Physical Structure Of Atmosphere
(Stratification of atmosphere or layering of atmosphere)
On the basis of the vertical temperature difference, the atmosphere can be divided into four horizontal layers or shells, namely.
A) Lower Atmosphere: 1. Troposphere and 2. Stratosphere
B) Upper Atmosphere: 1. Mesosphere and 2. Thermosphere.
A) Lower Atmosphere:
The altitude of the troposphere changes according to latitude. It has an elevation of about 16 km at the equator and only 8 km at the poles. Its average altitude is about 11 km. It contains near about 75% of the gaseous mass of the total atmosphere, water vapour and aerosols. It is the realm of clouds, storm and convective motion, The outstanding characteristic of the troposphere is the filmy uniform degree in temperature with increase in altitude until minimum temperature of -50% 0C——-600C is reached. The isothermal layer marking the end of temperature decrease is called tropopause and it separates troposphere and stratosphere. Through out the troposphere there is a general decrease of temperature with increase in height at a minimum rate of about 6.50C/km or 3.60F/1000 ft.0C
This is the second atmospheric layer above trop pause which extends upwards about 50 km. The stratosphere contains much of the total atmospheric ozone. The density of ozone is maximum at 22 to 245 km height approximately. The ozone at the upper layer of this sphere absorbs the ultraviolet rays from the Sun and temperature may exceed 00C. In stratosphere the temperature increases with increase in height.
B) Upper Atmosphere:
This is the third layer of atmosphere. A thin isothermal layer called a stratopause is the boundary layer, which separates stratosphere and mesosphere. Above the warm stratopause, temperature decreases with increase in height to a minimum of about-900C at about 80 km height Pressure in this layer is very low and decreases from 1 Mb at about 50 km to about 0.01 mb at 80 km nearly. The thin isothermal layer, which separates mesosphere from thermosphere, is called mesopause.
Outermost shell is known as thermosphere. It lies above 80 km height . In this sphere the atmospheric densities are extremely low. In this sphere temperature increases with increase in height due to absorption of ultraviolet radiation from the Sun. probably it reaches to 9500C at 350 km to 17000 C at an underfined upper limit but these temperatures are essentially theoretical. Such temperatures are not felt by the hands exposed by astronaut or the artificial satellite because of rarefied air.