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For Helicopter Pilots
Aviation Weather
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1 The Basics Page 11.1 The Atmosphere 1
1.2 The Atmosphere extends further above the Equator than the Poles 11.3 Subdivision of the Atmosphere 1
1.4 Composition of the Atmosphere 2
1.5 Humidity 2
1.6 Air Density 2
1.7 The Sun, Our Source Of Energy 2
1.8 Orbiting Around The Sun Effects Our Temperature 2
1.9 Heating From Solar Radiation Is Greater In The Tropics 3
1.10 Warm air rises, Cool air sinks 3
1.11 Terrestrial Radiation 3
1.12 Rotation of the earth on its axis 3
2 Temperature and Heat 42.1 Different surfaces heat differently 4
2.2 Cloud cover and its effect on surface heating and cooling 4
2.3 Transfer of heat energy 4
2.4 The Sea breeze 5
2.5 The land breeze 5
2.6 Katabatic Wind 6
2.7 The Anabatic Wind 6
2.8 Temperature is a measure of heat energy 6
2.9 Temperature Inversions 6
3 Atmospheric Pressure 7
3.1 Atmospheric Pressure 73.2 Atmospheric pressure can be measured by using: 7
3.3 Pressure Gradients 8
3.4 International Standard Atmosphere 8
4 Wind 94.1 What is wind? 9
4.2 How wind is defined 9
4.3 Veering and Backing 9
4.4 What causes a wind to blow? 9
4.5 The Pressure Gradient Force 10
4.6 Coriolis Force 10
4.7 The Geostrophic Wind 11
4.8 From High to Low Look Out Below. 11
4.9 Gradient wind blows around curved isobars 11
4.10 Surface winds 12
4.11 Diurnal Variation of the Surface Wind 12
4.12 Localised friction effects 12
4.13 Flight in turbulence 12
4.14 Windshear 13
4.15 Wind associated with Mountains 14
4.16 Wind in the Tropics 14
4.17 Microbursts 14
4.18 The Tropopause and wind 14
4.19 Jet Streams 15
4.20 Polar Front Jets 164.21 Other Jet Streams 17
4.22 Clear Air turbulence 17
Table of Contents
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5 Clouds Page 185.1 Cloud 18
5.2 Three States of Water 19
5.3 Latent Heat 19
5.4 Humidity 20
5.5 Relative Humidity 20
5.6 Wet and Dry Bulb Thermometer 20
5.7 Dew Point 205.8 Adiabatic Processes 21
5.9 Stable Air 21
5.9.1 Dry 21
5.9.2 Saturated 21
5.9.3 Absolute Stability 22
5.10 Unstable Air. 22
5.10.1 Dry 22
5.10.2 Saturated 22
5.10.3 Absolute Instability 22
5.11 Cloud formed by convection due to heating 23
5.12 Cloud formed by orographic uplift 23
5.13 Foehn wind 24
5.14 Cloud formed by Turbulence and mixing 24
5.15 Cloud formed by widespread ascent 24
5.16 Precipitation associated with cloud 24
5.17 Thunderstorms 25
5.17.1 Three Conditions Necessary For A Thunderstorm To Develop 25
5.18. Life Cycle Of A Thunderstorm 25
5.18.1 The Cumulus Stage 25
5.18.2 Mature Stage 25
5.18.3 Dissipating Stage 26
5.19 Dangers From A Thunderstorm 26
6 Air Masses and Fronts 27
6.1 Air Masses And Frontal Weather 276.2 Origin Of An Air Mass 27
6.3 Track Of An Air Mass 27
6.4 Convergence And Divergence 27
6.5 Types Of Air Masses That Affect Ireland and the British Isles 28
6.5.1 Typical Characteristics of Air Masses affecting Ireland and the British Isles 28
6.6 The Warm Front 29
6.6.1 The Warm Front As Seen By An Observer On The Ground 29
6.6.2 The General Characteristics Of A Warm Front 29
6.6.3 The Warm Front As Seen By A Pilot 30
6.7 The Cold Front 30
6.7.1 The General Characteristics Of A Cold Front 30
6.7.2 The Passage Of A Cold Front As Seen By An Observer On The Ground 316.7.3 The Cold Front As Seen By A Pilot 31
6.8 The Occluded Front 31
6.8.1 The characteristics of an Occluded Front 31
6.9 Depressions - Areas Of Low Pressure 32
6.9.1 The Three-Dimensional Pattern Of Airflow Near A Depression 32
6.9.2 Weather Associated With A Depression 32
6.9.3 Troughs Of Low Pressure 32
6.10 The Wave Or Frontal Depression 32
6.11 The Tropical Revolving Storm 32
6.12 Anticyclones Areas Of High Pressure 33
6.12.1 The three-dimensional flow of air associated with an Anticyclone 33
6.12.2 Weather Associated With A High 33
6.12.3 Ridge Of High Pressure 33
6.13 A Col. 33
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7 Icing Page 347.1 The Formation Of Ice 34
7.1.1 Icing Can Be Hazardous To Aviation 34
7.2 Supercooled water drops 34
7.3 Icing In Cloud 34
7.4 Hoar Frost 35
7.4.1 Synoptic situations which favour the formation of hoar frost 35
7.5 Rime And Glazed Or Clear Ice 357.5.1 Rime Ice 35
7.5.2 Clear Ice 36
7.6 Cloudy Or Mixed Ice 36
7.7 Intake or Impact Ice 36
7.8 Fuel Icing 36
7.9 Carburettor Icing 37
7.10 Points To Remeber 37
7.11 Icing of the pitot-static system 38
7.12 Ambient Conditions Conducive To The Formation Of Induction System Icing 38
8 Visiblity 39
8.1 Visibility 398.2 Slant Visibility 39
8.3 Runway Visual Range 40
8.4 Eye Observations By Day 40
8.5 Fog, mist and haze 40
8.6 Radiation Fog 40
8.7 Advection Fog 40
8.8 Eye Observations By Night 41
8.9 Upslope fog 41
8.10 Sea Fog 41
8.11 Smoke Pollution 41
8.12 Frontal Fog 41
8.13 Dust and sand 41
8.14 Precipitation And Visibility 428.15 Precipitation And Visual Perception 42
9 Weather Sources & Information 439.1 Aeronautical meteorological offices 43
9.2 Aerodrome meteorological offices 43
9.3 Meteorological services at aerodromes 43
9.4 Availability of periodic weather forecasts 43
9.5 Weather Information for Flight Planning 43
9.6 Special observations 43
9.7 Reports and forecasts for departure 43
9.7.1 En-route, destination and alternate(s) 43
9.8 Weather Forecasts and Reports 449.8.1 Special Forecasts 44
9.8.2 Aerodrome Forecasts (TAFs) 44
9.8.3 METARs 44
9.8.4 Trends (or Landing Forecasts) 44
9.8.5 VHF In-Flight weather Reports 44
9.9 Cloud Bases 44
9.10 CAVOK 45
9.11 Changing Weather in Forecasts 45
9.11.1 Temporary Change (TEMPO) 45
9.11.2 Lasting Changes 45
9.11.3 Probabil ity 45
9.12 Availability Of Ground Reports For Surface Conditions 45
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9.13 In Flight Meteorological Information Page 45
9.13.1 ATlS 45
9.13.2 VOLMET 46
9.13.3 Special Aerodrome Reports (SPEC) 46
9.13.4 SIGMET 46
9.13.4.1 Meteorological Abbreviations Used In Sigmets, Special Forecasts Etc 46
9.14 Weather Charts 47
9.14.1 Station Circle 47
9.14.2 Significant Weather Chart 48
9.14.3 Upper Wind Chart 49
9.15 Example of METARs and Short TAFs 50
9.16 METAR decoder 51
9.17 TAF decoder 52
Glossary of Terms 54
Index 61
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1.1 The Atmosphere.The Earth is a solid object, which a mixture of gases
surrounds. The force of gravity holds these gases to
the earth. We know this mixture of gases as AIR, and
the space that it occupies around the earth as the
Atmosphere.
1.2 The Atmosphere Extends Further
Above The Equator Than The Poles.The earth spins about its axis, carrying the
atmosphere with it and tending to throw the air to the
outside. Consequently, the atmosphere extendsfurther into space above the equator than the poles.
1.3 Subdivision of the AtmosphereThe atmosphere is divided vertically into four regions:
Troposphere;
Stratosphere;
Mesosphere, and the;
Thermosphere.Light aircraft fly in the Troposphere. High altitude jets
cruise in the Stratosphere. The boundary between the
two regions is known as the Tropopause. The
Tropopause occurs at a height of approximately20 000ft over the poles and at approximately 60 000ft
over the tropics.
In the "average" International Standard
Atmosphere the Tropopause is assumed to occur
at 36090ft.
Most of our "weather" occurs in the Troposphere.
Significant differences exist between the Stratosphere
and the Troposphere:
In the Troposphere temperaturedecreases with height (@ 1.98oC per
1000ft, up to 36 090ft) were it is constant
at -56.5oC in the Stratosphere.
There is a mark vertical movement of airin the Troposphere. Warm air rising and
cool air descending, on both large and
small scales.
Nearly all the water vapour in theatmosphere is contained in the
Troposphere. Cloud formation rarely
extends beyond the Tropopause. However
occasionally large cumulonimbus clouds
with strong and fast vertical development
may push into the Stratosphere.
1 Walker 2000 Aviation Weather
1. The Basics
Fig 1.1
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1.4 Composition of the Atmosphere.The atmosphere, or 'AIR' is a mixture of gases that
carries water vapour. Nitrogen is the largest
component, other components are;
Nitrogen 78% By Volume.
Oxygen 21%
Other Gases 1%Water Vapour
. . . . . . . . . Total 100%
Oxygen is needed to support life and combustion and
water vapour to produce weather. All air contains
some water vapour. It is the water vapour that
condenses out to form clouds, from these we get
precipitation (rain, hail, snow, etc.) which is so vital to
life on Earth. Maritime Air (air over an ocean) will
absorb moisture from the body of water and overall will
contain more water vapour than Continental Air(air
over a continent) particularly if the continent consists
mostly of deserts.
1.5 Humidity.Water molecules are very light molecules and their
presence in large numbers in 'AIR' lowers its density,
which affects the aerodynamic performance and the
power production from the engine of an aircraft.
Performance on a damp day will be poorer than on a
dry day. Carburettor icing is more likely on a day that
has high relative humidity, this is cause by the air
expanding as it cools while mixing with vapourising
fuel. The water vapour condenses out and sticks tothe carburettor casing as ice.
1.6 Air Density.Air Density Decreases With Altitude. The force of
gravity exists between each individual air molecule
and the Earth. This causes the air molecules to draw
closer together, particularly near the Earths surface
where they become very crowded. If we look at a
cubic metre of air at the surface, it will have twice the
molecules than a cubic metre of air at 40 000ft.
The Density (or mass per unit volume) of air at sealevel is 1225 grammes per cubic metre.
Why Is Air Density So Important To Pilots?
The required lift force can be generated ata lower true air speed.
More engine power is available.
Breathing is easier and more oxygen istaken into the lungs.
1.7 The Sun, Our Source Of EnergyThe Sun radiates electromagnetic energy and we
experience this energy as light and heat. These
wavelengths (short wave) of solar radiation are such
that a large percentage penetrates the Earth's
atmosphere and is absorbed by the Earth's surface.
This causes the temperature of the Earth's surface to
rise. The ground in turn heats any part of the
atmosphere that is in contact with or very close to it,
this causes any parcel of air that is warmer than the
surrounding air to rise.
1.8 Orbiting Around The Sun EffectsOur Temperature.The Earth's axis is tilted, and as it orbits around the
Sun the earth receives differing amounts of solar
radiation, this causes our four seasons. The solar
radiation received at a place in Summer is more
intense due to the surface being presented at a less
oblique angle.
Walker 20002 Aviation Weather
Fig 1.2
Fig 1.3
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2.1 Different surfaces heat differently.The temperature and the way different surfaces heat
depend upon several things.
1. The specific heat of a surface. Water
requires more heat energy than land to
raise its temperature by 10oC. Land will
heat more quickly by day and cool faster by
night. Compared with a large body of water
the land nearby will be warmer by day and
cooler by night. Water is said to have a
higher specific heat than land.
2. Reflectivity of a surface. When solar
radiation is reflected from a surface, it
cannot be absorbed. Areas covered by
snow or water will have a high reflectivity
and will not be heated as much as an area of
land, such as a ploughed field or dense
jungle.
3. Conductivity of a surface. Currents in the
ocean transfer heat through the motion of
the water, heating it to a greater depth than
a land surface.
2.2 Cloud cover and its effect on
surface heating and cooling.Cloud cover prevents solar radiation penetrating the
Earth's surface, which results in reduced heating of
the Earth and lower temperatures. Air in contact with
the surface is subjected too much less heating. At
night cloud cover will prevent heat from escaping into
the upper atmosphere and cause the atmosphere
below the cloud to have a higher temperature.
2.3 Transfer of heat energy.
Heat energy can be redistributed in a body ortransferred to another body by several means:
1. Radiation. All bodies transmit energy as
electromagnetic radiation. The higher the
temperature of the body, the shorter the
wave length. The Sun emits short wave
radiation and the Earth long waves.
2. Absorption. Any body in the path of radiation
will absorb some of its energy. The amount
depends upon the nature of the body and the
radiation. Densely forested areas will absorbmore solar radiation than snow-covered
mountains.
2. Temperature and Heat
I really need to fix my airconditioner
Fig 2.1
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3. Conduction. Heat energy may pass within
one body or from one body to another body in
contact with it. Metals are good conductors of
heat. Wood and air are not good conductors
of heat. A parcel of air heated at the Earths
surface by conduction will not transfer this
heat energy to a neighboring parcel of air.
This is a very significant factor in the
production of weather systems.
4. Convection. A body in motion carries its heat
energy with it. A parcel of air heated at the
Earth's surface will expand, become less
dense and rise. As it rises it will carry its heat
energy higher into the atmosphere.
5. Advection. As the air heated by convection
rises cooler air will move in to replace it, this
occurs in a horizontal plane. The body of air
will bring, with it, its own heat and moisture.
The general vertical circulation pattern of air flow that
occurs on a large scale around the Earth also happens
on a much smaller scale in localised areas.
2.4 The Sea breeze.The process known as a Sea Breeze occurs on sunny
days when the land is heated more quickly than the
sea causing the air over the land mass to becomewarm, expand and lose density. This warmed air will
then start to rise (convection). The cooler air from the
sea will move in to replace the warm air that has risen
(advection). The vertical extent of a sea breeze is
approximately 1000 to 2000 feet. Sea breezes may
affect operations of airfields near the coast. This
would be as windshear, or turbulence as the aircraft
passes over one body of air to another. Cooler air
moving in over warm land may cause fog or mist,
reducing visibility.
2.5 The land breeze.At night the land cools more quickly than the sea
causing the air above it to cool and subside. The air
over the sea is warmer and will rise. The effect of a
land breeze could hold sea fog offshore during the
night, but as the land warms during daylight a sea
breeze could develop and bring the fog inshore,
causing visibility problems.Fig 2.3
Fig 2.5
Fig 2.2
Fig 2.4
Fig 2.6
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2.6 Katabatic Wind.During night time the Earth loses much heat through
terrestrial radiation and cools down. This is particularly
noticeable on clear, cloudless nights. The air in
contact with the ground loses heat to it by conduction,
and cools down, becomes more dense and sinks. In
mountainous regions the cool air will flow down the
side of the mountain and into valleys, creating aKatabatic wind. These winds can reach speeds of 30
knots down the slopes of mountains. They will die out
as the solar radiation warms the surface and it starts
to re-radiate the energy.
2.7 The Anabatic Wind.Heating of a mountain slope during the day causes the
air mass in contact with it to warm, decreasing its
density and causing it to rise along the mountainslope. The force of gravity opposes the flow up the hill,
making the Anabatic Wind weaker than the night time
Katabatic Wind.
2.8 Temperature is a measure of heat
energy.
As a body of matter absorbs heat energy, itsmolecules become agitated. This agitation is
measured as temperature, which is used to measure
heat energy. The temperature at which no molecular
agitation occurs is called absolute zero and is
measured using a scale called Kelvin:
0oKelvin = -273oCelsius.
Temperature is measured using different scales
around the world. There are several scales of
temperature measurement is use around the world.
The scale most commonly used in aviation is the
CELSIUS scale. This divides the temperature that
water boils and freezes at 100 units. Using the Celsius
scale water boils at 100oC and freezes at 0oC. Some
countries still use the FAHRENHEIT scale, where
water boils at 212oF and freezes at 32oF.
There is a requirement for the pilot to be able to
convert from one scale to the other. The easiest and
most convenient way is to use the temperature
conversion scale on the flight computer. There aremathematical formulae that the pilot should commit to
memory;
1. Celsius to Fahrenheit.
oF = 9/5 x oC + 32
2. Fahrenheit to Celsius.
o
C = 5/9 x (oF - 32 )
2.9 Temperature Inversions.The general pattern of temperature distribution in the
atmosphere is that temperature decreases with
height. The rate at which this decrease takes place is
approximately 2oC per 1000 feet climbed in a
stationary air mass. On clear nights when the Earth
loses a great deal of heat by terrestrial radiation and
cools down, the air in contact with its surface also
cools by conduction. This cooler air sinks and does not
mix with the air at higher levels. This leads to the air at
the surface being cooler than the air above, creating a
temperature inversion. The inversion may exist for
only tens of feet or maybe hundreds of feet. There areby products of a temperature inversion important to a
pilot. Windshear or a ground fog.
Walker 20006 Aviation Weather
Fig 2.7 Fig 2.9
Fig 2.8
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7 Walker 2000 Aviation Weather
3.1 Atmospheric Pressure.The molecules making up the air move at very high
speeds and in random directions. They bounce off any
surface that they encounter, and the force they exert
on that surface we call Atmospheric Pressure. There
are fewer air molecules at higher altitude and less
weight of molecules pressing down from above.
Therefore, atmospheric pressure decreases with
height. An aircraft flying at 25 000 feet or a town in the
mountains at 5000 feet will experience a lower
pressure than at sea level.
3.2 Atmospheric pressure can be
measured by using:
1. Mercury Barometer.Atmospheric pressure
at sea level support a column of 26 inches of
mercury by pushing it into a partial vacuum;
or an
2. Aneroid Barometer.A flexible metal
chamber that is partially evacuated is
compressed by the atmospheric pressure.
This method is used in aircraft altimeters,
where changes in atmospheric pressure are
measured and converted to read changes in
the altitude.
At sea level on a standard day the atmospheric
pressure is 1013.2 hPa. Until recently the unit of
measurement for the atmospheric pressure was the
millibar, this has changed to the hectoPascal, there is
no difference between the two units. As height is
gained above sea level in the lower levels of the
atmosphere, the pressure drops at a rate of
approximately 1hPa per 30 feet of height gained. The
altimeter in an aircraft is calibrated to show this
pressure drop in feet above sea level or feet aboveground, depending on the pressure datum being used.
3. Atmospheric Pressure
Fig 3.2
Fig 3.1
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Walker 20008 Aviation Weather
The atmospheric pressure at a particular place is
continually changing, these changes maybe;
1. Irregular; due to the passage of pressuresystems, intensifying or weakening.
2. Regular; due to diurnal temperature variation
caused by the heating and cooling effects of
the Sun. These are known as semi-diurnal
variations of pressure.
3.3 Pressure Gradients.Readings are taken at many locations and converted
to sea level values for comparison purposes. The
places that are experiencing the same calculated sea
level pressures are then joined on a map with lines.
These lines are called isobars. These lines form
patterns on weather charts that are very meaningful.
Some will define areas of low pressure. Some will
define areas of high pressure. Others will be straight.
Variation of pressure over a horizontal distance is
called the Pressure Gradient. This occurs at right
angles to the isobars. Closely packed isobars will give
a rapid change in pressure, the pressure gradient is
said to be steep or strong then. Loosely packed or
widely spread isobars will give a flat or weak pressure
gradient. The natural tendency is for air to travel from
an area of high pressure to an area of low pressure,
the steeper the pressure gradient the stronger the flowof air. However, the flow is not directly from high to low
but somewhat modified due to the Earth's rotation.
When flying from one area or region, the pilot should
monitor the subscale setting on the altimeter. If the
aircraft is travelling from an area of high pressure
to an area of low pressure and the altimeter is not
reset for the new pressure the altimeter will over
read. The aircraft will be descending although the
altimeter still reads the correct height. The reverse
applies when flying from low pressure to high pressure
areas.
3.4 International Standard Atmosphere.A datum is needed to measure the actual atmosphere
against. This datum is called the International
Standard Atmosphere, and has been devised using
specific values against which everything relating to the
atmosphere is measured. The International Standard
Atmosphere is based on the following mean sea level
values:
i. Pressure = 1013.2 hPa;
ii. Temperature = +15 oC;
iii. Density 1225 gm / cubic metre;
iv. Lapse rate = 1.98 oC per 1000 feet up to
36 090 feet (Tropopause) where
temperature remains at -56.5oC;
v. Pressure falls at approximately 1 hPa per
30 feet.
In reality the actual atmosphere differs from ISA in
many ways. Sea level pressure varies from day to day,
even hour to hour. Temperature fluctuates betweenwide extremes at all levels. The variation vertically and
horizontally of ambient pressure affects the operation
of the altimeter.
Fig 3.6
Fig 3.5
Fig 3.3
Fig 3.4
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9 Walker 2000 Aviation Weather
4.1 What is wind?The term wind refers to the flow of air over the
Earths surface. This flow is practically all horizontal
with only about 1/1000th being vertical. The vertical
flow is important however to pilots as it is this flow that
creates cumuliform clouds and thunderstorms. The
vertical flow in can be of such strength that it can
destroy aircraft. When referring to wind in aviation we
are referring to the horizontal flow of air.
4.2 How wind is defined.There are two components of wind, its strength and
direction.
i. Wind direction is the direction from which the
wind is blowing. This is expressed in degrees
measured clockwise from North.
ii. Strength, which is expressed in knots (kt).
The two, direction and strength, together describe the
wind velocity. This is usually written in the form
300/30, i.e., wind from 300o at 30 knots.
A meteorologist relates wind direction to True North,
so all winds that appear on forecasts are in os (T).
Runways are aligned in their magnetic direction.
Winds are very important to aircraft when taking off
and landing, so winds given to the pilot from the tower
will be expressed in degrees magnetic.
4.3 Veering and Backing.When a wind direction changes it is said to have
veered or backed. These terms relate to a clockwise
or counter clockwise change in direction. If wind
changes in a clockwise direction (090/10 to 120/10) it
is said to have veered. Wind that has changed in a
counter clockwise direction is said to have backed
(270/10 to 200/10).
4.4 What causes a wind to blow?A change in velocity (speed or direction or both) is
called acceleration. Acceleration is caused by a force(or forces) acting on an object. The net or resultant
force acting on an object is the combined effect of all
the forces acting on that object. If all the forces acting
on an object, balance each other so that the resultant
force equals zero then the object will not accelerate. It
will continue to move at the same speed or remain
stationary. A steady wind velocity is called a balanced
flow. The forces that cause wind to blow are;
i. The pressure gradient force;
ii. The Coriolis force.
4. Wind
Fig 4.1
Fig 4.2
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4.5 The Pressure Gradient Force.The force that is usually responsible for getting a
parcel of air moving is the pressure gradient force.
This force acts by moving air from areas of high
pressure to areas of low pressure. On meteorology
charts the places of equal pressure are joined by
isobars. The pressure gradient force acts at right
angles to these lines of equal pressure moving the airfrom the high to the low pressure. The stronger the
pressure gradient (the greater the pressure difference
over a given distance) the greater the force will be, the
stronger the wind will blow. If the Pressure Gradient
force was the only force acting on a parcel of air, it
would continue to accelerate toward the low pressure.
Getting faster and faster and eventually the high and
low pressure areas would disappear because of the
transfer of air.
4.6 Coriolis ForceThis we know is not so, so another force must exist.
The other force working here is the Coriolis Force
cause by the rotation of the Earth. It is this force that
prevents the air from rushing from the high straight
into the low pressure area. The Coriolis Force is not
a real force but an apparent force that acts on a parcel
of air moving over the rotating Earth.
Imagine a parcel of air that is stationary over the point
A on the Equator. It is in fact moving with point A as
the Earth rotates on its axis from west to east. Now
suppose that a pressure gradient exists with a high
pressure at A and a low pressure at, directly North of
A. The parcel of air starts moving toward B, but still
with its motion toward the east due to the Earth's
rotation. The further North one goes the less is the
easterly motion of the Earth and so the earth will lag
behind the easterly motion of the parcel of air. Point B
will have moved to B1, but the parcel of air will have
moved to A2. The parcel of will have appeared to
turned right. This effect is due to Coriolis Force.
If the parcel was being accelerated Southerly from a
high pressure in the north toward a low pressure nearthe Equator, the Earths rotation would appear to get
away from the parcel of air as it travels south. The
parcel of air would again appear to have turn right,
having moved from B to B2 west of A1. The faster the
airflow the greater the Coriolis effect, no air flow
means no Coriolis effect. The Coriolis effect is greater
in higher latitudes toward the poles, where changes in
latitude cause more significant changes in speed at
which each point is moving toward the East. In the
Northern hemisphere the Coriolis effect deflects the
wind to the right and the reverse occurs in the
Southern hemisphere.
Coriolis force acts to the right in the Northernhemisphere.
Walker 200010 Aviation Weather
Fig 4.3Fig 4.4
Fig 4.5
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11 Walker 2000 Aviation Weather
4.8 From High to Low Look Out Below.If an aircraft is experiencing starboard drift when
flying, in the Northern hemisphere, the wind is from the
left and therefore, according to Buys Ballotss Law, the
aircraft is flying toward an area of low pressure. Low
pressure often has poor weather associated with it,such as low cloud, rain and poor visibility. Unless the
pilot resets the altimeter to the lower QNH the
altimeter is going to over read, not good.
When the aircraft experiences left drift this means that
it is heading into an area of higher pressure (applying
Buys Ballotss Law), higher pressure often suggests
more stable air and generally better weather (although
fog may occur). If the pilot does not reset the altimeter
to the higher regional QNH the altimeter is going to
under read.
4.9 Gradient wind blows around curved
isobars.Isobars are usually curved, for a wind to flow parallel
to these isobars it must be accelerated, in the sense
that its direction is being changed. In order for the air
to curve into the turn it must have a force acting on itto pull it into the turn. For a wind that is blowing around
a LOW (counter clockwise) in the Northern
hemisphere, the net force results from the Pressure
Gradient being greater than the Coriolis force, thereby
pulling the air flow into the LOW. For a wind blowing
around a HIGH (clockwise) in the Northern
hemisphere, the net results from the Coriolis force
being greater than the Pressure Gradient. Since the
Coriolis force increases with wind speed, it follows that
the wind around a HIGH will be faster than those
around a LOW with the same isobaric spacing.
4.7 The Geostrophic Wind.Two forces act on a moving airstream;
i. Pressure gradient;
ii. Coriolis force.
The pressure gradient force gets air moving and the Coriolis effect turns it right. This curving of the airflow over the
Earth's surface will continue until the pressure gradient force is balanced by the Coriolis force. Resulting in a wind
flow that is steady and blowing in a direction parallel to the isobars, this balanced flow is called the GeostrophicWind.
The Geostrophic wind flows in a direction parallel to the isobars with the low pressure on the left, and at a strength
directly proportional to the spacing of the isobars (proportional to the pressure gradient). The closer the isobars the
stronger the wind.
Fig 4.6
Fig 4.8
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In the Northern Hemisphere the result is a wind
flowing parallel to the isobars, clockwise around a high
(anticyclonic motion) and counter clockwise (cyclonic
motion) around a low. Balanced wind flow around
curved isobars is called the Gradient wind.
4.10 Surface winds.
Surface wind is the horizontal movement of air closeto the ground. It is measure by placing wind indicators
or wind socks at 30ft above the surface in flat open
spaces. Surface winds are important to pilots as they
directly affect takeoffs and landings.
Wind near the surface is usually less in strength than
winds at higher levels. The Gradient wind that flows
parallel to the isobars at higher levels is slowed by the
friction that exists between the lower level air and the
surface of the Earth. Coriolis effect is decreased due
to the slower wind speed, and the wind will back as a
result. The rougher the surface the greater the wind
will slow. Flat areas, like, deserts or oceans will affect
the wind less than hilly or city areas with many
obstructions.
A reduced wind speed results in a reduced Coriolis
force (since it depends upon speed). Therefore the
Pressure Gradient force will have more of an effect in
the lower levels, causing the wind to flow toward the
area of Low pressure and out toward the area of High
pressure. Instead of flowing parallel to the isobars.
The surface wind tends to back compared to the
Gradient wind. Over oceans the surface wind may
slow by one-third of the gradient wind and back by100. Over land the surface wind may slow by two-
thirds and back by 300. Frictional forces due to the
Earths surface decreases rapidly with height and are
negligible above 2000 feet above the ground level
(agl). The turbulence created by rough surfaces also
fades out at approximately the same
level.
4.11 Diurnal Variation of the Surface
Wind.Heating of the lower level air during the day will
promote vertical movement of this air. This causesmixing of the various level layers of air and the effect
of the Gradient wind will be brought closer the ground.
The surface wind by the day will resemble the
Gradient wind more closely than by night. The day
surface wind will be seen as a stronger wind that has
veered as compared to the night surface wind.
At night the mixing of the layers is reduced. The
Gradient wind will continue to blow at altitude, but its
effect will not be mixed with the air flow at the surface
to such an extent as during the day. The night wind at
surface level will drop in strength and the Corioliseffect will weaken. Compared with the day wind the
night wind will drop in strength and back in direction.
4.12 Localised friction effects.The surface wind will bear no resemblance to the
Gradient wind at 2000 feet agl and above if it has to
blow over and around obstacles such as hills, trees,
buildings, etc. The wind will form turbulent eddies.
The size and strength will depend upon both the size
of the obstacle and the wind strength.
Walker 200012 Aviation Weather
Fig 4.10
Fig 4.11
Fig 4.9
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4.13 Flight in turbulence.To a small degree some turbulence is always present
in the atmosphere and as pilots, one quickly becomes
used to its effect. Moderate to severe turbulence,
however, is uncomfortable and can be dangerous to
the aircraft and its occupants. Severe turbulence can
cause condition that may over stress the aircraft.
Vertical gusts will increase the angle of attack on arotor blade, causing an increase in the Lift generated
for that particular airspeed. If the angle of attack is
increased beyond the critical angle, the rotor blade will
stall. Load factor (or g-force) is a measure of the
stress on an aircraft and each category of aircraft is
built to take only a certain Load Factor. It is important
that these Load Factors are not exceeded. One
means of achieving this is to fly the aircraft at
"turbulence penetration speed" which is much slower
than the normal cruise speed.
When turbulence is encountered:
i. Slow, reduce air speed.
ii. Manipulate the controls to maintain a
steady altitude.
iii. Avoid flying close to hills or objects that
will create more turbulence.
iv. If turbulence is strong land as soon as
possible.
Avoiding turbulence is better, and to some extent
this is possible:
i. Do not fly if moderate to severe turbulence
is forecast.
ii. Avoid flying underneath, in or near
thunderstorms.
iii. Avoid flying under large cumulus cloud.
Large up drafts produce them.
iv. Do not fly in the lee of hills when strong
winds are blowing, they tumble over ridges
and create turbulence that your aircraft will
not be able to out perform.
v. Do not fly low over rough ground when
strong winds are blowing.
4.14 Windshear.Windshear is the variation of wind speed and/or
direction from place to place. Windshear is generally
present to some extent when an aircraft is
approaching the ground for landing, because of the
different speed and direction of the surface wind
compared with the Gradient wind aloft. Low level
windshear can be quite marked at night or in early
morning when there is little mixing of the lower layers,
for instance when an inversion exists. Windshear can
be expected when a Sea Breeze or a Land Breeze is
blowing, or near a Thunderstorm. Cumulonimbus
clouds have enormous updrafts associated with them.The effects of these can be felt up to 10 to 20 NM
away from the actual cloud. Windshear and
turbulence associated with a Thunderstorm can
destroy aircrafts.
Fig 4.12
Fig 4.13
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Walker 200014 Aviation Weather
4.15 Wind associated with Mountains.Wind that flows over mountains and down the lee side
can dangerous to aviation, not only because of the
turbulence, but because the aircraft must climb into it
to maintain the altitude. Wherever possible maintain
several thousand feet clearance when flying over
mountainous regions during strong winds.
Other winds that can be dangerous are, Katabatic
winds that flow down cool mountain slopes at night
and early morning, Valley winds. Valley winds can
change direction by as much as 180o to flow down a
valley. The valley acts as a venturi, and increases the
speed at which the wind flows. Large mountains or
mountain ranges cause an effect on wind that may
extend well above ground level, resulting in Mountain
Waves (Standing waves) possibly with associated
lenticular clouds.
The up-currents and down-currents associated with
mountain waves can be quite strong and may extend
for 30 to 40 NM downwind of the mountains. Rotor
areas may form beneath the crests of the nearer lee
waves, and are often characterised by Roll Cloud.
There may be severe turbulence in the rotor zone.
4.16 Wind in the Tropics.In tropical areas, Pressure Gradients are generally
weak and so will not cause the air to flow at high
speeds. Local effects, such as Land and Sea Breezes,
may have a stronger influence than the Pressure
Gradient. The Coriolis force that causes the air to flow
parallel to the isobars is very weak in the tropics since
the distance from the Earths axis remains constant.
The Pressure Gradient force, though relatively weak,
will dominate and the air will flow more from the high
pressure areas to the low pressure areas than parallel
to the isobars. Instead of using isobars (which join
areas of equal pressure) on tropical charts, it is more
common to use:
i. Streamlines to indicate wind direction,
which will be out-drafts from high
pressure, and in-drafts to low pressure.
ii. Isotachs, which are dotted lines joining
places of equal wind strength.
4.17 Microbursts.These are sudden local downdraughts from the base
of a thunderstorm; they hit the ground and spread out.
They are of particular concern when taking off or
landing. The speed out of the cloud is approximately
70 - 80 knots vertically down, this then spreads out in
all directions at a speed of approximately 60 knots
horizontally. Let us consider the take off case. The
pilot initiates the climb and is suddenly subjected to a
strong head wind; the aircraft's indicated airspeed
increases, the pilot slows the aircraft. The head wind
suddenly changes to a downdraught and the airspeed
is now low, also the aircraft is in a column of
descending air, the rate of climb could be zero. The
aircraft now experiences a tail wind. This could quite
easily take the aircraft below the stall speed and it
crashes. A similar outcome could be the result of
approaching to land through a microburst.
4.18 The Tropopause and windWhy is would a be pilot interested in the tropopause?
Temperature and wind vary greatly in the vicinity of the
tropopause affecting efficiency, comfort, and safety of
flight. Maximum winds generally occur at levels near
the tropopause. These strong winds create narrow
zones of wind shear which often generate hazardous
turbulence. Pre-flight knowledge of temperature, wind,
and wind shear is important to flight planning. The
tropopause is a thin layer forming the boundary
between the troposphere and stratosphere. Height ofthe tropopause varies from about 65,000 feet over the
Equator to 20,000 feet or lower over the poles. The
Fig 4.14
Fig 4.15
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15 Walker 2000 Aviation Weather
tropopause is not continuous but generally descends
step-wise from the Equator to the poles. These steps
occur as "breaks." An abrupt change in temperature
lapse rate characterises the tropopause. Note that the
temperature above the tropical tropopause increases
with height and over the polar tropopause,
temperature remains almost constant with height.
4.19 Jet StreamsWhen thermal effects are very strong jet streams can
form. These can be likened to a hollow flat tube
through which air passes at high speed. One open end
of the tube being an entry and the other an exit for the
passing air. The World Meteorology Organisation
defines a jet stream thus:
A strong narrow current concentrated along a
quasi-horizontal axis in the upper troposphere orstratosphere characterised by strong vertical and
lateral wind shears and featuring one or more
velocity maxima. The windspeed must be greater
than 60 kt.
Typical dimensions for a jet are 1500 nm long, 200 nm
wide and 12,000 ft deep.
The general shape together with typical isotach values
in a cross section diagram are shown in Fig 4.16. The
isotachs show that there are very strong windshears
on the cold or polar side of the jet and above the jetcore too. In these areas therefore and particularly on
the cold side there is strong clear air turbulence. Jet
streams in the troposphere have a general westerly
direction and speeds well above 100 kt are common.
In the region of east Asia and Japan speeds can be up
to 300 kt. There are two main locations for
tropospheric jet streams. These are the subtropical
jets and the polar front jets. In both cases the jet
streams form in the warm air below the tropical
tropopause.
4.20 Polar Front JetsAt the polar front there is the meeting between polar
and tropical air and therefore a strong north/south
thermal gradient. This produces a westerly jet stream
in both hemispheres in accord with Buys Ballot's Law.
The polar front is frequently separated into different
segments and contained within polar front
depressions. A cross section of a polar front low is at
Fig 17. The basic skeleton of this cross section is
shown at Fig 18 and it will be noted that there is adistinct increase in the mean temperature of a column
of air in the warm sector compared with the two
columns in the polar maritime air ahead of the warm
front and behind the cold front respectively. The
resulting strong thermal components cause jet
streams to form in the warm air below the tropical
tropopause. It will further be apparent that the jet is
more likely, or, likely to give a stronger wind, in
association with the cold front because of the shorter
horizontal distance between the warm and cold
columns.
Fig 4.16
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Walker 200016 Aviation Weather
The plan diagram of a polar front low at Fig 19 shows
that the two separate jets are in fact one jet stream
lying roughly parallel to the frontal surfaces. The
portion of the jet in association with the warm front is
some 400 nm ahead of the surface warm front position
and is parallel to the front. This results in a general
northwesterly jet. The jet behind the cold front surfaceposition by some 200 nm, is again parallel to the front
and usually from a generally southwesterly direction.
The isopleths of thickness are indicated to show the
thermal winds. Care should be taken when viewing
this plan diagram which can give the impression that
the jets are in the cold air. This is not the case as the
slope of the fronts allows the plan position to be
transposed to the warm air at height. With passage of
a polar front low from the west, the surface winds veer
from southerly through to northwesterly in the northern
hemisphere. The upper winds however back with
passage of a polar front low from the west, changingfrom northwesterly through westerly to southwesterly
behind the cold front. This can be appreciated from Fig
4.19. Polar front jets move with lows and are therefore
not as permanent as the subtropical jets. They are
more numerous and tend to be stronger in the wintermonths. The reasons are that there are more fronts in
winter and also during this season there are greater
mean temperature differences between the continents
and the oceans.
4.21 Other Jet StreamsBesides the main jet stream locations, there are upper
level winds well in excess of 60 kt in association with
increasing wind strength with increase of height, when
mountain waves are formed and these can spread into
the stratosphere. The zonal easterly winds in low
latitudes can sometimes produce tropospheric
easterly jets in the summer hemisphere, although
these tend to be fragmented in location. In the
stratosphere the easterly winds become jets with
speeds of 75 to 100 kn. These are positioned in bothhemispheres but are more pronounced in the summer
hemisphere.
Fig 4.19
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17 Walker 2000 Aviation Weather
4.22 Clear Air TurbulenceClear air turbulence is turbulence out of cloud which
does not include the turbulence in the friction layer.
Hence this is turbulence at all heights above a few
thousand feet. At the higher levels, CAT can cause
loss of control, stalling and airframe damage when it is
severe.
CAT is common in association with jet streams, for
around the boundaries of a jet, vertically and
horizontally, there are strong windshears in terms of
wind speed. The turbulence is more severe on top of
the jet and more particularly on the cold or polar side.
It is also more severe with stronger winds, with jets
which are curved and with those which occur above
and to the lee of mountain ranges. In this latter
instance the vertical movements caused by mountains
can speed up the jets and also enhance the shear in
speed.
Frontal jets can produce more severe turbulence than
the subtropical type because they move with the
movement of the front. This movement is roughly at
right angles to the direction of wind flow. The diagram
Fig 4.20 shows the different features of the turbulence
in association with jet streams and fronts.
Sharp directional windshears with upper level troughs
and sometimes with upper ridges can cause
turbulence. In these instances flight along the trough
or ridge line should be avoided if possible. The areas
of CAT are shown in Fig 4.21.
The same figure also shows turbulence in association
with CB cloud. The instability lifting inside the cloud
causes air from the sides to enter the uplift area
thereby causing turbulence all around the cloud.
There is often CAT above a CB cloud. This more
frequently occurs where the cloud tops have been
restricted due to the dryness of the air above.
Therefore the lifting is still present although the waterdroplets at the cloud top have evaporated. It has been
mentioned earlier in this chapter that clear air
turbulence must occur with mountain waves if the air
is dry and there is thus no cloud. A similar situation
applies with rotor streaming. Additionally there will be
CAT in association with the upper level jet stream
which occurs with mountain waves.
Where CAT occurs at high level (nominally above FL
150) and is not associated with cumuliform cloud or
thunderstorms it is reported as TURB. To reduce CAT
effects it is recommended that aircraft are flown at the'rough' air speed for the aircraft type and if possible
that areas where the terrain drops abruptly be
avoided. For the CAT associated with jet streams, with
a direct 'head-on' or 'tail-on' jet a change of flight level
or heading can be efficacious. For a cross track jet a
change of flight level only is worthwhile.
Fig 4.20
Fig 4.20
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5.1 Cloud.A cloud is the discernible assemblage of tiny water
droplets and/or ice crystals in the free air.
Classification of cloud types and individual clouds is
not straightforward. Clouds take on various forms,
many of which continuously change. It is important to
have an understanding of cloud classification as
meteorological forecasts and reports use this system
to give a picture of the weather for the Pilot.
Clouds are defined by four main groups:
1. Cirriform (fibrous)
2. Cumuliform (heaped)
3. Stratiform (layered)
4. Nimbus (rain bearing)
Clouds are further divided according to the level of
their bases above mean sea level, resulting in ten
basic types.
High Level Cloud. These clouds have a base above20 000ft and look fine and spidery. They are in a very
cold region, and are composed of ice crystals rather
than water particles.
1. Cirrus (Ci): Detached cloud in the form of
white delicate filaments. White patches or
narrow bands. These clouds have a fibrous
or silky appearance. They have little moisture
or turbulence and move across the sky with
little change of shape or form.
2. Cirrocumulus (Cc): Cirrus indicates high andcumulus indicates heaped or lumpy. Thin,
white patch, sheet or layer of cloud with no
shading, composed of very small elements in
the form of grain or ripples, joined together or
separate, and more or less regularly
arranged. These clouds are often referred to
as mackerel sky.
3. Cirrostratus (Cs): Cirrus indicates high and
stratus indicates sheets. Transparent veil of
fibrous or smooth appearance, totally or partly
covering the sky and generally producing a
halo effect around the Sun or the Moon.
Middle Level Cloud. These clouds have a base
above approximately 6500ft.
4. Altocumulus (Ac):Alto means middle level
and cumulus means heaped or lumpy. A layer
of cloud composed of flattened globular
masses or rolls. They are arranged in groups
or lines or waves which may be joined to form
a continuous layer or appear in broken
patches and are shaded either white or grey.
Forms a Corona around the Sun or Moon.
Vertical development of Ac may be sufficient
to produce precipitation in the form of Virga orslight showers.
5. Altostratus (As):Alto means middle level
and stratus means layer. Greyish or bluish
cloud sheet of fibrous or uniform appearance
totally or partly covering the sky and having
parts thin enough to reveal the Sun through
vaguely, possibly as though through ground
glass.
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5. Clouds
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19 Walker 2000 Aviation Weather
Low Level Cloud. These clouds have a base below
approximately 6500ft.
6. Nimbostratus (Ns): Nimbo means rain
bearing and stratus means layer. Sometimes
confused with As but is darker grey and has a
lack of a distinct lower surface. Dark grey
cloud layer generally covering the whole sky
and thick enough throughout to block the Sun
or Moon. The base is diffuse due to more or
less continuously falling rain or snow.
7. Stratocumulus (Sc.): Stratus means layered
and cumulus means heaped. Grey or whitish
patch or sheet of cloud which has dark parts
composed of rounded masses or rolls which
may be joined or show breaks between the
thicker areas. Associated weather if any, is
very light rain, drizzle or snow.
8. Stratus (St.): Stratus means layer. Greycloud layer with fairly uniform base. May give
precipitation in the form of drizzle. When the
Sun is visible through the cloud its outline is
clearly discernible.
9. Cumulus (Cu): Cumulus means heaped.
Detached clouds, generally dense and with
sharp outlines. Developing vertically in the
form of rising mounds, domes or towers, of
which the upper part often resembles a
cauliflower. The sunlit parts of these clouds
are mostly brilliant white while the base isrelatively dark as sunlight may not reach it.
Precipitation in the form of snow or rain may
occur with large Cumulus.
10. Cumulonimbus (Cb): Cumulo means
heaped and nimbus means rain bearing.
Heavy and dense cloud with considerable
vertical extent in the form of a mountain or
huge tower. At least part of the upper portion
is usually fibrous or striated, often appearing
as an anvil or vast plume. The base appears
dark and stormy. Low ragged cloud clouds
are frequently observed below the base and
generally other varieties of low cloud such as,
Cu, Sc are joined to or in close proximity to the
Cb. Lightning, thunder and hail are
characteristic of this type of cloud, while
associated weather with this type of cloud
may be moderate to heavy showers of rain,
snow or hail.
Above are the ten main cloud classifications, there are
certain variations that may be mentioned.
Stratus fractus and cumulus fractus observed asshreds or fragments below nimbostratus or
altostratus.
Castellanus, a number of small cumuliform clouds
sharing a common base and indicating the growth of
middle level clouds in an unstable atmosphere.
Lenticularis, lens-shaped clouds formed in standing
waves over mountains caused by strong winds aloft
and often associated with cumuliform cloud.
Noting the type of precipitation will help in determining
a particular type of cloud. Showers that start and stop
suddenly followed by clear skies only occur with
convective clouds such as Cumulus and
Cumulonimbus.
precipitation which usually starts and finishes
gradually over a long period is associated with
stratiform cloud.
Drizzle from Stratus and Stratocumulus, heavy
continuous rain or snow from Nimbostratus and rain
from Altostratus.
Cloud is formed from the water vapour contained in
the atmosphere. This water vapour is taken up into
the atmosphere by evaporation from oceans and other
bodies where water is present.
5.2 Three States of Water.Water can exist in three states, gas (vapour), liquid
(water) and solid (ice). Water as a vapour (gas) is not
visible, but when this vapour condenses out (liquid) it
forms water droplets which we see as cloud, fog, mist,
rain or dew. When water exists in its solid form (ice)we see it as snow, hail, frost and ice.
5.3 Latent Heat.
Any change of state involves a heat transaction withno change in temperature. The amount of heat energy
required to raise one gram of water one degree
Fig 5.1
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centigrade is one calorie. If 10 calories of heat energy
are applied to one gram of ice at -10oC the
temperature of the ice would rise to 0oC. A further 80
calories of heat energy are now required to change the
state of one gram of water from its solid to its liquid
form without changing the temperature. Having
changed the state of the water from its solid to its
liquid form, a further 100 calories are required to raise
the temperature to 100oC. To change the state of one
gram of water from liquid to vapour form, without
changing the temperature, will now require a further
540 calories if heat energy. The heat energy
required to change the state of the water from a
solid to a liquid, and then from a liquid to a gas,
without change of temperature, is known as latent
heat. This energy is store in the water and is released
as the water vapour changes back to liquid and then
to ice.
5.4 Humidity.The amount of water vapour present in the air is called
humidity, but the actual amount is not as important as
whether the air can support that water vapour or not.
5.5 Relative Humidity.When a parcel of air is supporting as much water
vapour as it can, it is said to be saturated, and has a
relative humidity of 100%. If it is supporting less water
vapour than its full capacity it is said to be
unsaturated, and its relative humidity will be less than
100%. Air that is supporting only 50% of its capacity
is said to have a relative humidity of 50%. There are
many ranges of relative humidity from 0% to 100%. Incloud and fog it is 100%, over a desert it may be 20%.
Relative Humidity is defined as the ratio of water
vapour actually in a parcel of air relative to what it
can hold at a particular temperature and pressure.
Temperature largely determines the maximum amountof water vapour air can hold. Warm air can hold more
water vapour than cold air.
5.6 Wet and Dry Bulb Thermometer.A wet and dry bulb system is used to determine the
surface air temperature, relative humidity and the dew
point. The wet bulb temperature is not the dew point
temperature, except when the air is saturated. The
dry bulb thermometer measures the temperature of
the free air. A wet bulb thermometer is a normal
thermometer, the bulb of which is wrapped in a singlelayer of muslin. It is kept continuously moist by
distilled water through a short wick. Any evaporation
is shown by a lower wet bulb temperature, due to the
extraction of latent heat of evaporation, from the bulb.
The drier the air, the greater the evaporation, and the
larger the amount of heat removed. A large difference
between dry and wet bulb temperatures therefore
indicates dry air, or low relative humidity. Identical
temperatures indicate no evaporation and therefore
saturated air or 100% relative humidity. Wet bulb
temperature may be defined as the lowest
temperature to which air may be cooled by the
evaporation of water.
5.7 Dew Point.Dew point is the temperature to which air must be
cooled to become saturated by the water vapour
already present in the air. Aviation weather reports
normally include the air temperature and dew pointtemperature. Dew point when related to air
temperature reveals qualitatively how close the air is
to saturation. The difference between air temperature
and dew point temperature is called the spread. As
spread becomes less, relative humidity increases, and
it is 100% when temperature and dew point are the
same. Surface temperature-dew point spread is
important for determining fog, but has little bearing on
precipitation. To support precipitation, air must be
saturated through thick layers aloft. Sometimes the
spread at ground level may be quite large, but at
higher altitudes the air is saturated and clouds form.Some rain may reach the ground or it may evaporate
as it falls into the dryer air.
Walker 200020 Aviation Weather
Fig 5.3
Fig 5.4
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21 Walker 2000 Aviation Weather
Relative humidity depends on both temperature and
water vapour. In this figure, water vapour is constant
but temperature varies. On the left, relative humidity
is 50%, the warmer air could hold twice as much water
vapour than is actually present. As the air cools,
centre and right, relative humidity increases. As the
air cools to the dew point, its capacity to hold water is
reduced to the amount actually present. Relative
humidity is 100% and the air is saturated.
Note at 100% humidity. Temperature and dew
point are the same. When the air cooled to
saturation, it cooled to the dew point.
5.8 Adiabatic Processes.
An adiabatic process is one in which heat is neitheradded nor removed from the system. The expansion
and compression of gases are adiabatic processes
where, although heat is neither added nor removed,
the temperature of the system may change, e.g.,
placing your finger over a bicycle pump will illustrate
that compressing air increases its temperature.
Conversely, air that is compressed and stored at room
temperature will feel cool if released to the
atmosphere and allowed to expand. Reducing
pressure will lower the temperature. A very common
adiabatic process that involves the expansion of a gas
and its cooling is when a parcel of air rises in theatmosphere. This can be initiated by the heating of a
parcel of air, causing it to expand and become less
dense than the surrounding air, hence it will rise. A
parcel of air can also be forced aloft as it blows over a
mountain range.
The change of temperature which occurs solely
because of change of pressure is known as
adiabatic heating or cooling as appropriate.
When considering adiabatic lapse rates it is assumed
that no heat energy will flow between the parcel of air
and the surrounding environment. Unsaturated air willcool adiabatically at about 3oC / 1000 ft as it rises.
This is known as the Dry Adiabatic Lapse Rate
(DALR). Cooler air can support less water vapour, so,
as a parcel of air rises and cools, its relative humidity
will increase. At the height where its temperature is
reduced to the dew point (100% relative humidity)
water will start to condense out and form cloud.
Above this height the now saturated air will continue to
cool as it rises, but, because latent heat is now given-
off as the water vapour condenses into the lower
energy state, the cooling will not be as great. The rate
at which saturated air cools as it rises is known as the
Saturated Adiabatic Lapse Rate (SALR) and may be
assumed to have a value of approximately half the
DALR, i.e., 1.5oC/1000 ft.
The rate of temperature change in the surrounding
atmosphere is called the Environmental Lapse Rate
(ELR) and its relationship to the DALR and SALR is a
main factor in determining the levels of the bases and
tops of clouds that form. The ISA assumes an ELR of
2oC / 1000ft.
5.9 Stable Air.Stable air is air which when displaced vertically
will tend to return to its original level.
5.9.1 Dry.
Consider the following simplified tephigram (Fig 5.7).
Dry air is lifted manually to 5000 ft. It will cool at the
DALR (3oC / 1000 ft). The parcel of air will therefore
cool from +15oC to 0oC in this example. The ELR (2oC
/ 1000 ft), the actual temperature of the surrounding air
is to the right of the DALR, this shows a temperature
of +5oC at 5000 ft. The parcel of air is cooler and moredense than its surrounding air and will sink back to its
original level when the lifting force is removed. This
air is STABLE.
5.9.2 Saturated.
The parcel of air (Fig 5.8) is saturated throughout the
ascent to 5000 ft and has cooled at the SALR
(1.5oC/1000 ft) from +10oC at mean sea level to 2.5oC
at 5000 ft. The ELR, is to the right of the SALR. The
temperature of the surrounding air is greater than the
parcel. The parcel of air is cooler and more dense
than its surrounding air and will sink back to its originallevel when the lifting force is removed. This air is
STABLE.
Fig 5.6
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5.9.3 Absolute Stability.
Fig 5.9 shows the ELR lying to the right of both the
DALR and the SALR lines. This means that the
atmosphere is stable regardless of whether or not the
parcel of air is dry or saturated when lifted. The parcel
of air will be cooler and more dense at the top of the
lifting force and will sink.
5.10 Unstable Air.Unstable air is air which when displaced vertically
will continue to ascend until it reaches air with an
equally low density.
5.10.1 Dry.
Fig 5.10 shows dry unstable air. The ELR line lies to
the left of the DALR line. The air will remain dry
throughout its ascent within the lifting layer. At the
upper limit of the lifting layer (5000 ft) the air is warmerand less dense than the surrounding air and it will
continue to rise seeking air of the same equally low
density.
5.10.2 Saturated.
Fig 5.11 shows saturated air. The ELR line lies to the
left of the SALR line. The air will remain saturated
throughout its ascent within the lifting layer. At the
upper limit of the lifting layer (5000 ft) the air is warmer
and less dense than the surrounding air and it will
continue to rise seeking air of the same equally low
density.
5.10.3 Absolute Instability.
Fig 5.12 shows absolute instability. The ELR line lies
to the left of both the DALR and SALR lines. The air
will continue to rise regardless of whether it is dry,
saturated or starts dry and becomes saturated during
its ascent within the lifting layer.
The type of cloud which forms depends upon the
Atmospheric Environment. The nature and extent of
any cloud which forms depends upon the nature of the
surrounding atmosphere through which it is
ascending. As long as the parcel of air is warmer than
its surroundings, it will continue to rise. An
atmosphere in which a parcel of air, when given
vertical motion, continues to move away from its
original level is called Unstable.
Cumiliform clouds may form in such an atmospheric
situation, the more moisture in the air, the higher its
dew point temperature. If the surrounding air is
warmer than the parcel of air it will stop rising as its
density is greater than its surroundings. Anatmosphere in which air tends to remain at the one
level is called Stable.
Walker 200022 Aviation Weather
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5.11 Cloud formed by convection due to
heating.Suppose that a parcel of air overlying a large ploughed
field heats up to +17oC whereas the air in the
surrounding area is only 12oC. The heated parcel of
air will start to rise, due to lower density, and cool at
the DALR of 3oC/1000 ft. If the ELR happens to be
1o
C/1000 ft, then the local area air through which theheated parcel is rising will cool at 1oC/1000 ft. The
moisture content of the parcel of air is such that it will
reach dew point when it cools to 11oC. By 2000 ft agl
the rising parcel of air will have reached saturation and
the water vapour will start to condense out and form
cloud. At 2000 ft agl the local area air will have cooled
to 10oC, the parcel of rising air is still warmer 11oC
(less dense) and will continue to rise. As the air
continues to rise above the level at which cloud first
started to form, latent heat will be given-off as more
and more vapour condenses into liquid water. The
rising air now cools at the reduced SALR of
1.5oC/1000 ft. Cooling at the new rate, the parcel of air
will have cooled to the same temperature 8oC as the
surrounding air at 4000 ft agl and will stop rising. This
produces a Cumulus cloud with base at 2000 ft and
tops of 4000 ft.
5.12 Cloud formed by orographic uplift.Air flowing over mountains rises and cools
adiabatically. If it cools below its dew point
temperature, water vapour will condense out and
clouds will form. Descending on the other side the air
warms adiabatically, once the air temperature exceeds
the dew point temperature the water vapour will no
longer condense out. The liquid water drops now start
to vapourise, and the cloud will cease to exist below
this level. A cloud that forms as a cap over the top of
a mountain is known as lenticular cloud. It will remain
more or less stationary whilst the air flows through it.
Sometimes when an air stream flows over a mountain
range and there is a stable layer of air above standing
waves occur. This is a wavy pattern as the air flow
settles back into more steady flow and, if the air is
moist, lenticular clouds may form in the crest of the lee
waves, and rotor clouds may form at a low level. The
level at which the cloud base forms depends upon the
moisture content of the parcel of air and its dew point
temperature. The cloud base may be below the
mountain tops or well above depending upon the
situation. Once having started to form the cloud may
sit low over the mountain as stratiform cloud (if the air
is stable) or as (if the air is unstable)the cloud will becumiliform and may rise to high levels.
Fig 5.13
Fig 5.14
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5.13 Foehn wind.If air rising up a mountain slope is moist enough to
have a dewpoint temperature and is cooled to it before
reaching the summit of the mountain, cloud will form
on the windward side, the air will now cool at the
SALR. If precipitaion occurs, moisture will be lost.
This will raise the dewpoint temperature causing the
cloud base to be higher on the lee side of the
mountain. As the air descends on the lee side it will
warm at the DALR which is at a greater rate than the
air cooling in the cloud. This results in a warmer, drier
wind on the lee side of the mountain known as a
Foehn Wind.
5.14 Cloud formed by Turbulence and
mixing.As air flows over the Earths surface friction causes
eddies to set up which causes mixing in the lower
levels. A very strong wind and rough surface will give
strong eddy currents a much deeper mixing layer. Theair in the rising currents will cool and if the turbulence
extends to a sufficient height and cooling to the airs
dew point may occur. Water vapour will condense out
into water droplets and clouds will form. The
descending air currents in the turbulent layer will
warm, if it warms above the dew point temperature the
liquid water droplets will return to vapour and the cloud
will not exist below this level. With turbulent mixing,
stratiform cloud may form over very large areas,
possibly with an undulating base. It may be
continuous Stratus or broken Stratocumulus.
5.15 Cloud formed by widespread
ascent.When two large air masses of differing temperature
meet, the warmer and less dense air will flow over or
be undercut by the cooler more dense air. As the
warmer air mass is forced aloft it will cool and if thedewpoint is reached cloud will form. The boundery
layer between the two air masses is called the Front
(see 6 Air Masses and Fronts).
5.16 Precipitation associated with cloud.Precipitation is falling water that reaches the ground.
This includes;
Rain consisting of liquid water drops;
Drizzle consisting of fine water drops;
Snow consisting of branched and star shapedice crystals;
Hail consisting of small balls of ice;
Freezing rain or drizzle which freezes oncontact with a cold surface (may be the
ground or an aircraft in flight).
Continuous rain or snow is often associated with
Nimbostratus and Altostratus clouds. Intermittent rain
or snow with Altostratus or Stratocumulus. Rain or
snow showers are associated with cumuliform clouds,
such as, Cumulonimbus, Cumulus and Altocumulus,
extremely heavy showers and/or hail coming from
Foehn Wind
Fig 5.15
Fig 5.16
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Cumulonimbus. Fine drizzle or snow is associated
with Stratus or Stratocumulus. It is possible to identify
cloud types by their precipitation. Showery
precipitation generally falls from cumuliform clouds
and non-showery precipitation from stratiform clouds,
mainly Altostratus and Nimbostratus. Rain that falls
from the base of a cloud but evaporates before
reaching the ground is called Virga.
5.17 Thunderstorms.Thunderstorms are a very severe weather hazard to
aviation, they generate spectacular weather which is
usually accompanied with, lightning, thunder, heavy
rain showers, hail, squalls and possibly tornadoes.
Thunderstorms are associated with Cumulonimbus
clouds and there may be several thunderstorm cells in
the one cloud.
5.17.1 Three Conditions Necessary For A
Thunderstorm To Develop.
Deep instability in the atmosphere, once airstarts to rise it will continue rising. A steep
lapse rate with warm air in the lower level and
cold air in the upper levels.
A moisture content.
A trigger action to start the air rising; a frontforcing air aloft; a mountain forcing air aloft;
strong heating action of the air in contact with
the Earth; heating of the lower layers of a
polar air mass as it moves to lower latitudes.
5.18. Life Cycle Of A Thunderstorm.
5.18.1 The Cumulus Stage.
Moist air heated from below begins to rise, as it rises
it cools at the DALR to its dewpoint temperature.
Water vapour will condense out as liquid drops and
cloud forms. The air continues to rise releasing latent
but now cools at the SALR. In the early stage of
development of a Thunderstorm, there are strong
updrafts over an area of one or two miles in diameter
with no significant down drafts. Air being drawn into
the cloud at all levels causes the updrafts to becomeeven stronger with height. The updrafts frequently
occur at a rate that an aircraft cannot out perform.
Temperature inside the cloud is higher than the
surrounding air causing the cloud to continue to build
higher and higher. The strong warm updrafts carry
water droplets higher and higher to levels often well
above the freezing level. They may freeze or continue
as super cooled water droplets. The water droplets
will coalesce to form bigger drops. This Cumulus
stage lasts approximately 10 to 20 minutes.
5.18.2 Mature Stage.
The water droplets will become too large and heavy
for the updrafts to support and will start to fall. As they
fall in great numbers, they will drag air inside the cloud
down with them causing down drafts. The first
lightning flashes and rain from the bottom of the cloud
usually occur at this stage. The descending air will
warm adiabatically, but the cold water drops will slow
down the rate at which it warms. Resulting in very
cool down drafts compared to the updrafts. Heavy
rain and/or hail will fall from the base of the cloud,
being heaviest in the first five minutes.
Fig 5.18
Fig 5.17
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The top of the cloud may reach the Tropopause and in
temperate latitudes may reach 20 000 ft and in the
tropics as high as 50 000 ft. The top may spread out
into an anvil shape, caused by the upper winds. The
updrafts and down drafts are of such a magnitude
(more than 5000 ft per minute) that they can out
perform a jet aircraft and cause structural damage.
The rapidly changing direction of the air flow can
cause an aircraft wing to stall. When the cold down
drafts flows out from the bottom of the cloud they
change direction and flow along the ground
horizontally. This produces a strong windshear. The
out flowing cold air will undercut the inflowing warm
air, which causes a mini cold front. A gusty wind and
sudden drop in air temperature may precede the
storm. A roll cloud may form slightly ahead of the
base of the cloud where the cold down drafts and the
warm updrafts pass. The mature stage lasts typically
20 to 40 minutes.
5.18.3 Dissipating Stage.The cold down drafts gradually causes the warm
updrafts to weaken and the supply of warm moist air
to the upper levels is reduced. The cold down drafts
continues and spread out over the whole cloud which
starts to collapse from the top. Eventually the
temperature of the cloud will warm to reach that of its
surroundings and the cloud will collapse into a
stratiform cloud.
Thunderstorms are HAZARDOUS to aviation. The
danger from a Thunderstorm does not exist just in orunder the cloud, but for some distance around it.
Thunderstorms should be avoided by at least 10nm
and in severe situations 20nm or more. Large aircraft
are equipped with weather radar to enable pilots to do
this. The VFR pilot must use their eyes and common
sense.
5.19 Dangers From A Thunderstorm Severe windshear (causes flight path
deviations, handling problems, loss of
airspeed and possibly structural damage);
Severe turbulence (causing loss of controland possibly structural damage);
Severe icing; Hail damage (airframe andcockpit windows);
Poor visibility;
Lightning strikes (causing damage toelectrical equipment and/or airframes);
Static (causing interference to radios andradio navigation equipment).
Fig 5.19
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6.1 Air Masses And Frontal Weather.An air mass is a large parcel of air with consistent
properties (such as temperature and moisture
content) throughout. Air masses are usually classed
by origin, its path over the Earths surface and whether
it is diverging or converging.
6.2 Origin Of An Air Mass.Maritime air flowing over oceans will absorb moisture
and tend to become saturated, in its lower levels.
Continental air flowing over a land mass will remain
relatively dry since little water is available forevaporation.
6.3 Track Of An Air Mass.Polar air flowing towards the lower latitudes will be
warmed from below and become unstable. Tropical
air flowing to the higher latitudes will be cooled from
below and become more stable.
6.4 Convergence And Divergence.An air mass influenced by the divergence of air flowing
out of a HIGH pressure system at the Earths surface
will slowly sink (known as subsidence) and become
warmer, drier and more stable. An air mass influenced
by convergence as air flows into a LOW pressure
system at the surface will be forced to rise slowly,
becoming cooler, moister and less stable.
6. Air Masses and Fronts
When you said watch the front I thought
you meant the weather.
Fig 6.1Fig 6.2
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6.5 Types Of Air Masses That Affect Ireland and the British Isles.Frontal Weather Air Masses have different characteristics, depending upon their origin and the type of surface over
which they have been passing. Because of these differences there is usually a distinct division between adjacent
air masses. These divisions are known as 'Fronts', and there are two basic types:- Cold Fronts and Warm Fronts.
'Frontal Activity' describes the interaction between the air masses, as one mass replaces the other.
6.5.1 Typical Characteristics of Air Masses affecting Ireland and the British Isles.
Air Mass Summer Winter
Warm & Humid. Fog or low ST & perhaps drizzle, may
persist to SW & over sea but break to lee of hills & inland
by day, giving moderate visibil ity.
S to SW, can be strong with risk of gales
Mild, moist & stable. Poor visiblity, extensive fog, ST &
drizzle; a few breaks to E & N of hills. Strong winds lift fog
to ST.
Tropical
Maritime (Tm)
Winds
Tropical
Continental
Tc
Winds
Hot, dry & hazy. Often clear skies. Risk of fog or ST on
S coast. Perhaps high level CB & Thunder if mixed with
moist Tm from Biscay.
Mild, dry & hazy. Some ST. Not common in winter.
Mostly S to SE, usually light, occaisionally moderate
Polar
Continental
Pc
Winds
Warm or very warm except near E coast where some ST
or fog may occur. Rather hazy but mostly clear skies
inland.
Mostly between NNE and SE
Very cold. Wintry showers from CU/SC near E coast if sea
track long enough. Risk of freezing PPN if mild Westerlies
aloft. Frost, perhaps severe, may last all day.
Artic Maritime
(Am)
Winds
Cold & Unstable; CU, perhaps CB; showers. Often clear
skies to S of high ground. May be frost in places.
Exceptionally good visibility.
Mainly NNW to NNE
Very cold. Vigorous instability of limited depth. Sleet, snow
or hail showers in the North & on E Coast. Clear inland.
Servere frost, perhaps all day.
Polar Maritime
(Pm)
Winds
Returning
Polar Maritime
(rPm)
Winds
Mild in winter, cool in summer. PM tracking south of Irish lattitude over the Atlantic becoming unstable, then tracking
north over cooler sea, becoming stable at low levels. Stability variable depending on length of track. Some fog or ST,
especially in the SW, but SC inland with risk of CU, CB, showers or even thunder developing. Snow is rare. Visibility
moderate by day but variable at night.
Mostly between S and W.
Cool & unstable; moderate humidity. CU, CB over sea &
NW facing coasts; forming inland by day. Showery, some
hail & thunder. Good visibility; bumoy flying. Servere
icing & turbulence in CB.
WSW to N, often blustery and strong.
Cold; unstable, sea temperatures giving CU, CB &
showers (snow over hills & in North). Few showers well
inland. Overnight frost, and perhaps fog if light winds.
Note: Wind directions are those most likely with the stated airmass. the type of airmass cannot be infered
soley from the wind direction.
Fig 6.3
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6.6 The Warm FrontIf two air masses meet so that the warmer air replaces
the cooler air at the surface, a Warm Front is said to
exist. The boundary at the Earth's surface between
the two air masses is represented on a weather chart
as a line with semi-circles pointed in the direction of
movement. The slope formed in a Warm Front as the
warm air slides up over the cold air is fairly shallowand so the cloud that forms in the usually quite stable
rising warm air is likely to be stratifo