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Chapter: 11th 12th std standard Geography earth space Higher secondary school College Notes

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Layers of the Atmosphere

The atmosphere consists of five layers: The troposphere, the stratosphere, the mesosphere, the thermosphere, and the exosphere. The thickness of these layers is slightly different around the globe, and also varies according to temperature and season.

Layers of the Atmosphere

 

The atmosphere consists of five layers: The troposphere, the stratosphere, the mesosphere, the thermosphere, and the exosphere. The thickness of these layers is slightly different around the globe, and also varies according to temperature and season. In this discussion, we will focus primarily on the troposphere and the stratosphere because human-made pollutants affect the function of the lower layers.

 

1.The Troposphere: The troposphere is the layer closest to the Earth's surface. It is a layer of air approximately 10 to 15 kilometers thick. The conditions in this layer determine practically all of the global weather patterns. It consists of gases like nitrogen, oxygen and carbon dioxide. In addition to these gases, water vapour and dust are found in this year.

Consequently, weather phenomena such as clouds, lightning, thunder, storms and rain take place in this layer.

 

Carbon dioxide is much heavier than the other gases. So the amount of carbon dioxide is higher in this lower layer of the atmosphere. It tends to decrease as the altitude increases. Therefore the temperature decreases with the increasing altitude at the rate of 1'C per 165 metres. This ratio of decrease in temperature with the increase of altitude is called the lapse rate.

 

2.The stratosphere : The stratosphere is the layer just above the troposphere. It is approximately 40 kilometers thick. In contrast to the troposphere, water vapour is not found in this layer. Therefore weather changes do not take place in this layer. Pollutants tend to remain long periods of time in the stratosphere. Moreover, the wind blows horizontally and therefore the conditions are conducive for airplanes to fly with great speed and a long distance.

 

3. Thermosphere : The mesosphere spreads above the stratosphere from 50 kms to 80 kms. The meteors, which fall on the Earth due to the gravitational force, burn due to the friction with the wind at this level. This layer protects the Earth from the falling meteors, otherwise these meteors will damage the surface of the Earth.

 

4. Thermosphere : The thermosphere lies above the mesosphere. It extends approximately from 80 kms to 1600 kms. It contains hydrogen and helium in large amounts. Below this layer ions are found from 500 kms. We can enjoy radio programs that are broadcast from the radio station only because of the reflection done by the ions.

 

5. Exosphere : The exosphere extends from 1600 kms to 10,000 kms. Winds having very low density are found here. It mingles in the space and only vacuum is found above this layer.

 

Among the above said layers of the atmosphere, the stratosphere filters the ultraviolet rays and protects the Earth. Let us now learn the significance of ozone layer and its depletion.

 

Ozone layer and its depletion : Ozone is a form of oxygen. Ozone is made of three oxygen atoms (O3). The oxygen  we find in the atmosphere is made up of two oxygen atoms (O2). Because of its chemical formulation, a single atom of oxygen (O) is unstable. That is, it wants to combine with something else. That is why oxygen is almost always found in pairs, in its diatomic form, where it is more stable. O3 is less stable than, because it wants to return to the diatomic state by giving up an oxygen atom. When enough ozone molecules are present, it forms a pale blue gas. Ozone has the same chemical structure whether it is found in the stratosphere or thetroposphere.

 

In the troposphere, 'bad' ozone is an air pollutant . It damages human health and vegetation. It is a key ingredient of urban smog. In the stratosphere, we find the 'good' ozone that protects life on earth from the harmful effects of the sun's ultraviolet rays.

 

Ozone is constantly being formed in the earth's atmosphere by the action of the sun's ultraviolet radiation on oxygen molecules. Ultraviolet light splits the molecules apart by breaking the bonds between the atoms. A highly reactive free oxygen atom then collides with another oxygen molecule to form an ozone molecule. Because ozone is unstable, ultraviolet radiation quickly breaks it up, and he process begins again.

About 90% of the zone in the earth's atmosphere lies in the stratosphere. Ozone forms a very thin layer in the stratosphere, where it is more concentrated than anywhere else. While both oxygen and ozone together absorb 95 to 99.9% of the sun's ultraviolet radiation, only ozone effectively absorbs the most energetic ultraviolet light, which causes biological damage.


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