Atmosphere is a thick gaseous envelope that surrounds the earth and extends thousands of kilometers above the earth’s surface. Much of the life on the earth exists because of the atmosphere otherwise the earth would have been barren. Nitrogen and Oxygen comprise 99% of the total volume of the atmosphere.
Structure of the Atmosphere
The atmosphere consists of almost concentric layers of air with varying density and temperature.
- Lowest layer of the atmosphere.
- The height of troposphere is 16 km thick over the equator and 10 km thick at the poles.
- All weather phenomena are confined to troposphere (e.g. fog, cloud, frost, rainfall, storms, etc.)
- Temperature decreases with height in this layer roughly at the rate of 6.5° per 1000 metres, which is called normal lapse rate.
- Upper limit of the troposphere is called tropopause which is about 1.5 km.
- b) Stratosphere:
- The stratosphere is more or less devoid of major weather phenomenon but there is circulation of feeble winds and cirrus cloud in the lower stratosphere.
- Jet aircrafts fly through the lower stratosphere because it provides perfect flying conditions.
- Ozone layer lies within the stratosphere mostly at the altitude of 15 to 35 km above earth’s surface.
- Ozone layer acts as a protective cover as it absorbs ultra-voilet rays of solar radiation.
- Depletion of ozone may result in rise of temperature of ground surface and lower atmosphere.
- Temperature rises from -60°C at the base of the stratosphere to its upper boundary as it absorbs ultra-voilet rays.
- Upper limit of the Stratosphere is called stratopause.
- c) Mesosphere
- Mesosphere extends to the height of 50-90 km.
- Temperature decreases with height. It reaches a minimum of -80°C at an altitude of 80-90 km
- The upper limit is called mesopause.
- d) Thermosphere
- It lies at 80 km to 640 km above the earth’s surface.
- It is also known as ionosphere
- Temperature increases rapidly with increasing height.
- It is an electrically charged layer. This layer is produced due to interaction of solar radiation and the chemicals present, thus disappears with the sunset.
- There are a number of layers in thermosphere e.g. D-layer, E-layer, F-layer and G-layer.
- Radio waves transmitted from earth are reflected back to the earth by these layers.
- e) Exosphere
- This is the uppermost layer of the atmosphere extending beyond the ionosphere.
- The density is very low and temperature becomes 5568°C.
- This layer merges with the outer space.
At heights of 80 km (50 miles), the gas is so thin that free electrons can exist for short periods of time before they are captured by a nearby positive ion. This portion of the atmosphere is ionized and contains plasma which is referred to as the ionosphere. The Ultraviolet (UV), X-Ray and shorter wavelengths of solar radiation ionizes the atmosphere. The ionosphere is broken down into the D, E and F regions.