Aviation Meteorology
Aviation Meteorology
Meteorology is the study of the atmosphere and the weather processes that occur in it. Since an aircraft
is flown through the medium of the atmosphere, an aviator must have adequate knowledge of
meteorology and an appreciation of the effect of weather on all aspects of flying.
The Atmosphere
The invisible and odorless gas which we breathe, which sustains life and produces an infinite variety of
phenomena is what we call air. The envelope of air surrounding the earth and extending to great heights
is the atmosphere where vast physical processes occur, giving rise to the ever-changing weather
phenomena.
Composition
Air is a mechanical mixture of a variety of gases. The main constituents of this mixture are nitrogen and
oxygen, accounting for almost 99% of the whole, with roughly three parts of nitrogen to one part of
oxygen. There are small amounts or traces of other gases. The composition is more or less the same upto
about 60 kilometers. The percentage composition of dry air (by volume) is in the proportions shown
below:
Nitrogen
78.09
Oxygen
20.95
Argon
0.93
Carbon dioxide
0.03
Traces of Neon, Ozone, Krypton, Sulphur dioxide, Helium, Nitrogen dioxide, Xenon, Ammonia, Hydrogen,
Carbon monoxide, Methane, Iodine and Nitrous oxide.
The atmosphere is never completely dry. Water vapour is always present in varying amounts. Water
vapour also behaves as a gas. It is the change in the amount and state of the water vapour (solid, liquid
and gas) which is important in the physics of the weather processes in the atmosphere. Apart from water
vapour, suspended particles like dust, smoke and other impurities affect the transparency of the
atmosphere. In the higher layers, there is concentration of ozone between 30 and 50 kilometers. At
heights of above 70 kilometers, the effect of molecular diffusion becomes significant and the effect of
gravitational separation can be noticed in the case of the lighter gases, which finally escape into space.
Solid particles
• Apart from water, solid particles are present in their varied size, shape and composition. In size, they
range from sub microscopic to large elements that can be detected by naked eyes. They may be
organic such as seeds, pollen, spores or bacteria or inorganic like soil, Industrial pollutants, smoke or
salt from ocean spray.
• These particles reduce visibility through the air.
• These are important in the process of condensation (change from Gaseous state to water droplet) and
sublimation (change from gaseous state to ice crystal or vice versa).
• Even when air is cooled below the saturation temperature, condensation and sublimation process will
not take place unless a small particle called condensation or sublimation nuclei is contacted.
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Aviation Meteorology
• Concentration of these particles is changes with location. Number of particles per unit volume is much
higher near cities and industrial areas than in the rural areas or over the sea.
Structure
The average conditions below about 25km have been well explored by the aid of balloons and
instrumented aircraft. The properties of the higher layers are not so well known. However, studies by
means of rockets, sound and radio waves, aurorae, meteors etc. have given fairly reliable information of
the characteristics of the atmosphere upto almost 100 Km.
The Troposphere
This is the lowest layer of atmosphere nearest to the earth. It extends 8 to 9km at poles and 16 to 18km at
the Equator (i.e. the average height of The Tropopause is taken as 11km).
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This is the first layer of the atmosphere
All the weather occurs in this region due to
the presence of water vapour and large- scale
vertical currents of the air in this part.
About
75%
of
the
air
mass
concentration.
Majority of the aviation activities take place
here
Pressure falls @ of 1hPa /27 feet with height
Temperature decreases with increase in
height at a constant rate independent of
latitude and altitude.
Rate of fall of temperature is known as lapse
rate (‫ )ץ‬of temperature.
Top of troposphere is called Tropopause.
Average height of Tropopause is 16 – 18km at
equator and 8 – 10km at poles.
Equatorial Tropopause is colder than polar
Tropopause being twice higher than it.
Height of Tropopause varies with latitude and from season to season.
During winter, the Tropopause comes down to very low heights and in summer it goes up and it
is higher in warm latitudes rather than cold latitudes (highest over equator and lowest over the
poles).
Sometimes breaks occur in middle latitudes with overlapping Tropopause.
In these breaks high velocity winds (complex in structure due to narrow bands and rapid
movement) called Jet Streams are found.
Winds with speed more than 60kts. are termed jet streams and can reach their maximum speeds
of 200 knots in the upper cold portion of Tropopause.
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Stratosphere
This layer extends from Tropopause up to 50 km from the surface of the earth.
•
Layer is very deep at poles as stratopause is circular.
•
Temperature
remains
steady
between Tropopause and up to
about 20 km and beyond that it
increases with height (0°C at
stratopause)
•
Ozone concentration is found from
15km up to 35 kms from the earth
surface
and
above
which
temperature rises due to absorbed
solar ultra violate radiation by
ozone layer. If this ultraviolet
radiation were to reach the earth
surface it would kill all life on the
earth. This layer of Ozone is called
Ozonosphere.
•
Stratosphere is Stable region of very
low humidity, excellent visibility, no
turbulence and no Weather. Rarely,
a type of cloud ‘nacreous’ (mother
of pearl) Occurs at about 20 to 30
km in winter and are thought to
consist of ice crystals.
Ozone causes sickness in human being and corrodes metals. Precaution must be taken while flying within
Ozonosphere.
Mesosphere
This layer of atmosphere extends from 50 to 85km from the surface of the earth.
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Temperature decreases with height.
At Mesopause temperature is of the order of -90°C (the lowest temperature of the atmosphere).
Pressure in mesosphere is very low decreasing from about 1 hPa at 50 km to .01 hPa at 90 km
Mesopause is situated at 90 km from the surface
The Thermosphere
This layer of temperature extends from 85km from the surface of earth upwards.
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Aviation Meteorology
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The upper limit of thermosphere is not specified.
Above 90km temperature again increases dramatically to thousands of degrees Celsius.
Atmospheric density is very low in this region. Due to the thinness of the air the temperature is not
felt on human body and only a kinetic temperature which governs the speed of the molecules in the
thermosphere.
The aurora is found in this layer and is caused by particles from the sun causing molecules of Oxygen,
Hydrogen and Nitrogen to fluoresce.
Upper layer or top of thermosphere is undefined. However, at about 700 Km, the gravitational pull of the
earth is practically absent and the particles can escape from the atmosphere into space. This is often
referred to as exosphere. There are intense radiation belts known as Van Allen belts at about 3,000 Km
and 15,000 Km above earth’s surface. In the thermosphere, the temperature increases rapidly with
height to about 200 Km and reaches values of the order of 600 °C.
International Standard Atmosphere
A standard average atmosphere has to be specified for various purposes like the design and testing of
aircraft, evaluation of aircraft performance, calibration of pressure altimeters, etc. For this purpose, a
standard atmosphere is defined and used as a basis of reference. The standard atmosphere is a
hypothetical atmosphere, corresponding approximately to the average state of the real atmosphere.
The most widely used atmosphere for reference purposes is the one defined by the ICAO, known as the
International Standard Atmosphere (ISA) whose specifications are:
Mean sea level temperature
Mean sea level pressure
Surface density
Acceleration due to gravity
Temperature Lapse Rate up to 11km
15°C(288.15 °K)
1013.25mb
3
1225 g/m
980.665 cm/sec2
6.5°C/km
Temperature assumed constant at —56.5°C (216.65°K) with an isothermal lower stratosphere above
11km up to 20km – From 20 to 30km there is a rise in temperature at the rate of 1° C/km with a
temperature of -44.5°C (228.65°K) at 32km.
Temperature
Temperature is a measure of the degree of warmth of a substance. It is measured by a thermometer
which works on the property of the expansion of a liquid when the temperature increases.
Thermometers
Thermometers may be either alcohol-in-glass, spirit in glass or mercury-in-glass type. The mercury-in-glass
thermometers are more accurate because mercury responds more quickly to changes in temperature
than alcohol. The thermometers commonly in use for meteorological purposes are:
Dry bulb thermometer, wet bulb thermometer, maximum thermometer and minimum thermometer.
The names indicate the type of readings, registered by them. The minimum thermometer is alcohol-inglass type while the others are mercury-in-glass.
Scale of Measurement
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Aviation Meteorology
Temperature is expressed in degrees. There are several scales of temperature in use. Three commonly
used scales are: —
(a) Fahrenheit — in which the temperature of the melting point of ice is 32 degrees (°F) and of the
boiling point of water 212 degrees (°F).
(b) Celsius (Centigrade) — in which the melting point of ice is 0 degrees (°C) and the boiling point of
water is 100 degrees (°C).
(c) Absolute — in which the melting point of ice is 273 degrees (°A) and the boiling point of water is
373 degrees (°A). This is also known as degrees Kelvin (°K).
In meteorology, the Celsius (centigrade) scale is used. The absolute scale is used in theoretical calculations
as in the case of the behaviour of air under different conditions of temperature and pressure.
For converting values of temperature obtained on one scale to those of another, the following formulae
may be used:
F = 32+(9/5) C
C = 5/9 (F - 32)
A = 273+C = 273+5/9 (F - 32)
Diurnal variation in temperature – Maximum temperature is reached about 2 hours after midday. At
night the ground cools because the earth emits long wave radiation. At the time of the minimum
temperature, the ground is colder than the air close to it sometimes by about 5°C when the sky is clear
and radiation effect is at its maximum. Minimum temperature is reached near about sunrise time.
Atmospheric Pressure
The weight of a vertical column of air standing on unit area and extending to
the uppermost levels of the atmosphere is known as atmospheric pressure.
Pressure can be expressed in terms of different units like pounds per square
inch, grams per square centimeter etc. Pressure in the atmosphere decreases
with height
The instruments used for measuring pressure are called Barometers. They are
of two types:
1.
2.
Mercury Barometer
Aneroid Barometer
Aneroid Barometer
Mercury barometers are delicate instruments and are unsuitable as mobile
units. An instrument which is sturdy and compact, though less accurate, is the
aneroid barometer. It is illustrated in the figure below:
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Aviation Meteorology
Aneroid Barometer
It consists of a chamber made of two corrugated lids, hermetically sealed after removing air from the
inside.
When the atmospheric pressure increases, the lids are pressed closer together. When the pressure
decreases, the lids are displaced away from each other. The movements are magnified and transmitted to
a needle which moves along a graduated dial, on which the values of pressure are indicated. The aneroid
barometer is initially adjusted by comparison with a mercury barometer.
Aircraft altimeters are actually aneroid barometers in which the dial graduations are in terms of heights in
the atmosphere instead of in units of pressure.
Unit of Measurement
Although for many purposes pressure continues to be expressed in terms of inches or millimetres of
mercury, the unit of pressure commonly used in meteorology is millibar (mb).
A millibar is 1000 times the c.g.s. unit of pressure (dyne per square centimeter) used in physics. Thus, 1mb
2
= 103 dynes/cm .
Mercury barometers as well as aneroid barometers are nowadays graduated directly in millibars. The
standard sea level pressure of 29.92 inches or 760 millimetres of mercury is equivalent to 1013.2mb. The
term millibar has been changed to hectopascals(hPa) and 1hPa=1mb
Diurnal Variation of Pressure
Superimposed on irregular changes in pressure due to movement, etc. of pressure systems, is the
rhythmic oscillation of pressure which gives rise to two maxima and two minima of pressure every day.
The maxima are at about 1000 and 2200 hours local time and the minima at about 0400 and 1600 hours
local time. The range between the maxima and minima is highest at the equator (about 3 - 4mb) and
negligible at the poles.
QNH
The most convenient setting of the sub-scale while in the vicinity of an airfield is QNH or altimeter setting.
QNH is defined as the pressure over the airfield reduced to mean sea level assuming that the rate of
variation of pressure between the airfield level and sea level is the same as in the ISA.
Thus an ICAO altimeter whose sub-scale is set to the current value of QNH, will read, while on ground, the
elevation of the airfield above sea level. While flying in the immediate neighborhood of the airfield, the
altimeter will indicate reasonably true altitudes. Current values of QNH are supplied to aircraft in flight by
ATC in hPa, inches or illimeters as required.
QFE
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Aviation Meteorology
Another setting which may be used while in the vicinity of the airfield is QFE. This is the current value of
the pressure reduced to the level of the airfield, (i.e. the airfield reference point). An altimeter set to
QFE will read, while on ground, zero height. While in flight in the vicinity of the airfield, altimeters set to
QFE indicate reasonably true heights above the airfield level. As has been explained earlier, that pressure
altitudes or indicated altitudes are not necessarily true altitudes. In fact they are rarely so. This should be
borne in mind while flying over mountainous terrain.
QFF
QFF is the value of pressure reduced to mean sea level according to standard meteorological practice. This
value is used only in the preparation of surface charts.
QNE
QNE is defined as the height (in feet) indicated by an aircraft altimeter on ground, when the subscale of
altimeter is set to a value of 1013.25hPa
Air Density
Density is defined as mass per unit volume. The density of air in the atmosphere is not measured directly.
It is calculated from the well known gas equation:
Where, P is pressure, ρ is density, T is temperature in °A and R is the gas constant for air.
Variation in Surface Density
At a given pressure, density is inversely proportional to the absolute temperature. Hence, warm air is
comparatively lighter than cold air.
Density Altitude
This is defined as the altitude in the ISA at which, air density is the same as the observed density.
Obviously, a higher density altitude means lower air density. In aircraft manuals density altitude (instead
of density itself) is used as a parameter in monograms for computation of permissible take-off loads.
Density altitude is calculated by the formula: DA = (DB-MSLT) x 120’
Vertical Variation of Density
Pressure decreases with height much more rapidly than the temperature. Density decreases with height
at all levels. Air density at about 6.0 Km is about 50%, 25% at 12.0 Km and at 20.0 Km 10% of the standard
density at sea level.
Relationship between temperature, pressure and density:Temperature is inversely proportionate to pressure, that means if temperature is higher pressure will be
lower because warm air will expand and there will be less weight of air at a particular area and vice versa.
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Aviation Meteorology
Temperature and density are also inversely proportionate to each other. That means if temperature is
lower, the air will be cooler and denser and if temperature is higher, air density will drop or will be lower.
Pressure and density are directly proportionate to each other i.e. higher the density, higher will be
pressure and lower the density, lower will be the atmospheric pressure and vice versa.
Wind
Air in horizontal motion, is called, wind.
Wind is measured by using anemometer which gives wind speed
in kms and knots and wind vane which gives direction of wind in
O
360 of compass.(Direction of wind is the direction from which the
wind is coming or blowing.)
Pressure and Wind
Like any other fluid, the tendency of air is to move from
a region of surplus (high pressure) to a region of deficit (low
pressure). The rate of horizontal movement, i.e. speed of wind, is
directly proportional to the gradient of pressure. If the pressure
gradient were the only force, the wind would be directed from
the high pressure area to the low pressure area and at right
angles to the isobars.
Buys Ballot's Law
A chart on which isobars are drawn and prevailing winds
are plotted shows that the winds do not blow across the isobars.
They usually blow nearly parallel to the isobars, with the low
pressure to the left in the northern hemisphere. This rule is
known is Buys Ballot's Law which is stated as:
If an observer stands with his back to the wind the lower pressure is to his left in the northern
hemisphere and to his right in the southern hemisphere.
Veering and Backing
A wind is said to veer when its direction changes in a clockwise sense; it is said to back when its direction
changes in an anti-clockwise sense.
Winds at Higher Levels
At higher levels the temperatures decrease from the poles to the equator. Thus the thermal wind would
be a westerly wind. Observations support this. In the middle latitudes, westerly winds increase in speed
progressively with height until the Tropopause is reached.
Land and Sea Breeze
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Aviation Meteorology
Large change in wind direction occurs, during the day and night near the coastal areas, even when no
major synoptic situation or steep pressure gradient exists. The wind blows from sea to land during day
and land to sea during night, irrespective of prevailing pressure pattern. These are referred to as, land and
sea breeze. The diurnal difference of temperature between land and sea is the reason for these breezes.
Sea Breeze
During the day, air rises from the land due to increasing temperature and decreasing pressure, the cool
and heavy air of the sea starts moving from the sea to land in the lower levels of the atmosphere
establishing the sea breeze. A counter current at higher levels from the land to sea with rising air over
land subsiding over sea completes the circulation.
Land Breeze
During the night the process is reversed. The land gets cooled faster, while the sea remains warmer. The
off shore wind starts blowing from the land to sea at ground level with the counter current from the sea
to land aloft. In both the cases upper air flow is spread over considerable depth.
In general, sea breeze is more prominent than the land breeze. Its onset is sudden and is accompanied by
a sharp fall in temperature and a sharp rise in humidity. Sea breeze generally set in before noon. On
occasions when a steep pressure gradient exists, the sea breeze may be delayed considerably. On the rare
occasions it may not set in at all. The sea breeze penetrates in land 25 miles or more. In comparison the
land breeze is weaker than sea breeze.
Katabatic Wind
At night, slope of a hill cools more rapidly than the air; consequently the air close to any point on the
slope is cooler and hence denser than the free air at the corresponding level. The denser air close to the
slope slides down and blows as “Katabatic wind”. These winds are characteristic features of hilly areas
and are of great importance on the Himalayan areas and NE India. It causes early morning
thundershowers in the valleys.
Anabatic Wind
During the day time the hill slope gets heated up
more quickly than the free air and the air close to
the slope become warmer than the air at the
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Aviation Meteorology
corresponding level resulting into an up slope wind. This wind is termed as “Anabatic wind”.
Fohn Wind
On meeting a mountain barrier, a fairly steady and
strong wind containing moisture surmounts the
barriers and descends on the lee side. During
ascends to the top, the air cools and cloud
formation takes place which results rain. While
descending on lee ward side, the air is dry and
warms at a greater rate than during ascends on
the wind ward side, resulting into a warm dry air
on the lee side of a mountain. It is known as,
“Fohn wind”.
Valley Wind
The direction of the wind at low level tends, up to some extent to conform to the ground contours. Thus a
stream of air blowing towards mountain ranges gets deflected parallel to it. If the range is broken by a
pass or valley, the air will be forced through it with enhanced velocity and very high wind speed may be
attained locally. The channeling of wind through the valley is known as, funnel effect or valley wind.
Clouds
Definition of Cloud
A cloud may be defined as a visible aggregate of minute particles of water or ice or both, in the free air.
From a brief observation of the sky two fundamental characteristics of clouds become apparent, first,
their infinite variety of form and second, their continual change m appearance. Clouds form in sky,
develop, take different shapes and dissolve. Each process is an indication of some physical state or
process in the atmosphere.
Formation of Cloud
The various mechanisms which result in ascent of air are:
a) Turbulence
b) Orographic Lifting
c) Convection
d) Convergence
e) Frontal Lift
Either singly or in combination they can cause, the formation of clouds if the ascending air reaches the
condensation level. The further build up of the cloud depends on the humidity and the lapse rate aloft.
Instability and a high degree of humidity can cause the build-up of clouds upto great heights; on the other
hand, stable lapse rate and low humidity aloft restrict further growth.
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Classification of Clouds
Families: Clouds are divided into four families according to the average height and development, as
shown below:
Family Height
High
Medium
Low
Clouds of Vertical
Development
Km
3-8
2-4
Height ranges in region
Polar
Temperate
Tropical
Thousand feet
Km
Thousand feet
Km
Thousand feet
10-15
5-11
16-35
5-18
16-60
6-13
2-7
6-23
2-8
6-25
From the Earth's surface to 2km (6500 feet)
Base 0.5 - 2km (1500 - 6500’), Tops up to 18km (55,000’)
Nomenclature
Clouds are given descriptive names which depend mainly on their appearance. They are based on the
classification proposed by Luke Howard in 1803. The three fundamental forms in his classification are:
(a)
Stratus:
Stratiform clouds or sheet layer type
(b)
Cumulus:
Cumuliform clouds ; like a heap of cotton or cauliflower
(c)
Cirrus:
Fibrous or Cirriform clouds ; like threads
Despite their infinite variety of forms, it is possible to define ten basic types of clouds of worldwide
occurrence. These are taken as genera. Most genera possess several species and many of these, in turn,
occur in a number of varieties, sometimes accompanied by supplementary features and accessory clouds.
Description of the genera is given below:
High Clouds
Cirrus (CI): Detached clouds in the form of white, delicate
filaments or white or mostly white patches or narrow
bands. This clouds has a fibrous (hair like) appearance or a
silky sheen or both.
Cirrocumulus(CC): Thin white patch, sheet or layer of cloud
without shading, composed of very small elements in the
form of grains, ripples etc., merged or separate and more or
less regularly arranged; most of the elements have an
apparent width of less than one degree.
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Cirrostratus (CS): Transparent whitish cloud veil of fibrous or
smooth appearance, totally or partly covering the sky and
generally producing hallow phenomena.
Medium Clouds
Altocumulus(AC): White or grey or both patch, sheet or
layer of cloud, generally with shading, composed of luminate,
rounded masses, rolls etc., which are sometimes partly
fibrous or diffused and which may or may not be merged;
most the regularly arranged small elements usually have an
apparent width of between 1 and 5 degrees.
(b)
Altostratus(AS) : Greyish or bluish cloud sheet or
layer of striated, fibrous or uniform appearance, totally or
partially covering the sky and having parts thin enough to
reveal the sun at least vaguely and through ground glass.
Altostratus does not show hallow phenomena but many a
time gives rise to corona.
Low Clouds
(a)
Nimbostratus (NS):
Grey cloud layer, often
dark, the appearance of which is rendered diffuse by more or
less continually falling rain or snow which on most cases
reaches the ground. It is thick throughout, enough to blot
out the sun. Low ragged clouds frequently occur below the
layer with which they may or may not merge. NS often
merges with AS.
(b)
Stratocumulus(SC) :
Grey or whitish, or both
grey and whitish patch, sheet or layer of cloud which almost
always has dark parts composed of tessellation, rounded
masses etc, which are non-fibrous and which may or may not
be merged; most of the regularly arranged small elements
have an apparent white of more than five degrees.
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Stratus (ST): Generally grey cloud layer with a fairly uniform
base, which may give drizzle, ice prisms or snow grains.
When the sun is visible through the cloud, its outline is clearly
discernable. Stratus does not produce hallow phenomena
(except possibly at very low temperature).
Clouds of Vertical Development
Cumulus (CU): Detached clouds, generally dense with sharp
outline, developing vertically in the form of rising mounds,
domes or towers, of which the bulging upper parts often
resemble a cauliflower. The sunlit part of this could are
mostly brilliant white; their bases are relatively dark and
nearly horizontal.
Cumulonimbus (CB):
Heavy and dense cloud, with
considerable vertical extent, in the form of a mountain or
huge tower. At least part of its upper portion is usually
smooth or fibrous or striated and nearly always flattened; this
part often spread out in the shape of an anvil or vast plume.
Under the base of this cloud, which is often very dark, there
are frequently low ragged cloud either merged with it or not
and precipitation.
Special Type of Clouds
Apart from the ten fundamental type of cloud listed above, there are some kind of clouds which form
either by the peculiarity of topography or due to special meteorological situation. Some of them are
described below:
Fracto Cloud: The prefix Fracto is used for a cloud when it is broken into ragged type of fragments. Fracto
Stratus and Fracto Cumulus are indicative of turbulence at the base of the cloud leading to fragmentation
of the lower portions.
Castellanus: When an Altocumulus or Stratocumulus cloud has a turreted shape, these are known as
Castellanus. These are an indication of instability at higher levels.
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Lenticular Cloud: Lens shaped clouds, seen at times near mountain tops, are on the lee side and are
indicative of the crest of the waves.
Line Squall Cloud: Roll clouds of dark color in the shape of an arc, slightly ahead of a long line of the
Cumulonimbus cloud. Their approach indicates impending severe squall/thunderstorm.
Rotor Cloud: Roll type of cloud which sometimes forms on the leeward side of a mountain in a zone of
severe turbulence.
SC
ST
AS
AC
SC
SC
NS
CU
CI
CC
cs
CU
CU
CB
Flying in Clouds
Flying in clouds has to be done under Instrument Flight Rules unless the clouds are thin or of patchy
nature. Before entering any cloud aircrew must be thoroughly sure of the kind of cloud, its approximate
thickness and horizontal coverage. Flying through cloud is associated with the following hazards:
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Aviation Meteorology
(a) Poor Visibility. In cloud, the vertical as well as horizontal visibility is poor. The horizontal visibility
ranges from 1 kilometer in cirrus clouds to less than 10 meters in well-developed cumulus or
cumulonimbus. In general, rain-giving clouds, which have a concentration of larger water drops, are
associated with much less visibility than the non-precipitating types.
(b) Turbulence. All clouds are associated with vertical motion of air, because this is a pre-requisite for their
formation. The vertical motion may be in the form of columns of rising or falling air or in the form of
smaller eddies with irregular random motions. When their sizes are comparable to the dimensions of
an aircraft, they cause bumpiness in the motion of the aircraft; the turbulence may range from slight to
severe type. In general, cumuliform clouds give more turbulence than stratiform clouds. Clouds associated
with instability give rise to severe turbulence.
(c) Ice accretion. Ice may form on the aircraft surface disturbing the aerodynamic properties, control
surfaces and in the carburetor.
Precipitation
Precipitation is the general term used for the fall of liquid water drops or ice crystals on to the ground
from clouds. It thus covers the phenomena of drizzle, rain, shower, hail, sleet and snow.
Definition
Precipitation is a general term used for the fall
of liquid water drops or ice crystals to the
ground from clouds.
It thus covers the
phenomena of drizzle, rain, shower, hail, sleet
and snow. Water droplets or ice crystal in a
cloud are usually of such small dimensions that
they are kept suspended in mid air by the vertical
currents at the base of the clouds. These vertical
currents are necessary for the formation of
clouds and their maintenance. For the water
droplets or ice crystal to overcome the vertical
current and fall under the force of gravity, their
diameter should be of the order of a millimeter
or more.
Theories of Precipitation
The exact process by which the minute cloud particles attain sizes large enough to overcome the vertical
current is not yet fully known. The following theories have been put forward:
Ice Crystal Theory: This is also known as Bergeron theory. This theory was proposed by Bergeron in 1935.
As per this theory the precipitation occurs from clouds which grow beyond the freezing level. In such
clouds super cooled water drops and ice crystals co-exist above the freezing level. Due to difference in
vapour pressure over water and ice, the super cooled water droplets evaporate and condenses on the icecrystals. Thus the ice crystals grow at the expense of water drops and soon attain sufficient size to escape
from the cloud. When they pass through temperature above 0°C they melt and fall as rain.
Coalescence Theory: The Bergeron Theory assumes that precipitation is initiated by the presence of ice
crystals. But in the tropical region precipitation occurs from clouds which do not grow beyond freezing
level and thus do not contain any ice crystals. In order to explain principle of precipitation from such
clouds Irving Longmuir proposed the Coalescence theory. According to this theory the clouds drops
initially grow in size due to condensation and subsequently grow due to collision and coalescence with
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Aviation Meteorology
other smaller droplets. A comparatively larger cloud drop (radius 50mm) while falling through the cloud,
overtakes, collides and fuses with some of the smaller droplets on its path and grow to large size. Due to
air resistance these drops break up and again grow by coalescence and break up. A chain reaction is thus
set in and cloud drop grow in size till it falls as precipitation.
Types of Precipitation
The following are the types of precipitation:
(a) Drizzle: Liquid precipitation in the form water drops with a diameter less than 0.5 mm usually
reaches the ground from the clouds such as Stratus and Stratocumulus. Falling drops are very
close to one another.
(b) Rain: Liquid precipitation in the form of drops of appreciable size (more than 0.5 mm in
diameter) reaching the ground from AS, NS, CU and CB clouds.
(c) Shower: Large water drops with a diameter of 5 mm or more which occur usually from vertical
type of clouds for short duration.
(d) Sleet: Partly melted snow flakes or rain and snow falling together.
(e) Hail: Small balls, pieces of ice with diameter 5 – 50mm some times more, falling either
separately or conglomerated into lumps.
(f) Snow: White opaque pellets (2 – 5mm in diameter) or very small white opaque grain of ice (less
than 1 mm). These may be spherical, conical, and flat or elongated. These occur from NS, AS, SC
and CB clouds.
(g) Freezing Precipitation: Rain or water drops which freeze on impact with the ground.
(h) Thundershowers: Thunderstorm with precipitation is known as thundershowers. It occurs from
CB clouds.
Nature of Clouds and Precipitation
The type of precipitation from a cloud depends on the strength of the vertical current which it has to
overcome, and this in turn depends on the mechanism by which the cloud is formed. The types of
precipitation encountered with different clouds are:
(a) Stratus cloud is formed due to frictional eddies near ground level. The strength of the vertical
current in these eddies is small. Thus even minute droplets are able to overcome them and fall to
the ground as drizzle.
(b) Altostratus and Nimbostratus cloud are formed by frontal ascent or ascent of air in its own zone
of convergence. The vertical current involved are of moderate strength. Here medium size drops
are able to overcome upward current to fall as rain.
(c) Cumulus of great vertical development or Cumulonimbus cloud form due to rigorous convection
coupled with instability. The associated vertical currents are strong. Precipitation from these
clouds is therefore in the form of large shower drops or even hail.
Orographic Rain
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Aviation Meteorology
In certain parts of the world, purely orographic rain occurs in considerable amount. Elsewhere, orographic
features serve to increase the precipitation in circumstances where rain is already more or less general
due to other causes.
The leeward side of a mountain is associated with predominantly downward motion of air. Further due to
orographic rain on the windward side the air is denuded of a good amount of moisture. The precipitation
on the leeward is thus much less than on the windward side. The area immediately to the lee of the
mountain, therefore, forms a rain shadow area.
Thunderstorms
Cumulonimbus is a cumulus cloud which develops upto great heights due to instability and a high degree
of humidity in a deep layer of air. The release of energy due to the over-turning of air in the unstable
layers gives rise to a storm. The electrical charges developed in the cloud give rise to lightning and
thunder. Thunderstorms are one or more convective cells in which electrical discharges are seen as
lightning or heard as thunder.
Conditions Favorable for Cb Formation
The necessary conditions for the formation of Cb clouds are:
(a) A lapse rate steeper than the SALR throughout a layer at least 5 to 6 kilometres in depth,
permitting development of clouds to heights at which the temperature is below 0°C.
(b) An adequate supply of moisture from below.
(c) A process which produces saturation in the region of the steep lapse rate or a triggering
mechanism.
As the instability cloud grows upwards, some of the surrounding unsaturated air is entrained into the
cloud mass. Consequently, some of the cloud droplets evaporate. If the humidity of the surrounding air is
very low, evaporation becomes dominant and arrests further growth of the cloud. Well developed Cbs are
thus possible only when the humidity aloft is sufficiently high.
The triggering mechanism which sets off a thunderstorm may be:
(a) Local convection (insolation)
(b) Orographic lifting
(c) Convergence
(d) Frontal lifting
(e) Radiational or katabatic cooling
Stages of a Thunderstorm
There are three stages of a thunderstorm:
(a) Growing
(b) Mature
(c) Dissipating
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Formative (or developing or Cumulus) Stage: This the first stage when one or more cumulus clouds begin
to grow into a large Cumulus.
•
•
Throughout the cloud only up draughts prevail
An extreme velocity of up draughts can reach 100 ft/sec; they increase with height and are stronger
in the middle than those near the edges. The strongest up draughts are found near the top pars of
the cloud.
Mature Stage: This stage begins with fall of rain from the cloud
•
The fall of rain causes a viscous drag on the surrounding air initiating an onset of down draughts,
which once started in this manner maintains on their own accord due to descending air being colder
than surrounding. Rain falls and first gust or squall reaches the ground.
•
In this stage down draughts are strongest up draughts are also as strong as they were in the
formative stage. Maximum up and down draught velocities are encountered in the middle of the
cloud.
•
Extreme down draughts velocities can reach 40 ft/sec. After rain starts to fall. Anvil extends in the
direction of movement of cloud. In between up and down draughts there is severe turbulence. Down
draughts produce squall on ground.
•
It is in this stage that lightning and thunder occurs.
•
Hail forms above freezing level and their size depends on the strength of up draughts. Bigger hail size
is found in stronger up draughts.
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•
Due to splitting of water droplets negative and positive charges accumulate in the cloud which causes
lightning.
•
Thunder occurs when air in the path of lightning gets heated up and expands and contracted by the
cooler air in surrounding. This sudden expansion and contraction of air results in clapping, producing
sound waves which are heard on the ground as thunder.
Dissipating Stage
Gradually, the downdraughts spread across the lower level of
the cloud and up draughts which are restricted to the upper
part of the cloud become of secondary importance. The
lower part of the cloud descends with the down draughts and
ultimately dissipate away some times leaving behind a
stratified layer of clouds at higher level. This may continue for
some time to give rain after lower part of the cloud has
dissipated.
Usual life cycle of a Cb cell is 2 to 3 hours but the most active
period comprising of the first and second stages lasts for 30
to 40 minutes.
The thunder storm comprises of a number of cells. The weather on the ground depends upon a particular
cell, whichever is over head and its stage of development. Unicellular Cb is rather common.
Flying Hazards in Thunderstorms
Thunderstorms pose a variety of hazards to aircraft in flight. The more important ones are:
(a) Squall: Sudden increase in wind speed and possibly change of direction.
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(b) Heavy showers: These may reduce ground visibility to very low values for short durations.
(c) Fractostratus clouds: These clouds are actually ragged fragments of the base of the Cb; the
fragmentation occurs due to turbulence caused by the squall in the region between the ground
and the cloud base. At times the base of these fragments may be as low as 100 meters, even
over comparatively plain ground, while in the case of high terrain, the hills may be completely in
cloud.
(d) Poor in-flight visibility: In the interior of a Cb cloud the in flight visibility is practically nil. Cb
clouds have a larger amount of water content per unit volume as compared to other clouds, both
the concentration as well as the size of drops being higher.
(e) Draughts: Within the cloud, strong updraughts and downdraughts may cause sudden and large
variations in the attitude of an aircraft. In general the updraughts are stronger than the
downdraughts, both being predominant in the middle and upper parts of the cloud. In the
Thunderstorm Project in the USA, the traverses were made at a cruising speed of about 160kt.
The highest vertical displacement encountered was 6000 ft. in the upward direction and 1400 ft.
in the downward direction, both at a flight level of about 25,000 ft. The storms traversed were of
the sub tropical variety. It is possible that in the summer thunderstorms in the tropics, the
displacements may be of a higher order.
(f) Gusts: Near the borders of the updraughts and downdraughts the large difference in the
velocities creates friction. This friction is responsible for vigorous eddies, which once formed,
travel horizontally with the wind and also in the vertical direction with draughts. They are mainly
responsible for the extreme bumpiness within the cloud. The effects of the frictional eddies on
an aircraft are expressed in terms of effective gust velocity on the readings of an accelerometer
fitted in the aircraft. In the USA Project the most severe gust velocity was 73 ft. per second
experienced on the edge of an up draught. The effectiveness of the gusts due to frictional eddies
in producing bumpiness is not dependent on gust velocity alone, but also on the frequency of the
gusts encountered. Pilots report severe bumpiness when gusts exceeding 14 ft. per second are
encountered with a frequency of more than four such gusts in 3000 ft. of traverse. The severity
of bumpiness increases with height upto the middle part of the cloud and remains constant upto
about 10,000 ft. below the top. Further aloft it decreases rapidly. Bumpiness is much less in the
cloud-filled lanes between two adjacent cells of the same storm.
(g) Ice accretion: It may be emphasized that unlike other clouds, Cb clouds can have super-cooled
water drops and hence chances of icing upto great heights. The water drops are carried up
rapidly in the updraughts and do not readily freeze until they reach very low temperatures.
(h) Hail: Hail forms in well-developed Cb clouds. These may strike the aircraft.
(i) Lightning: Apart from distracting aircrew and temporarily dazzling their eyes, lightning may
interfere with radio communication and may seriously affect the magnetic compass
performance. The accompanying thunder may make listening-in on intercom difficult. The
aircraft itself does not, however, get damaged due to lightning strike, since it is bonded.
Visibility
Visibility is a measure of the degree of transparency of the atmosphere. It is expressed in terms of the
distance in meters/kilometers upto which objects are visible to the naked eye and can be recognized as
such. During daytime, visibility is estimated with reference to landmarks at known distances. At night, the
method adopted is to estimate the equivalent day-time visibility. This is done by installation of lights of
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Aviation Meteorology
standard candle power (100 CP) at different specified distances. From careful experiments, the equivalent
day-time visibilities have been computed. If such lights are not available, the visibility is estimated by
using existing lights and their distances.
(a) Haze: Atmospheric obscurity due to moisture, dust or smoke, wherein visibility is reduced to 5
kilometers or below.
(b) Mist: Moist haze wherein visibility ranges from 1 to 5kilometers.
(c) Fog: Moist haze which is so thick that visibility is less than 1 kilometers
(d) Smog: Moist haze which is so thick that visibility is less than 1 kilometer with smoke and dust
QNH
The most convenient setting of the sub-scale while in the vicinity of an airfield is QNH or altimeter setting.
QNH is defined as the pressure over the airfield reduced to mean sea level assuming that the rate of
variation of pressure between the airfield level and sea level is the same as in the ISA. Thus an ICAN
altimeter whose sub-scale is set to the current value of QNH, will read, while on ground, the elevation of
the airfield above sea level. While flying in the immediate neighborhood of the airfield, the altimeter will
indicate reasonably true altitudes. Current values of QNH are supplied to aircraft in flight by ATC in
millibars, inches or millimetres as required.
QFE
Another setting which may be used while in the vicinity of the airfield is QFE. This is the current value of
the pressure reduced to the level of the airfield, (i.e. the airfield reference point). An altimeter set to QFE
will read, while on ground, zero height. While in flight in the vicinity of the airfield, altimeters set to QFE
indicate reasonably true heights above the airfield level. As has been explained earlier, that pressure
altitudes or indicated altitudes are not necessarily true altitudes. In fact they are rarely so. This should be
borne in mind while flying over mountainous terrain.
QFF
QFF is the value of pressure reduced to mean sea level according to standard meteorological practice. This
value is used only in the preparation of surface charts.
QNE
QNE is the value of height indicated by ICAN altimeter when the aircraft is on ground and sub scale is set
to the value of SPS (1013.25hPa)
Q codes
Weather reports may also be passed on R/T in the International Q code. A few important Q codes in
common use are given below:
QAN
QNT
QBA
QBB
QAM
QAO
QFE
QNH
QNE
Surface wind
Maximum speed of gust
Visibility
Cloud type, amount and height of base
Weather
Upper wind at specified level
Pressure at airfield level
Altimeter setting
Altimeter setting: Mean Sea Level (ICAO Atmosphere)
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Aviation Meteorology
METAR & SPECI
Aviation routine weather reports use the symbolic form METAR.
These are issued hourly or half hourly. Special weather reports are issued when there is sudden
deterioration or improvement of weather (in relation to certain laid down criteria) at an airfield. These are
called Aviation selected special weather reports and use the symbolic form SPECI. The symbolic forms and
explanations are given below:
METAR GGgg CCCC dddff/fmfm VVVV w'w' (NsCC hshshs) or CAVOK (T'T'/Td' Td') (PHPHPHPH)
The groups in brackets may be omitted.
In civil practice, whenever the visibility is 10km or more and there is no significant cloud layer below 3km,
a group CAVOK (cloud and visibility OK) is included.
An explanation is needed here for the meaning of the term significant cloud layer. It covers the following:
(a) Lowest cloud layer
(b) Next higher layer if amount 3 oktas or more
(c) Next higher layer of amount 5 okta or more
(d) Cb cloud, irrespective of amount and height, if not reported already.
Cloud groups may have to be repeated depending on the number of significant cloud layers. On no
occasion can there be more than 4 cloud groups in the message.
However, in Air Force practice, all layers of cloud are reported so as to give a realistic picture of the sky
condition. Also, the total amount of clouds is indicated by an additional group TTL (total amount).
SPECI Code
The code form used for messages indicating sudden deterioration in any element and subsequent
improvement is SPECI. This code form is the same as METAR, except that temperature and pressure
groups are omitted.
Aerodrome Warnings
A warning message is a notification of the occurrence or expected occurrence, not previously notified, of
specified meteorological conditions which may affect safety of aircraft. Thus any meteorological
phenomenon which presents a potential hazard to flight safety forms the subject of a warning.
PANSMET lays down that warnings for the protection of parked and moored aircraft are to be issued in
respect of the following phenomena: Gale, squall, thunderstorm, sand storm, rising sand or dust devil,
frost, rime, snow, freezing precipitation, rough sea and swell. These warning messages are normally not
passed outside the aerodrome of issue or the aerodrome for which issued (in case the meteorological
service is provided by a Met Office not located at the concerned aerodrome).
Use of CAVOK
When the following conditions occur simultaneously at the time of observation:
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Aviation Meteorology
(a) Visibility, 10km or more;
(b) No cloud below 1500 m (5000 ft) or below the highest minimum sector altitude, whichever is
greater, and no cumulonimbus;
(c) No weather of significance to aviation as given in 4.10.10.3 and 4.10.10.4; then, information on
visibility, runway visual range, present weather and cloud amount, type and height shall be
replaced in all meteorological reports by the term “CAVOK”.
Criteria for issuance of local special reports and SPECI
Local special report and SPECI shall be issued whenever changes in accordance with the following criteria
occur:
Surface Wind
0
(a) When the mean surface wind direction has changed by 60 or more from that given in the latest
report, the mean wind speed before and/or after the change being 10 knots or more;
(b) When the mean surface wind speed has changed by 10 knots or more from that given in the
latest report;
(c) When the variation from the mean surface wind speed (gusts) has increased by 10 knots or more
from that given in the latest report, the mean speed before and/or after the change being 15
knots or more.
Visibility
When the visibility is improving and changes to or passes through one or more of the following values, or
when the visibility is deteriorating and changes to or passes through one or more of the following values:
800, 1500, 3000 or 5000 meters.
Runway Visual Range (RVR)
When the runway visual range is improving and changes to or passes through one or more of the
following values, or when the runway visual range is deteriorating and passes through one or more of the
following values: 150, 350, 600 or 800 meters.
These SPECIs are to be issued by all offices equipped with instrumental recording facilities of RVR.
Present Weather
When the onset, cessation or change in intensity of any of the following weather phenomena or
combinations thereof occurs:
Freezing precipitation, freezing fog, moderate or heavy precipitation, (including showers thereof), low
drifting dust, sand or snow, blowing dust, sand or snow (including snowstorm), dust storm, sandstorm, ice
crystals, thunderstorms (with or without precipitation), squall, funnel cloud (tornado or waterspout).
Cloud
(a) When the height of base of the lowest cloud layer of BKN or OVC extent is lifting and changes to
or passes through one or more of the following values, or when the height of base of the lowest
cloud layer of BKN or OVC extent is lowering and changes to or passes through one or more of
the following values: 30, 60, 150, 300, or 450 m (100, 200, 500, 1000 or 1500 ft.)
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Aviation Meteorology
(b) When the amount of a cloud layer below 450 m (1500 ft) changes from SKC, FEW or SCT to BKN
or OVC; or from BKN or OVC to SKC, FEW or SCT.
Vertical Visibility
When the sky is obscured and the vertical visibility changes to or passes through one or more of the
following values: 30, 60, 150 or 300M (100, 200, 300, 1000 feet)
Air temperature
0
When air temperature has increased by 2 C or more from that given in the latest report.
METAR/SPECI
--------------------------------------------------------------------------------------------------------------------------------------------STATION
DATE
TIME OF OBSERVATION
O
SURFACE WIND
--------- /-----------kt/kmph/mps
Max Wind
----------------------kt/kmph/mps
Visibility
----------------------meters/km
Present Weather
------------------------------------
Clouds
st
Amt/Type
Height
1 layer
----/--------------m/feet
nd
rd
2 Layer
------/--------------m/feet
3 Layer
-----/-------------m/feet
Total Amount:
----------------------/8
Temperature:
Dew point Temperature:
---------------------- C
o
---------------------- C
QNH:
----------------------hPa/Inches/mms
QFE:
----------------------hPa/Inches/mms
Recent Weather:
----------------------
Wind shear:
----------------------
Trend:
----------------------
o
Remarks:
-----------------------------------------------------------------------------------------------------------------------------------------------------------------Time of issue
------------------------------------------- Signature of Observer
---------------------------------------------------------------------------------------------------------------------------------------------
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Aviation Meteorology
Instruments and methods used to measure Meteorological Elements.
Instrument or Method
1. Mercury Barometer
2. Aneroid Barometer
Element
Units
Atmospheric Pressure
Millibar/Hactopascle
1. Anemometer
Wind Speed
Knots
2. Wind Vane
Surface Wind Direction
°True
3. Wind shock
Surface Wind Direction
Surface Wind Speed
Approx °Magnetic
approx Knots
1. Transmissometer
2. Skopograph
3. By estimation
1. Visibility
2. RVR is taken when Visibility falls below 1500m
meters/Kilometres
meters
1. By estimation
Amount of clouds
Octas
1. Alidade apparatus
2. Ceilometers
3. RADAR
4. By estimation
5. Aircraft Observation
6. Balloon Assent
Height of Cloud base
Feet/meters
1. Radio Sonde
it is released with a balloon ascent
Upper air -Temperature & humidity)
°C
%age
1.
2.
3.
4.
U/ Wind direction
Upper Wind speed
°true
knots
DEW POINT
°C
Rain
Inches/mms
RAWIN
Balloon Trajectory
Aircraft observation
Nephoscope
WET and DRY bulb
temperature Thermometer
Rain gauge
85