from an angle and the situation may
be constantly changing. However, in thunderstorms, substantial shafts of air which can be moving
either up or down may be encountered with no warning; such shafts may be virtually side by side
and the shear very marked and violent.

Entering a vertical updraught or downdraught from a horizontal airflow, the aeroplane's
momentum at first keeps it on its original path relative to the new direction of flow. In addition to a
loss of airspeed, also realise that the shift of relative airflow affects the angle of attack of the wing,

Meteorology 19-5

Chapter 19 Windshear and Turbulence

which may result in either increased or decreased lift. A slight increase of angle may not cause
much concern. However, if the aircraft is already on the approach with a high angle of attack, an
increase might put the wing near the stall and any decrease will bring about a loss of lift.
Normally, below 1000 ft, the risk of a downdraught is more likely than an updraught.

Having described the combination of increasing headwind followed by downdraught followed by
increasing tailwind consider, that this is the sequence which might be encountered in a microburst
on the approach or following a take-off. This may be a rare occurrence in the United Kingdom or
Europe, but it needs to be appreciated by those flying to the USA. Even on this side of the
Atlantic, an encounter with a downburst, a headwind followed by downdraught, or a downdraught
followed by tailwind is possible and may cause problems.

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Windshear and Turbulence Chapter 19

A Energy gain
Increasing headwind
Airspeed rising
Rate of descent reduced
Tendency to go high on glide path

B Energy loss
Reducing headwind and downdraught
Airspeed falling
Rate of descent increased
Tendency to go low on glide path

C Energy loss
Increasing tailwind
Airspeed still falling
Rate of descent checked by missed approach
Success depends upon power, height and speed reserves available

An aircraft, approaching on a 3° ILS glidepath, might see ahead an area of heavy rain. Ideally this
might alert the pilot to possible danger, and a missed approach could be executed in good time,
though even this might take the aircraft into the microburst. Then, however, the aircraft will have
gained precious extra height.

Given that the approach continues towards the microburst, the leading edge can produce a
rapidly increasing headwind; the airspeed increases and the aircraft goes high on the glidepath.
The likely reaction is to reduce power to increase the rate of descent and adjust attitude to reduce
airspeed. Then comes the downdraught when the rate of descent increases rapidly and the
aircraft passes through and below the glidepath, still possibly with the nose high and the power
low.

Power is re-applied, but it takes time to spool up the engines, meanwhile the aircraft passes from
downdraughts to increasing tail wind with the airspeed dropping. The rate of descent is not
checked and the nose is high while power increases.

No figures are attached to this description, merely the likely sequence of events. A very strong
microburst has a more pronounced effect on the rise and fall of airspeed and extremes of rate of
descent. The power reserves available and the rate at which they can be applied and built up to
give maximum thrust, determine the aircraft’s ability to counteract the energy loss of downdraught
and increasing tailwind.

Strong wind buffeting, the lashing of rain, and possibly blinding flashes of lightning may
accompany this dynamic sequence of events. If this is a black picture, it matches the descriptions
of those that have flown through a microburst and would probably be echoed by some who have
tried but failed to fly through one. The aim must be to avoid severe windshear at all costs.

It might be thought that an encounter with windshear from a microburst after take-off is likely to be
less hazardous than when approaching to land. The aircraft is at high power and is not
constrained by the need to hold a precise glide path. The temporary energy gain from meeting
the increasing headwind, with a burst of higher air-speed and rate of climb may seem positively
beneficial.

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Chapter 19 Windshear and Turbulence

The transition to downdraught soon kills any rise in airspeed; it may even drop. The rate of climb
may lessen or even show a rate of descent enhanced by the shift to increasing tailwind, when the
airspeed (with the aircraft close to the ground) may drop further. Any benefits of high power may
be balanced by higher aircraft weight. There may be a small power reserve in hand and this may,
or may not, be sufficient to enable the aircraft to fly through the microburst or downburst, together
with other measures described later.

TECHNIQUES TO COUNTER THE EFFECTS OF WINDSHEAR

Windshear can vary enormously in its impact and effect. There is as yet no international
agreement on definitions for grading windshear, but clearly some shears are more severe and
consequently more dangerous than others. In discussing guidance on countering the effects of
windshear, one must inevitably deal with the “worse case” situation. If the golden rule of
avoidance fails for whatever reason, it is impossible to predict at the first stages of a windshear
encounter how severe it will be and it is not bad advice to suggest that recovery action should
anticipate the worst.

No pilot who studies the meteorological situation carefully in advance and updates his knowledge
with the latest reports during flight should be taken totally by surprise by windshear. If
thunderstorms are forecast in the vicinity of the planned destination and then are reported as
being active and are seen on the weather radar or visually, then a mental Windshear Alert should
register. At this stage, depending on the evidence, a diversion might be considered, as windshear
avoidance is the safest course.

If it is decided to continue to the destination, then the crew should consider a few basic measures
to anticipate a possible windshear encounter. One of these is to increase the airspeed on the
approach. The amount of airspeed increase to be recommended is less easy to assess, as what
might be suitable for a light twin-piston engined aeroplane might be quite inappropriate for a
swept-wing jet.

Rule of thumb guidance includes adding half the headwind component of the reported surface
wind to VAT, or, half the mean wind speed plus half the gust factor, in each case up to a maximum
of 20 kt. This may be satisfactory for a strong but turbulent wind, but may not meet the
thunderstorm case, where it is not uncommon for light and variable winds to precede the
onslaught of a gust front or downburst.

The unpredictability of windshear is such that, if it does not materialise, the aircraft can arrive at
threshold with excessive speed to be shed and that could be embarrassing on a short runway.
Because the amount of airspeed “margin” is related to the aircraft's acceleration potential, the
relatively slow propeller driven aircraft is probably at an advantage over a faster jet aircraft.
Remember that the rate of shear is important and the aircraft which penetrates the shear zone
slower experiences a lower rate of shear — the rapid response of propeller driven airflow over a
wing also helps.

The windshear encounter which produces a sudden increase in airspeed (temporary energy gain)
on the approach destabilises it to a greater or lesser extent, which calls for some control
adjustment. The normal reaction to the rise above the glidepath is to reduce power to regain the
glidepath and as the deviation was sudden, the power reduction will probably be more than just a
slight one. The pilot must then be alert to the need to re-appIy power in good time to avoid
dropping below the glidepath. If the wind component then stabilises, leaving the aircraft merely
with a stronger headwind, a further power adjustment will be needed to a higher setting than the
initial one which had given a stable airspeed and rate of descent.

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Windshear and Turbulence Chapter 19

When an aircraft on the glidepath in the later stages of an approach runs into an “energy loss”
windshear, it can be much more hazardous.

A building or line of trees obstructing the windflow might cause the shear, and the resulting drop
in the wind speed might bring about a very sudden drop in airspeed with a consequent increase
in the rate of descent. To avoid a heavy and premature landing, a rapid and positive increase in
power is needed. Another likely effect is for the nose to drop initially, requiring a check with an
increase in pitch attitude - but not so much that this causes a further loss of airspeed; as always
power and attitude adjustments must be coordinated. These actions may enable the aircraft to
regain the glidepath and continue the approach.

Anticipate the power reduction to avoid flying through the glidepath and expect to set slightly less
power than that originally used, to continue the approach. If the approach has been badly de-
stabilised, full missed approach action may be the wiser and safer option, with a second
approach made with an airspeed “margin” to counter the anticipated windshear effect.

Vital Actions to counter loss of airspeed caused by windshear near the ground:

¾ Briskly increase power (full go round power if necessary)
¾ Raise the nose to check descent
¾ Co-ordinate power and pitch
¾ Be prepared to carry out a missed approach rather than risk landing from a de-

stabilised approach

To counter the effect of a downburst or microburst on an approach or take-off calls for more
stringent measures. It must be stressed that any well-founded report of either phenomenon must
be treated seriously and the approach or take-off delayed until the danger has passed. If there is
an inadvertent encounter, the aircraft may be affected by wind from any flank by the descending
and outflowing column of air, but again the worst case will be considered - entry on one side,
through the centre and exit through the other side. It will be a turbulent and unpleasant
experience which can tax the abilities of the most skillful pilots.

The presence of thunderstorms should be known and obvious, so the increase in speed caused
by the rising headwind should be seen as the forerunner of a downburst or microburst; any hope
of a stabilised approach is abandoned and a missed approach is the only safe course of action -
the technique is to make it as safe as possible.

The initial rise in airspeed and rise above the approach path should be seen as a bonus and
capitalised. Without hesitation, increase to go-around power, being prepared to go to maximum
power if necessary, select a pitch angle consistent with a missed approach, typically about 15°
and hold it against turbulence and buffeting.

The next phase may well see the initial advantages of increased airspeed and rate of climb
rapidly eroded. The downdraught now strikes, airspeed may be lost and the aircraft may start to
descend despite the high power and pitch angle. It will be impossible to gauge the true angle of
attack, so there is a possibility that the stick shaker (if fitted) may be triggered; only then should
the attempt to hold the pitch angle normally be relaxed.

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Chapter 19 Windshear and Turbulence

The point at which downdraught begins to change to increasing tailwind may well be the most
critical period. The rate of descent may lessen, but the airspeed may still continue to fall; the
height loss may have cut seriously into ground obstacle clearance margins. Given that maximum
thrust is already applied, as an extreme measure if the risk of striking the ground or an obstacle
still exists, it may be necessary to increase the pitch angle further and deliberately raise the nose
until stick shaker is felt. Then an easing forward of the control column to try and hold this higher
pitch angle should be made, until the situation eases with the aircraft beginning to escape from
the effects of the microburst.

When there is an indefinite risk of shear, it may be possible to use a longer runway, or one that
points away from an area of potential threat. It may also be an option to rotate at a slightly higher
speed, provided this does not cause undue tyre stress or any handling problems.

The high power setting and high pitch angle after rotate have already put the aircraft into a good
configuration should a microburst then be encountered. The aircraft is, however, very low where
there is little safety margin and the ride can be rough. If there is still extra power available, it
should be used without hesitation. Ignore noise abatement procedures and maintain the high
pitch angles, watching out for stick shaker indications as a signal to ease the controls forward.

In both approach and take-off cases, vital actions are:

¾ Use the maximum power available as soon as possible.
¾ Adopt a pitch angle of around 15° and try and hold that attitude. Do not chase

airspeed.
¾ Be guided by stick shaker indications when holding or increasing pitch attitude,

easing the back pressure as required to attain and hold a slightly lower attitude.

Windshear warning can be provided in several ways:

¾ Meteorological warning
¾ ATS warning
¾ Pilot warning
¾ On board pre-encounter warning
¾ On board encounter warning and/or guidance

ICAO DEFINITIONS

The following windshear reporting system is used to give pilots a common understanding of the
problem of windshear:

Intensity of Vertical Horizontal Up or down Effect on flight
windshear wind- wind- draught altitude

Light shear/100 ft shear/2000 ft 0 – 4 kt Small
Moderate 4 – 8 kt Significant
0 – 4 kt 0 – 4 kt 8 – 12 kt Hazardous
Strong 4 – 8 kt 4 – 8 kt > 12 kt Highly dangerous
Severe 8 – 12 kt 8 – 12 kt
> 12 kt > 12 kt

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Windshear and Turbulence Chapter 19

NATURE OF TURBULENCE

The small-scale vortices that constitute turbulence, form:

¾ When the air-flow is disturbed by an obstruction, (e.g. the ground surface).
¾ When two air-flows of different direction and/or speed adjoin each other.
¾ When the speed of the air changes rapidly within the same air-flow.

Turbulence transfers momentum from one volume of air to another by exchanging small amounts
of air. The wind speed, for example, can be accelerated or retarded.

To describe this we use the words gust and lull.

Gust is an increase of the wind speed of short duration.
Lull is a short-lived decrease of the wind speed.

TURBULENCE, METEOROLOGICAL FACTORS

Windshear caused by ascending and subsiding thermals, convection, results in the aircraft
bouncing along through the thermals, which creates thermal turbulence.

THERMAL TURBULENCE

Thermal turbulence is generated by heated thermals ascending through the air, causing a return
flow at the sides. During a flight, this causes severe bumps, and during the landing phase the up-
and downdrafts may disturb the approach.

Thermal turbulence is marked over warm surfaces, such as tarmac, concrete, mountains, sand,
or dark ground surfaces.

As a matter of fact, it is often a question of a combination of up- and down-winds with a clear local
character. Thermal turbulence occurs:

¾ Above land in the daytime and generally in association with convective clouds.
¾ In the autumn/winter above seas by day and night.

Except during the landing phase thermal turbulence does not constitute any major problem in
Northern Europe. In extreme cases, however, the aircraft can be bumped into exceptional flight
attitudes, and it may be rather uncomfortable to fly in areas with severe thermal turbulence

MECHANICAL/ FRICTIONAL TURBULENCE

Windshear and turbulence occur because of friction against the ground surface at high wind
speeds.

The mechanical effect depends on the structure of the surface and the wind speed, see the table
below. The consequence is very uncomfortable flight up to 2000 — 3000 ft above the terrain with
the aircraft being subjected to accelerations of several “g”.

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Chapter 19 Windshear and Turbulence

Criteria of mechanical turbulence:

Surface Wind< 15 kt 15-30 kt >30 kt
Sea Light Moderate Mod/severe
Plain Moderate
Only light Severe
Broken terrain Light -moderate Severe Extreme

Mechanical turbulence occurs throughout the year, when the prevailing wind is high. The more
unstable the air, the more severe the turbulence - this applies to both thermal and mechanical
turbulence.

MOUNTAIN WAVES

FLIGHT OVER AND IN THE VICINITY OF HIGH GROUND

Air flow is more disturbed and turbulent over high ground than over level country and the forced
ascent of air over high ground often leads to the formation of cloud on or near the surface. This
sometimes extends through a substantial part of the troposphere if the air is moist enough.

Forced ascent also increases instability so that thunderstorms embedded in widespread layer
cloud may occur over high ground, even when no convective clouds form over low ground. When
the air is generally unstable, cloud development is greater, icing in the clouds is more severe and
turbulence in the friction layer and in cloud is intensified over high ground.

The air flowing over high ground may be so dry that, even when it is forced to rise, little or no
cloud is formed. The absence of cloud over high ground does not imply the absence of vertical air
currents and turbulence.

Strong down currents are caused by the air descending the lee slope and it is, therefore,
especially hazardous to fly towards high ground when experiencing a headwind.

On some occasions, the disturbance of a transverse airflow by high ground creates an organised
flow pattern of waves and large scale eddies in which strong up-draughts and downdraughts and
turbulence frequently occur. These organised flow patterns are usually called mountain waves
but may also be referred to as lee waves or standing waves. These can be associated with
relatively low hills and ridges as well as with high mountains.

CONDITIONS

Conditions favourable for the formation of mountain waves are:

¾ A wind blowing within about 30° of a direction at right angles to a substantial ridge.

¾ The wind must increase with height with little change in direction (strong waves are
often associated with jet streams).

¾ A wind speed of more than 15 kt at the crest of the ridge is also usually necessary.

¾ A marked stable layer (approaching isothermal, or an inversion), with less stable air
above and below, between crest level and a few thousand feet above.

Mountain wave systems may extend for many miles downwind of the initiating high ground.
Satellite photographs have shown wave clouds extending more than 250 nm from the Pennines
in the UK; 50 to 100 nm is a more usual extent of wave systems in most areas. Wave systems,
on occasion, extend well into the stratosphere.

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Windshear and Turbulence Chapter 19

The average wavelength of mountain waves in the troposphere is about 15 miles, but much
longer waves occur. Derive a good estimate of the wave length using the following formula:

Wavelength = mean troposphere wind ÷ 7

Disturbances in the stratosphere are often irregular features located very near or just over the
initiating mountains. When waves to the lee of the high ground are evident, their length is usually
greater than in the troposphere. A typical wavelength is 15 nm, but wavelengths of 60 nm have
been measured.

The amplitude of waves is much more difficult to determine. In general, the higher the mountain
and the stronger the airflow, the greater the resulting disturbance. The most severe conditions
occur when the natural frequency of the waves is tuned to the ground profile.

In the troposphere, the double amplitude (peak-to-trough) of waves is commonly 1500 ft with
vertical velocities about 1000 ft/min. However, double amplitudes of about 20 000 ft and vertical
velocities aver 5000 fpm have been measured.

VISUAL DETECTION OF MOUNTAIN WAVES

The clouds which owe their appearance to the nature of wave flow are a valuable indicator of the
existence of wave formation. Provided there is sufficient moisture available, the ascent of air
leads to condensation and formation of characteristic clouds. These clouds form in the crest of
standing waves and therefore remain more or less stationary.

They occur at all heights from the surface to cirrus level.

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Chapter 19 Windshear and Turbulence

Lenticular Clouds provide the most unmistakable evidence of the existence of mountain waves.
They form within stable layers in the crusts of standing waves. Air streams through them, the
clouds forming at the up-wind edges and dissipating downwind. They have a characteristically
smooth, lens-shaped outline and may appear at several levels, sometimes resulting in an
appearance reminiscent of a stack of inverted saucers.

Lenticular clouds usually appear up to a few thousand feet above the mountain crests, but are
also seen at any level up to the tropopause and even above. Mother-of-pearl clouds, seen on rare
occasions over mountains, are a form of wave-cloud at an altitude of 80 000 ft. Air flow through
these clouds is usually smooth unless the edges of the cloud take on a ragged appearance,
which is an indication of turbulence.

Rotor Clouds, or roll clouds, appear as ragged cumulus or stratocumulus parallel to and
downwind of the ridge. On closer inspection, these clouds rotate about a horizontal axis. Rotor
clouds are produced by local breakdown of the flow into violent turbulence. They occur under the
crests of strong waves beneath the stable layers associated with the waves. The strongest rotor
normally forms in the first wave downwind of the ridge and is usually near or somewhat above the
level of the ridge crest. There are usually no more than one or two rotor clouds in the lee of the
ridge.

Cap Clouds form on the ridge crest. Strong surface winds which are commonly found sweeping
down the lee slope may extend the cap cloud down the slope.

Although cloud often provides the most useful visible evidence of disturbances to the airflow,
other cloud systems, particularly frontal cloud, sometimes obscures the characteristic cloud types.

TURBULENCE

TURBULENCE AT LOW AND MEDIUM LEVELS

A strong wind over irregular terrain produces low-level turbulence which increases in depth and
intensity with increasing wind speed and terrain irregularity.

In a well developed wave system, the rotor zone and the area below are strongly turbulent and
reversed flow is often observed at the surface. Strong winds confined to the lower troposphere,
with reversed or no flow in the middle and higher troposphere, produce the most turbulent
conditions at low levels. These are sometimes accompanied by rotor streaming, comprised of
violent rotors which are generated intermittently near lee slopes and move downwind. These low-
level travelling rotors are distinct from the stationary rotors which form at higher levels in
association with strong mountain waves.

TURBULENCE IN THE ROTOR ZONE

Rotors lie beneath the crests of lee waves and are often marked by roll-cloud. The most powerful
rotor lies beneath the first wave crest down-stream of the mountains. Rotors give rise to the most
severe turbulence found in the air flow over high ground. On occasions it may be as violent as
that in the worst thunderstorms.

TURBULENCE IN WAVES

Although flight in waves is often remarkably smooth, severe turbulence can occur. The transition
from smooth to bumpy flight can be abrupt. Very occasionally, violent turbulence may result,
sometimes attributed to the wave breaking.

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Windshear and Turbulence Chapter 19

TURBULENCE AT HIGH LEVELS (NEAR AND ABOVE THE TROPOPAUSE)

TURBULENCE NEAR THE JET STREAM
Turbulence in jet streams is frequently greatly increased in extent and intensity over high ground.
Strong vertical windshears are often concentrated in a few stable layers just above and below the
core of the jet stream. Distortion of these layers when the jet stream flows over high ground,
particularly when mountain waves form, can produce local enhancements of the shears so that
the flow in those regions breaks down into turbulence. Usually the cold side of the jet stream is
more prone to turbulence, but mountain waves may be more pronounced on the warm side.

TURBULENCE IN THE STRATOSPHERE
Flight experience shows that in the stratosphere, moderate or severe turbulence is encountered
over high ground about four times more frequently than over plains and about seven times more
frequently than over the oceans.

DOWNDRAUGHTS

Whether or not a well developed wave system exists, if the air is stable a strong surface air flow
over high ground produces a substantial and sustained downdraught and/or turbulence on the lee
side. Such downdraughts may, on occasion, be strong enough to defeat the rate of climb
capability of some aircraft. In a wave system, a series of downdraughts and updraughts exists,
the most powerful being those nearest the high ground.

ICING

Adiabatic cooling caused by the forced ascent of air over high ground generally results in a
lowering of the freezing level and an increase of liquid water concentration in clouds. Thus, when
extensive cloud is present, airframe icing is likely to be more severe than at the same altitude
over lower ground. This hazard is at a maximum a few thousand feet above the freezing level, but
in general is unlikely to be serious at altitudes above 20 000 ft except in cumulonimbus clouds.

FLYING ASPECTS

The effects of the airflow over high ground on aircraft in flight depends on the magnitude of the
disturbance to the airflow; in other words, the altitude and the aircraft’s speed and direction in
relation to the wave system. A broad distinction may be made between low-level hazards (below
about 20 000 ft) and high-level hazards (above 20 000 ft).

LOW ALTITUDE FLIGHT

The main hazards arise in low altitude flight from severe turbulence in the rotor zone, from
downdraughts and from icing. The presence of roll clouds in the rotor zone may warn pilots of the
region of most severe turbulence, but characteristic cloud formations are not always present or, if
they are present, may lose definition in other clouds. Similarly, the updraughts and downdraughts
are, in general, not visible. If an aircraft remains for any length of time in a downdraught, which
may be remarkably smooth (e.g. by flying parallel to the mountains in the descending portion of
the wave), serious loss of height may occur.

During upwind flight, the aircraft’s height variations are normally out of phase with the waves; the
aircraft is, therefore, liable to be at its lowest height when over the highest ground. The aircraft
may also be driven down into a roll-cloud over which ample height clearance previously appeared
to be available.

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Chapter 19 Windshear and Turbulence

Downwind flight may be safer. Height variations are usually in phase with waves, but it must be
appreciated that the relative speed of an accidental entry into the rotor zone is greater than in up-
wind flight because the rotor zone is stationary with regard to the ground. Thus, the structural
loads imposed on the airframe when gusts are encountered are likely to be greater, and there will
probably be less warning of possible handling difficulties.

HIGH ALTITUDE FLIGHT

The primary danger at high altitude is that of a sudden encounter with localised disturbances
(i.e. turbulence or sudden large wind and temperature changes) at high penetration speeds. This
is particularly relevant at cruising levels above FL 300 where the buffet-free margin between the
Mach number for 1g buffet and the stall is restricted. In this respect, flight downwind is likely to be
more critical than flight up-wind, especially when the wind is strong.

As in the case of low altitude flight, the waves are stationary relative to the ground. The higher the
relative speed on accidentally encountering a standing wave while flying downwind, the greater
the likelihood of greater loads on the airframe. There is often no advance warning of wave activity
from preliminary variations in flight instrument readings, or from turbulence. Although
downdraughts are present, they are probably not hazardous and icing and rotor zone turbulence
are unlikely.

INVERSIONS

Inversions on the leeward side of a mountain range can prevent the down-slope wind from
reaching the ground. A very powerful shear is generated from about 300 ft up to 1500 ft above
the ground. When the downdraught moves over the inversion, a low level jet forms.

Fresh winds over a mountain but light winds at the airport on the leeward side of the mountain
indicate strong low-level windshear.

MARKED TEMPERATURE INVERSION

The marked temperature inversion occurs during cloudless nights due to terrestrial radiation. The
situation is enhanced if the aerodrome is situated in a valley. A pocket of cold air is trapped under
higher warm air. A low level jet can form just below the top of a strong radiation inversion on clear
nights.

At certain airfields, a warning of marked temperature inversion is issued when a temperature
difference of 10°C or more exists between the surface and any point up to 1000 ft above the
surface.

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Windshear and Turbulence Chapter 19

REPORTING TURBULENCE

CAT remains an important operational factor at all levels of flying but particularly above FL 150.
Pilots encountering CAT are requested to report time, location, level and intensity, and aircraft
type to the appropriate ATS unit. This is done as a Special Observations Report. The criteria
required are:

INCIDENCE
¾ OCCASIONAL — less than 1/3 of the time.
¾ INTERMITTENT — 1/3 to 2/3.
¾ CONTINUOUS — more than 2/3.

INTENSITY
LIGHT
¾ Light Turbulence — IAS fluctuates 5 - 15 kt, turbulence that momentarily causes
slight erratic changes in attitude and/or altitude.
¾ Light Chop — Turbulence that causes slight rapid rhythmic bumping without
appreciable changes in altitude or attitude. No IAS fluctuations.
¾ Reaction Inside Aircraft — Occupants may feel a slight strain against seat belts or
shoulder straps. Unsecured objects may be displaced slightly. Food service may be
conducted and little or no difficulty is encountered when walking.

MODERATE
¾ Moderate Turbulence — IAS fluctuates 15 - 25 kt, turbulence that is similar to light

turbulence but of greater intensity. Changes in altitude and/or attitude can occur but
the aircraft remains in positive control at all times.
¾ Moderate Chop — Turbulence that is similar to light chop but of greater intensity.
Rapid bumps or jolts without appreciable changes in altitude or attitude. IAS may
fluctuate slightly.
¾ Reaction Inside Aircraft — Occupants feel definite strains against seat belts or
shoulder straps. Unsecured objects are dislodged. Food service and walking are
difficult.

SEVERE
¾ Severe Turbulence — IAS fluctuates more than 25 kt; turbulence that causes large,

abrupt changes in altitude and/or attitude. The aircraft may be momentarily out of
control.
¾ Reaction Inside Aircraft — Occupants are forced violently against seat belts or
shoulder straps. Unsecured objects are tossed about. Food service and walking
impossible.

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Chapter 19 Windshear and Turbulence

19-18 Meteorology

INTRODUCTION

Air pressure varies considerably between positions on the Earth’s surface. These pressure
differences are important to the Earth’s weather and winds. On the meteorological charts the
pressure pattern is shown by isobars, enclosing areas of different pressure.

LOW, CYCLONE OR DEPRESSION, AND TROUGH

For a low pressure system, the isobars are generally closely spaced which results in windy
weather. The centre of the low pressure system experiences calm winds. Convergence occurs
and air is forced upward and cools adiabatically. If the air is humid, condensation occurs and
clouds form.

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Chapter 20 Non-Frontal Pressure Systems

In the Northern Hemisphere, the wind follows a left-hand circuit parallel to the isobars. Friction
acts as a brake on the wind in the friction layer, and the wind blows in toward the low pressure
centre. The result is a general lifting of the air within the low pressure area. For the Southern
Hemisphere the rotation is in the opposite direction.

In a low-pressure area, convective movement is strengthened, and CB are likely to form if the air
is unstable. If the air is stable but humid, clouds form. In this, extensive stratiform cloud layers
form. Visibility at low levels is generally better than in an anticyclone, due to a stronger mixing of
the air.

LOW PRESSURE TYPES

As in the case of the high pressures, there are two basic types of lows, warm and cold.

Dynamic Low Cold low, the low deepens at altitude and the winds are
Thermal Low increasing.

Warm low, the low weakens aloft and turns into a high pressure.

SECONDARY DEPRESSION

The secondary depression forms in the circulation of a larger primary depression. The secondary
depression can be frontal or non-frontal depending upon how it forms. Secondary depression
movement depends upon the hemisphere.

¾ In the Northern Hemisphere, a secondary depression moves anti-clockwise around
the primary depression.

¾ In the Southern Hemisphere, a secondary depression moves clockwise around the
primary depression.

The secondary depression can form:

¾ On the tip of an occlusion.
¾ On unstable waves on a trailing cold front.
¾ Inside the primary depression circulatory system.

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Non-Frontal Pressure Systems Chapter 20

The life cycle and weather patterns associated with secondary depressions are similar to those of
a primary depression. As the secondary depression deepens, the depression may become the
dominant feature. In this case, the old primary depression becomes the secondary depression
and starts circulating around the new primary depression. This process is known as “dumb
belling.”

The weather in a secondary depression is often more severe than in a primary depression.

The worst weather associated with a non-frontal secondary depression usually occurs on the side
of the secondary furthest from the primary.

ICELANDIC LOW

The Icelandic low, as shown on the January and July mean value surface pressure charts below,
is a dynamic system.

The adiabatic cooling (due to the expansion of the air) leads to extensive clouds in the low
pressure area. In temperate latitudes there is a transport of unstable cold air in the northern and
western areas of the low, while there is an airflow of more stable warm air in the southern and
eastern areas. Showers are more frequent in the north-western parts of the low. Apart from the
showers, visibility is good.

If the lifted air is humid, extended AS and AC with embedded areas of light rain can form.

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Chapter 20 Non-Frontal Pressure Systems
JANUARY

JULY

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Non-Frontal Pressure Systems Chapter 20

THE ORIGIN OF LOW PRESSURES AND WEATHER

In aviation, dynamic and thermal classifications are rarely used. Normally depressions are
classes as Non-Frontal or Frontal:

Examples of non-frontal depressions are:

¾ Orographic lows ¾ Mediterranean low
¾ Thermal lows ¾ Polar low
¾ Summer lows over land or ¾ Baltic Sea low
¾ Monsoon low ¾ Cold air pool
¾ Equatorial low or trough ¾ Tropical revolving storm
¾ Instability lows ¾ Easterly waves
¾ Winter lows over sea ¾ Whirlwind or Tornado

The last three items are discussed in Chapter 27 — Tropical Storms and Tornadoes.

OROGRAPHIC OR LEE SIDE LOWS OR TROUGHS

If a current of air flows perpendicular to a mountain range, the barrier will force the air to
compress on the windward side and over the mountain. The air on the leeward side of the
mountain seems to “stretch.” There will be a tendency for anticyclonic curvature over the
mountain with closely spaced pressure surfaces, and on the lee-side there will be a clearly visible
cyclonic curvature.

Falling air pressure on the leeward side forms a depression. This is known as a lee-depression or
a lee-trough. The lee-trough is usually stationary if the airflow remains the same and no
deepening low forms.

The lee-low causes the pressure surfaces to slope down towards the mountain and become
closely packed over the mountain.

666

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Chapter 20 Non-Frontal Pressure Systems

On the leeward side, foehn winds prevail, and the weather typically is fine. Humid air may be
sucked into a lee trough giving clouds and sometimes precipitation.

When a cold front encloses warmer air on the leeward side of the mountain, rapid development of
the system occurs. The low deepens and intense cumulonimbus clouds form.

A cold front may be activated/intensified when passing a range of mountains.

When the cold air sweeps around the sides of the mountain and across it, the warm air on the
leeward side acts as a warm sector and a wave forms on the front.

This wave normally develops rapidly which leads to an occlusion-like process, and storms move
away from the mountain.

The most severe Orographic lows that form over north-west Italy and affect the Mediterranean
form when the western Alps stop cold fronts.

¾ The lower portion of the cold front is slowed by the mountains.
¾ The slope of the frontal surface increases.
¾ Eventually the cold air spills over into the warm air on the lee side of the Alps.
¾ The warm air is undercut by the cold air causing severe instability.

Similar phenomena appear on the Skagerak when the Scandinavian mountains impede cold
fronts. In this particular case, air sweeps around the southern edge of the mountains, giving
strong winds. Humidity and temperature increase in the air that travels around the mountain at
low levels. Travelling over a relatively warmer water surface and causing increased instability,
rapid cyclonic development on the lee side occurs, often giving clusters of showers.

THERMAL DEPRESSIONS

Thermal depressions form over warm surfaces. The heated air rises through convection and
turbulence. A high pressure aloft is formed causing an outflow of air at height. The air pressure at
the surface begins to decrease and a circulation similar to the sea breeze occurs.

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Non-Frontal Pressure Systems Chapter 20

An influx of air occurs at the surface low and an ascending motion is generated, strengthening the
convective clouds in the area, if any.

The most predominant thermal depressions are:

¾ The monsoon low pressures in Asia.
¾ The Equatorial low pressure belt.
¾ The summer lows in south-western USA.
¾ The lows of north-east Africa.

Less intense thermal lows are common on the weather charts in the summer, especially over
France and Spain. These smaller cyclones are shallow and do not affect weather to any greater
extent.

In the winter, thermal depressions can form over “warm” water surfaces such as the Baltic Sea,
the Skagerak, the Black Sea, and the Mediterranean. These are referred to as instability lows.

If the air is dry, thermal lows bring good flying weather with some cloud and moderate to good
visibility.

If the air is humid, however, convective CB are likely to form, and heat thunderstorms or squalls
will also appear. This is a common feature in France and on the Iberian Peninsula. Thermal lows
generated in these areas may drift towards north-western Europe and Scandinavia.

INSTABILITY LOWS

If large scale organised convection occurs in an area where there is already a lee low, a
development may take place that looks similar to a thermal low. This is an instability low.

The same process that created the thermal low also influences the instability low. A significant
amount of the energy is derived from the released latent heat of the condensation process.
According to the hydrostatic equation, heating causes the distance between two pressure
surfaces to increase. As a result a high pressure is generated aloft resulting in an outflow of air
and falling pressure at the ground. If divergence already exists at height, the effect will be
strengthened and a rapid pressure fall can occur at the surface level. This generates a spiral flow
in toward the centre.

Instability lows can be very intense, particularly in the Tropics. In mid-latitudes the humidity
content is low and the lows are thus less intense.

MEDITERRANEAN LOW

A typical winter low that forms over the sea when cold polar air reaches the warm Mediterranean
water. A separate low forms in which clusters of convective cloud are found.

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Chapter 20 Non-Frontal Pressure Systems

POLAR LOWS

Instability lows often form when cold polar or arctic air moves south over a gradually warmer sea
or major water area. Common from November to March in the Northern Hemisphere sea areas.
The air transforms due to an intense heating and vapour increase in the lower levels resulting in
intense convection caused by the southerly travel of the airmass.

Between the two highs there is a tendency to a cyclonic airflow and the formation of lee-lows off
the south-eastern coast of Norway. When the cold air reaches the warmer water, small intense
instability lows develop.

A similar type of instability low forms in the winter on the Bay of Genoa, generated when the cold
Mistral wind sweeps down over the warm Mediterranean.

BALTIC SEA CYCLONES

If energy is released in an area where a low pressure already exists, the low deepens or
intensifies. This is common in the autumn on the Baltic Sea, when lows from the continent in the
south and east move out over relatively warm water. It can also occur when a low has passed
Scandinavia from the west.

More precipitation and lower cloud bases than predicted in a forecast affect the Baltic Sea isles
and coasts. Heavy northerly squalls may develop.

CELLS OF COLD AIR ALOFT (COLD POOLS)

The general theories about the long waves encircling our globe, separating the cold polar from
the warm tropical air, are discussed in Climatology. The waves are usually zonal moving from
west to east along a latitude line. Cold air outbreaks can cut off from the main stream air and
generate a pool of cold air at height in a position south of the normal Polar front. This cold pool
can remain for several days constituting a potential area of instability at height.

In the summer, thermal lows form over the continents and may develop into instability lows. This
happens when cold air is carried in over the low (by the upper airflow) or when a cold pool
already exists at height. In these conditions, the atmosphere becomes unstable, and a major area
of thunderstorms may develop.

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Non-Frontal Pressure Systems Chapter 20

ANTICYCLONE OR HIGH, AND RIDGE OR WEDGE

A high pressure system is an area enclosed by isobars that decrease in value with distance from
the centre.

NATURE OF A HIGH
Isobars are normally well spaced resulting in light winds. Where a high is adjacent to a low the
pressure gradient can become steep, leading to moderate or strong winds.

In a high pressure cell, mass convergence at height and divergence at low levels creates
subsidence within the core of the anticyclone with an outflow at low level. The subsidence is
checked above the ground, due to the thermal mixing in the surface layer and a subsidence
inversion is formed. The height of the subsidence inversion depends on the intensity of the
anticyclone, the degree of thermal mixing, and the distance from the core.

Inversions form from 2000 to 5000 ft in cold anticyclones and up to FL 100 in warm anticyclones.

Above the friction layer, in the Northern Hemisphere, the wind blows in a right-hand circuit parallel
to the isobars. In the friction layer, friction slows the wind and it blows at an angle out from the
centre. The outflow at the bottom of the high leads to a sinking motion of air, which is compressed
and adiabatically heated. The subsidence inversion forms, the temperature rises significantly and
the humidity decreases. In the Southern Hemisphere, the rotation is reversed.

The air above the inversion is dry, while the air below may or may not be dry depending on the
circumstances that prevail. Air pollution collects below the inversion, and this leads to a drop in
visibility at the lowest levels. If the inversion persists, clouds can form in the inversion.

At high latitudes, the increased loss of terrestrial radiation due to the drying at height creates
nocturnal inversions at the surface. Large areas with SC and ST may form in maritime air
masses.

In the winter these clouds can persist for several days.

Where the humidity is high and the lower levels are cold, fog forms below the subsidence
inversion.

In the summer, or at lower latitudes, SC often dissipates during the day and returns at night. If the
air below the subsidence inversion is unstable or conditionally unstable, CU may form below the
inversion during the day.

In continental air masses, the humidity content is low, but visibility is still limited below the
inversion. If the air passes over a major water feature, moisture is rapidly absorbed and cloud
forms.

Maritime airmasses dry out with an extensive passage over a major land surface.

The weather above the subsidence inversion is normally fine; cloudless with good visibility.

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Chapter 20 Non-Frontal Pressure Systems

HIGH PRESSURE SYSTEMS

There are two main high pressure systems, depending on whether they consist of warm or cold
air.

SUBTROPICAL HIGHS (WARM ANTICYCLONES)

Subtropical highs are formed by air from the equatorial regions travelling away from the equator
at altitudes around the Equatorial tropopause. They are deflected by Coriolis which generates a
subtropical jet stream and an accumulation of air around the 30º latitude. At low levels the air
pressure increases and there is an outflow of air from the system. In the subtropical high,
subsidence from aloft occurs. The subsidence inversion in these cells is sometimes called a
Trade Inversion.

These anticyclones are often stationary or move in a seasonal manner and are therefore referred
to as permanent highs.

Europe’s nearest subtropical anticyclone is the Azores High, which is the source region of
maritime tropical air.

The air below the subsidence inversion is humid and unstable; above it is dry and stable. CU
dominates the weather below the inversion.

The height to the inversion varies within the high pressure cell. The highest values are found in

the western areas nearest the Equator (5000 − 7000 ft) and the lowest in the north-eastern areas

(1500 − 2000 ft). Tropical showers are more likely to develop in the western part of an ocean than
in the eastern.

As the low level air travels away from the equator, the humidity increases. The sea temperature
decreases and the air is cooled from below. In winter, this frequently leads to vast areas of low
clouds, drizzle, and fog over NW Europe.

In summer, the anticyclone occasionally intensifies over the North Atlantic. This causes lows and
the associated rain areas to move in a wide arc north of Scandinavia, forming a blockage
(a “blocking high”) with dry and sunny weather over western Europe.

CONTINENTAL HIGHS (COLD ANTICYCLONES)

Consisting of polar air, the cold anticyclone forms over the cold continents in the winter. They
seldom reach higher than 700 hPa (FL 100), but the horizontal extension may be considerable.

Thermal highs are not as stable as dynamic highs, and travelling depressions can break them
down.

The Siberian and the Canadian highs consist of, and are the source regions of, continental polar
air. In midwinter they also constitute the source region of arctic air from within the Arctic and
Antarctic permanent cold anticyclones.

If the pressure system spreads over a coastal area, there will be convection and snow showers
over the open water surface with fog and mist below the inversion inland.

If the air is dry and there is no advection from open water, the weather can be cold, bright, and
cloudless. In clear and extremely cold areas, ice fog or diamond dust may form.

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Non-Frontal Pressure Systems Chapter 20

HIGH PRESSURES AND HIGH PRESSURE RIDGES (OR WEDGES) IN
SERIES OF TRAVELLING DEPRESSIONS

The third type of high pressure forms between the lows of a family of depressions.

The ridges, or temporary high, forms as cold air sweeps behind a frontal low. This type of high is
thermal, and as a consequence it is not visible on an upper air chart.

High pressure ridges follow low pressure systems in their movements and constitute a break in
the storms associated with the frontal systems of the lows.

The ridge can be subdivided into three weather zones:

¾ Ahead of the axis of the ridge (just behind the cold front)
¾ Along the axis of the ridge
¾ Behind the ridge (in front of the next warm front)

There is a high risk of showers, often troughs with low pressure and line squall
showers/thunderstorms well ahead of the ridge axis. CB turns to CU and SC closer to the ridge
axis. In winter, terrestrial radiation from the Earth is high, and nocturnal radiation fog is likely to
form if the wind is light. ST or SC form if the wind is stronger at the border of the ridge/cold high.
When the ridge passes, the air is humidified in the prevailing SW wind, which again leads to
increased cloud with CU and SC at lower levels while the frontal cloud deck thickens at height.

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Chapter 20 Non-Frontal Pressure Systems

20-12 Meteorology

TYPES OF SERVICE

PRE-FLIGHT BRIEFING

The primary method of meteorological briefing for flight crews is self briefing. An alternate method
for obtaining information is from the Meteorological Information Self Briefing Terminal (MIST).
Where the primary method is not available, then special forecasts are often provided.

If the personal advice of a forecaster is required, then information is only given on the
understanding that full use is made of all available information.

Note: Meteorological observations and forecasts have certain expected tolerances of
accuracy.

METEOROLOGICAL CHARTS

Meteorological information is available on various charts which are routinely transmitted over the
METFAX network to major aerodromes. They provide information under the following headings:

Low and medium level flights within the UK and to near Europe
Surface Weather Chart for: Surface − 15 000 ft amsl. (Form 215)
Spot Wind/ Temperature Chart for 1000 ft − 24 000 ft amsl, (Form 214)

Medium and high level flights to Europe and the Mediterranean
Significant Weather/Tropopause/Maximum Wind Charts for FL 100 − FL 450.
Upper Wind and Temperature Charts for FLs: 50, 100, 180, 240, 300, 340, 390
and 450.

High level flights to North America
Significant Weather/Tropopause/MaximumWind Chart for FL 250 − FL 630.
Upper Wind and Temperature chart for FLs: 180, 240, 300, 340, 390, 450 and
530.

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Chapter 21 Meteorological Observations and Meteorological Services

High level flights to Middle/Far East
Significant Weather/Tropopause/MaximumWind Chart for FL 250 − FL 450.
Upper Wind and Temperature chart for FLs: 180, 240, 300, 340, 390, 450 and
530.

High level flights to Africa, The Caribbean and South America
Significant Weather/Tropopause/MaximumWind Chart for FL 250 and above.
Upper Wind and Temperature charts for FLs: 180, 240, 300, 340, 390, 450 and
530.

Other area charts or additional Flight Level information, which are not routinely available by
METFAX, are requested from the LONDON/Heathrow forecast office, subject to available
METFAX capacity. Amended charts are issued when forecast conditions change significantly.

BROADCAST TEXT METEOROLOGICAL INFORMATION

The following reports are broadcast by teleprinter:

METAR
Aerodrome meteorological reports. METARs are routinely broadcast every ½ hour during
aerodrome opening hours. Exceptionally they are sometimes broadcast every 1 hour.

TAF
Aerodrome Forecasts. FC denotes a TAF valid for a period less than 12 hours, usually 9
hours, which is issued every 3 hours. FT denotes a TAF valid between 12 and 24 hours
which is issued every 6 hours. Amendments are broadcast between routine times as
required.

SIGMET
Warnings of weather significant to flight safety these are available for areas within 1000
nm of the UK.

The following are additions that may be added to the METAR:

¾ Short term landing forecasts (TREND), which are valid for 2 hours.
¾ Information on runway state when weather conditions require and continue until

conditions cease. Special Aerodrome Meteorological Reports are issued when
conditions change through specific limits.

SPECIAL AERODROME METEOROLOGICAL REPORTS (SPECI)

Special Aerodrome Meteorological Reports are issued when conditions change significantly.
Selected Special Reports (SPECI) are defined as Special Reports disseminated beyond the
aerodrome of origin. The UK does not normally issue Selected Special Reports.

TERMINAL AERODROMES FORECAST (TAF)

TAFs are normally provided only for those aerodromes where official meteorological observations
are made. For other aerodromes, Local Area Forecasts are made. Amended TAFs or Local Area
Forecasts are issued when forecast conditions change significantly.

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Meteorological Observations and Meteorological Services Chapter 21

SPECIAL FORECASTS AND SPECIALISED INFORMATION

For departures from an aerodrome where the standard pre-flight meteorological briefing is
inadequate for the intended flight, a special forecast may be issued. Normally a Special Flight
Forecast is supplied from the last UK departure point to the first transit aerodrome outside the
coverage of standard documentation. By prior arrangement, forecasts are provided for other legs
if the initial ETD to final ETA does not exceed 6 hours and no stops longer than 60 minutes are
planned.

Forecast offices normally require prior notification for special forecasts. For flights up to 500 nm at
least 2 hours is required before the time of collection. For flights over 500 nm at least 4 hours is
required before the time of collection.

SIGMET SERVICE

Aircraft can be supplied with information in flight. MWOs are responsible for the preparation and
issue of SIGMETs to the appropriate ATC unit. Aircraft in flight are warned of the occurrence or
expectation of one or more of the following SIGMET phenomena for the route ahead, for up to
500 nm or 2 hours flying time:

a. At Subsonic Cruising Levels (SIGMET)
i. Thunderstorm (See Note)
ii. Heavy hail (See Note)
iii. Tropical cyclone
iv. Freezing rain
v. Severe turbulence (not associated with convective cloud)
vi. Severe icing (not associated with convective cloud)
vii. Severe mountain waves
viii. Heavy sand/dust storms
ix. Volcanic ash cloud

Note: Thunderstorm does not refer to isolated or occasional thunderstorms not embedded in
cloud layers or concealed by haze. This refers only to thunderstorms widespread, including if
necessary CB which is not accompanied by a TS, within an area:

With little or no separation FRQ
Along a line with little or no separation SQL
Embedded in cloud layers EMBD
Or concealed in cloud layers or concealed by haze OBSC

TS and tropical cyclones each imply:

Moderate or severe turbulence
Moderate or severe icing and hail
Heavy hail HVYGR is used as a further description of the TS as necessary

b. At Transonic and Supersonic Cruising Levels (SIGMET SST) 21-3
i. Moderate or severe turbulence
ii. Cumulonimbus cloud
iii. Hail
iv. Volcanic ash cloud

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Chapter 21 Meteorological Observations and Meteorological Services

In general SIGMET messages are identified by the letters WS at the beginning of the header line.
Tropical Cyclones and Volcanic Ash will be identified by WC and WV respectively.

AIRCRAFT REPORTS

SIGMETs are not usually valid for more than 4 hours, except volcanic ash clouds where the
period is upwards of 12 hours. SIGMETs are sequentially numbered through the day

Flight levels for SIGMET SST are as follows:
¾ FL 250 − FL 600 London and Scottish UIRs
¾ FL 400 − FL 600 Shanwick OCA

ROUTINE AIRCRAFT OBSERVATIONS

Routine aircraft observations are not required in the London/Scottish FIR/UIR. In the Shanwick
OCA, aircraft are to conform with the requirements laid out in the ENR section or applicable
NOTAM.

SPECIAL AIRCRAFT OBSERVATIONS

Special observations are required in any UK FIR/UIR/OCA when:

a. Severe turbulence or severe icing is encountered.
or

b. Moderate turbulence, hail or cumulonimbus clouds are encountered during transonic
or supersonic flight.
or

c. Any factors which a pilot believes affects the safety of flight are encountered.
or

d. When requested by the meteorological office.
or

e. When there is an agreement between the meteorological office and the aircraft
operator.

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Meteorological Observations and Meteorological Services Chapter 21

CLEAR AIR TURBULENCE (CAT)

CAT remains an important operational factor at all levels of flying but particularly above FL 150.
Pilots encountering CAT are requested to report time, location, level, intensity, and aircraft type to
the ATS unit they are operating with. This is done as a Special Observations Report. The
criteria required are:

INCIDENCE
¾ OCCASIONAL — less than 1/3 of the time.
¾ INTERMITTENT — 1/3 to 2/3.
¾ CONTINUOUS — more than 2/3.

INTENSITY
LIGHT
¾ Light Turbulence — IAS fluctuates 5 − 15 kt, turbulence that momentarily causes
slight erratic changes in attitude and/or altitude.
¾ Light Chop — Turbulence that causes slight rapid rhythmic bumping without
appreciable changes in altitude or attitude. No IAS fluctuations.
¾ Reaction Inside Aircraft — Occupants may feel a slight strain against seat belts or
shoulder straps. Unsecured objects may be displaced slightly. Food service may be
conducted and little or no difficulty is encountered when walking.

MODERATE
¾ Moderate Turbulence — IAS fluctuates 15 − 25 kt, turbulence that is similar to light

turbulence but of greater intensity. Changes in altitude and/or attitude can occur but
the aircraft remains in positive control at all times.
¾ Moderate Chop — Turbulence that is similar to light chop but of greater intensity.
Rapid bumps or jolts without appreciable changes in altitude or attitude. IAS may
fluctuate slightly.
¾ Reaction Inside Aircraft — Occupants feel definite strains against seat belts or
shoulder straps. Unsecured objects are dislodged. Food service and walking are
difficult.

SEVERE
¾ Severe Turbulence — IAS fluctuates more than 25 kt; turbulence that causes large,

abrupt changes in altitude and/or attitude. The aircraft may be momentarily out of
control.
¾ Reaction Inside Aircraft — Occupants are forced violently against seat belts or
shoulder straps. Unsecured objects are tossed about. Food service and walking
impossible.

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Chapter 21 Meteorological Observations and Meteorological Services

AIRFRAME ICING

Any pilot encountering unforecast icing is requested to report the time, location, level, intensity,
icing type, and aircraft type to the ATS unit they are operating with. The following are reporting
definitions, and are not necessarily forecasting definitions.

Trace
Ice becomes perceptible. Rate of accumulation slightly greater than the rate of
sublimation. It is not hazardous even though 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
1 hour. Occasional use of de-icing/anti-icing equipment removes/prevents
accumulation. It does not present a problem if anti-icing equipment is used.

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.

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

AERODROME CLOSURE

The term SNOCLO is added to the end of an aerodrome report in a VOLMET radio broadcast
when it is unusable for take-off or landing due to heavy snow on the runways, or the runway is
blocked for snow clearance.

IN-FLIGHT PROCEDURES

An in-flight enroute service is available in exceptional circumstances by prior arrangement with
the meteorological office. Make applications for this service in advance stating:

1. The flight levels and route sector required.
2. The period of validity required.
3. The approximate time and position the request will be made.
4. The ATS unit the aircraft expects to be in contact.

Aircraft can obtain aerodrome weather information from any of the following sources:

¾ VOLMET broadcasts.
¾ Automatic Terminal Information Service (ATIS) broadcasts as described in the GEN

section.
¾ By request to an ATC unit.
¾ If an aircraft proposes to divert to an aerodrome for which no forecast is provided, the

commander may request the relevant information from the ATS unit serving the
aircraft.

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