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.

[Title: Structure of the Atmosphere]