For a PPL, CPL & ATPL aspirants who wants to become a good pilot, during
flying prior to departure or enroute, earth atmosphere plays a vital role for
safety of the aircraft. To ensure this,
pilot has to be well versant with earth atmosphere & other topics related
to aviation meteorology. At Gracious
Avatar during ground classes for PPL, CPL & ATPL aspirants, meteorology
will be covered by the expert ground instructors.
EARTH’S
ATMOSPHERE
Atmosphere
The atmosphere of the earth is an envelope of air around earth
and it is bound to the earth due to the gravitational pull of the earth.
Effectively, it is part of earth and moves with the earth as the earth rotates
on its axis. Air is a mixture of gases, and this mixture is quite homogeneous upto
about 80km due to turbulent flow, whilst above 80 km the atmosphere is
heterogeneous.
Composition
& Characteristics
1. Since atmosphere
is made up of air and air has weight, so the atmosphere has weight and it
exerts pressure. Atmosphere has no definite shape. It occupies space. Air is a
mixture of gases- the most prominent being Nitrogen and Oxygen.
The ratio of gases present in the air is :-
Nitrogen 78.08%, Oxygen 20.95%, CO2 0.035%, Argon (A2) 0.03% and
traces of Helium, Methane, Hydrogen, Nitrous Oxide, Ozone, Xenon etc.
2. Nitrogen And
Oxygen making up most of the atmosphere can be said to have a ratio which is :-
Nitrogen: Oxygen= 3:1 (By Weight)
Nitrogen: Oxygen= 4:1 (By Volume)
3. Even though
atmosphere is quite homogeneous, it shows remarkable variable quantities of
water vapour in the lower troposphere and of ozone in the lower stratosphere
(20-25km).
4. Water vapour,
Carbon dioxide and Ozone, though present in small quantities have significant
effect on weather and life. These gases are called Green House Gases. Ozone in
the stratosphere absorbs harmful ultra-violet radiation from the sun and
protects us.
5. Air which has no
water vapour is called dry air. A mixture of Dry air and water vapour is called
Moist Air. The air which has >4% water vapour (=100% RH) is called
saturated. Similarly, the air which has <4% water vapour (RH<100%) is
called unsaturated. In meteorology, all air that is unsaturated is called Dry
Air.
Vertical Distribution of Air
Due to the gravitational attraction of the
earth, most of the atmospheric mass is concentrated in the lower portion and
the density decreases rapidly with height. As a rough estimate :-
·
1/2 of the
atmospheric mass is contained in the lower 6 km
·
3/4 of the
atmospheric mass is contained in the lower 10 km
·
99% of the
atmospheric mass is contained in the lower 35 km
Thermal Structure of the Atmosphere
1. Sun is the main source of
energy for the earth. It gets heated up by the sun’s rays during day and then
heats the layers above by conduction, convection, radiation and latent heat.
2. Sensible Heat (Conduction,
Convection & Radiation) – 23%.
3. Latent Heat (Evaporation,
Condensation & Sublimation) 77%.
4. Due to absorption of solar
radiation the atmospheric layers close to ground are warmer than layers above.
5. The temperature falls with
height. This trend however does not continue till the end of atmosphere.
6. Based on temperature
distribution the atmosphere is divided into layers viz. troposphere,
stratosphere, mesosphere & Thermosphere (which includes ionosphere).
7. The top of the troposphere
where the temperature stops falling is called tropopause. It is a boundary
layer between troposphere & stratosphere.
8. Higher the temperature at
surface higher will be tropopause.
9. Above (not at) 8km there is a
reversal of temperature & density and above this height poles are warmer
than equator.
Troposphere
1.
Lowest layer of
atmosphere is troposphere.
2. Extends to 16-18 km over equator and 8-10 km over poles.
3. Temperature falls with height at a uniform rate of 6°
C/km.
4. In day to day conditions temp may fall (lapse), rise
(inversion) or stay constant (isothermal).Troposphere is generally unstable and
thus most weather occurs in this layer.
5. 70% of the atmosphere lies within this layer.
6. Tropopause is top of troposphere and a boundary between
troposphere and stratosphere.
7. At tropopause the temperature stops falling with height.
Lapse rate, if any, reduces to just 1-2°/km.
8. Height of tropopause is 16-18 km over equator and 8-10 km
over poles.
9. There are breaks in Tropopause where jet stream occurs.
10. Polar tropopause- Pole ward of 45-600 N 300
hPa.
11. Tropical tropopause- Equator to 350 N 100 hPa.
12.
Jet Stream at 200
hPa between the two tropopause.
Stratosphere
1.
The layer above troposphere
which is much stable and in which temperature rises with height is
stratosphere.
2.
Nacreous clouds or
mother of pearl clouds are seen in upper stratosphere in higher latitudes.
3.
The temperature in
lower stratosphere slowly decreases with height in high latitudes in winters,
remains nearly constant in the mid-latitudes and increases with height in lower
latitudes.
4. In the upper stratosphere (above 30 km) there is general
increase of temperature with height due to absorption of UV rays by Ozone and
Oxygen.
5.
The top of
stratosphere where once again the temperature has stopped rising and has
reached about 0° C is called Stratopause.
It occurs at a height of 50 km.
Mesosphere
1. Mesosphere extends from
Stratopause to about 80 km. Lack of absorption of solar radiation and weak
vertical mixing causes temperatures to fall with height in this layer.
2. On rare occasions Noctilucent
clouds are seen in upper Mesosphere.
3. The level at the top of
Mesosphere where temperatures after reaching about -90° C stop falling is
called Mesopause. It
generally occurs at about 80 km.
Thermosphere
1. The region extends from
Mesopause to the outermost fringes of the atmosphere in which temperature
increases with height up to 200 km.
2. In this layer air is very
thin and very few molecules are present. However, there are ionized particles
that move rapidly and strike each other thus increasing temperature.
3. The temperatures are about
600° C at 200 km which increases to 2000° C during sunspot maxima.
International
Standard Atmosphere
ISA
has been defined by ICAO. Its specifications are:-
1. Air is Dry.
2. Temp at MSL 15°C.
3. Pressure at MSL 1013.25 hPa.
4. Acceleration due to gravity
980.665 cm/sec2.
5. Lapse rate up to 11km 6.5°
C/km.
6. Temp assumed constant at
-56.5° C from 11km to 20 km.
7. From 20 to 30 km there is a
rise of temp at the rate of 1° C/km with a temp of -44.5° C at 32 km.
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