The weather conditions below 25 km have been well explored by the aid of balloons and instrumented aircraft. Studies by rockets, sound and radio waves, aurora, meteors etc. have given fairly reliable information of the characteristics of the atmosphere up to almost 100 Km. The properties of the higher levels of atmosphere, are not so well known.
The atmosphere can be divided into five regions depending on the variations of temperature with height.
- Troposphere : Up to about 11 – 16 Km
- Stratosphere : Up to about 50km above Troposphere
- Mesosphere : 50 – 85 Km
- Thermosphere : above 85 Km
The troposphere is the region nearest to the earth and it is generally known as the region where maximum weather occurs as shown in Figure below. It has a more or less uniform decrease of temperature with height. The rate of decrease of temperature with height known as ‘lapse rate’ is roughly 6.5oC/Km. However, isothermal (line joining regions of equal temperature) and inversion layer (temperature increase with altitude) can occur in the troposphere. The upper boundary of the troposphere is the tropopause whose height varies from equator to poles, being highest over the equator (16-18 Km) and loWest over the poles (8-10Km). The height of the tropopause varies from season to season also. The tropopause is not a continuous surface, but a layer with breaks, thus showing distinct polar, middle and tropical tropopause.
The stratosphere is the layer above tropopause and extends up to about 50 km above the surface of the earth. The temperature in this region is steady (Isothermal) or increases with height (Inversion). The increase being more noticeable at the top, where the temperature is of the order of 0oC. The upper boundary of the stratosphere is Stratopause. This is the region where maximum ozone concentration is found. Ozone absorbs ultra-violet radiation and this accounts for the increase in temperature in this region. The region from the tropopause to 20 km above, is referred as lower stratosphere, 20-30 km as middle stratosphere and 30-50 km as Upper stratosphere.
The layer above the stratosphere is the mesosphere, where the temperature again decreases with height. The boundary of the mesosphere is the mesospause, about 85 km high, where the loWest temperatures in the atmosphere are found (about – 90oC).
Above the mesosphere is the thermosphere. It is the region the temperature increases rapidly with height up to about 200 Km and reaches about 600oC under sun conditions and possibly to 2000oC during sunspot maxima. The upper limit of the 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. The space shuttle and the International Space Station both orbit Earth within the thermosphere. The temperatures referred to in this region are kinetic temperature, i.e. they are a measure of average kinetic energy of the gaseous molecules/atoms present in the atmosphere. Thus thermometers are of no use at these heights and therefore indirect methods are used for determination of temperature.
The lower thermosphere is in a highly ionised state. The resulting concentration of ions (charged particles) causes reflection of radio waves. This enables long wave radio communication possible. The highest concentration of ions is found at about 110 Km. the luminous aurora occur in this region, as shown in Fig-7.2.6. Aurora is highly colourful and spectacular display in the upper atmosphere in the height band of 65 to 1000 Km in the polar and sub-polar regions.
ICAO Standard Atmosphere
International Civil Aviation Organisation (ICAO) Standard Atmosphere (ISA) is a hypothetical atmosphere, corresponding approximately to the average state of real atmosphere in which the pressure and temperature are defined at all heights. Such an atmosphere is adopted internationally as the basis for the calibration of the altimeters, evaluation of aircraft, etc. Its chief specifications are:-
- Air is dry
- Mean Sea Level (MSL) Pressure : 1013.25 hPa
- MSL Temperature : +15.0ºC (288.15ºK)
- Density at MSL : 1225 g / m3
- ‘g’ value : 980.665 cm/sec2
- Temperature Lapse rate : Rate of drop in Temperature with increase in height is assumed to be 6.5°C/Km upto 11 Km height. From 11 Km to 20 Km, the temperature is constant at –56.5°C (216.65°K): From 20 to 30 Km there is a rise of temperature at the rate of 1°C/Km attaining a temperature of -44.5°C (228.65°K) at 32 Km.