Earth’s Atmosphere

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.  EduKwik eLearning module of Aviation Meteorology helps the PPL, CPL & ATPL aspirants to understand & apply the concept of Aviation Meteorology.

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.