H.2. Structure Of The Atmosphere
- The atmosphere consists of different layers with varying density and temperature.
- Density is highest near the surface of the earth and decreases with increasing altitude.
- The column of atmosphere is divided into five different layers depending upon the temperature condition.
- They are: troposphere, stratosphere, mesosphere, thermosphere and exosphere.
- The troposphere is the lowermost layer of the atmosphere.
- Its average height is 13 km and extends roughly to a height of 8 km near the poles and about 18 km at the equator.
- Thickness of the troposphere is greatest at the equator because heat is transported to great heights by strong convectional currents.
- This layer contains dust particles and water vapour.
- All changes in climate and weather take place in this layer.
- The temperature in this layer decreases at the rate of 1°C for every 165m of height. This is the most important layer for all biological activity.
- The zone separating the tropsophere from stratosphere is known as the tropopause.
- The air temperature at the tropopause is about minus 800C over the equator and about minus 45oC over the poles.
- The temperature here is nearly constant, and hence, it is called the tropopause.
- The stratosphere is found above the tropopause and extends up to a height of 50 km.
- One important feature of the stratosphere is that it contains the ozone layer.
- This layer absorbs ultra-violet radiation and shields life on the earth from intense, harmful form o energy.
- The mesosphere lies above the stratosphere, which extends up to a height of 80 km. In this layer, once again, temperature starts decreasing with the increase in altitude and reaches up to minus 100°C at the height of 80 km.
- The upper limit of mesosphere is known as the mesopause.
- The ionosphere is located between 80 and 400 km above the mesopause.
- It contains electrically charged particles known as ions, and hence, it is known as ionosphere.
- Radio waves transmitted from the earth are reflected back to the earth by this layer.
- Temperature here starts increasing with height. The uppermost layer of the atmosphere above.
[Image: Structure of the Atmosphere]