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Friday, 19 March 2021

Atmosphere Layers of Earth

The gaseous (air) covering surrounding the earth is called earth’s atmosphere. The earth’s atmosphere mainly consists of nitrogen (78%) and oxygen (21%). In addition to this a very small amount of argon, carbon dioxide, water vapour, hydrocarbon, sulphur compounds and dust particles are also present.

 

As we go up, the density and pressure of the atmosphere rapidly decrease. At 100 km height from the earth surface, the pressure is only 10-3 mm which is same as the vacuum attainable in the laboratory by a good vacuum pump. The mean free path of a molecule in this region is 10-4 in as compared to 10-7 m near the ground.

The earth's atmosphere is divided into four regions; (1) Troposphere (2) Stratosphere (3) Mesosphere and (4) Ionosphere based on the mode of temperature control. There is no sharp boundary between the regions.

 

1. Troposphere: 


Troposphere is the layer of the atmosphere closest to the earth. We live there and nearly all the earth’s weather occurs there. The upper boundary of the troposphere is called the tropopause. The tropopause lies about 16 kms over the equator and about 10 kms over the North and South poles. The stratosphere lies above the troposphere, followed by the mesosphere and the thermosphere. Within the troposphere, the temperature of the air generally decreases as the altitude increases. The temperature of the troposphere also varies with latitude. Troposphere region extends upto a height of 12 km from the surface of earth. The solar radiation which is responsible for heating of the troposphere by conventional air current is in the infrared, visible and ultraviolet region. In this portion of the atmosphere, the average temperature decreases with height from 17°C to —53°C. At the equator, the tropopause can as cold as –80°C. Above the tropopause, the temperature stops decreasing with altitude.

 

2. Stratosphere: 


In 1902 the French meteorologist Leon Teisserene de Bort proposed that the atmosphere is divided into two parts, with the lower layer containing all the weather activity (such as rain, clouds, winds). He called this the troposphere and the higher layer he named the stratosphere, beginning at 25,000 feet at the poles and 50,000 feet at the equator. The boundary between the two he called the tropopause. Stratosphere extends from 12 km upto 50 km from the surface of earth. Solar radiation is the controlling factor of temperature. The temperature of this region varies from —53°C to 7°C. At the base of the stratosphere the temperature is almost constant with height. In the stratosphere at a height of 15 km from the surface of the earth, the density is about one-eighth of that at the ground. From spectroscopic studies, Hartley concluded that Ozone (O3) is present in the stratosphere and it is concentrated in the region near 25 km to 50 km from the surface of earth. This region is called Ozone layer. It is this ozone layer which absorbs large portion of uv radiation emitted by the sun.

 

3. Mesosphere: 


Mesosphere is a layer of the earth’s atmosphere. It lies between the stratosphere and the thermosphere, the upper most layer of the atmosphere. This region of atmosphere extends from 50 km upto 80 km from the surface of the earth. Mesosphere is very cold. The temperature of the air in the mesosphere generally decreases as the altitude increases. The temperature of this region fluctuates from 7°C to -93°C. The lowest temperature in the earth’s atmosphere occurs at the top of the mesosphere called the mesopause. In the mesopause over the North and South poles, the air temperature may drop as low as –109°C. Shooting stars burns up here. The coldest mesopause temperatures at a pole occur when it is summer there.

 

4. Ionosphere:

 

Ionosphere is a part of the earth’s atmosphere that has many ions and free electrons. Cosmic rays and radiation from the sun produce those ions. The ionosphere extends through the layers of the atmosphere known as the mesosphere and the thermosphere. Several ionized regions make up the ionosphere. The lowest one, called the D region, begins at an altitude of 55 to 89 kilometres. The F region begins at 80 kilometres and extends up to about 400 kilometres. The height and ionization of these regions vary from day to night and with changes in solar radiation. For example, the D region almost disappears at night. The other regions rise and become less strongly ionized at night because no solar radiation reaches the atmosphere. In 1902 Kennelly in America and Heaviside in England almost simultaneously postulated the existence of this conducting layer in the upper atmosphere. By the ionisation action of solar radiation positive and negative ions are produced in the atmosphere which reflect the radio waves (wavelength around 10 metre). Later Ionosphere was discovered in 1922 by English physicist Sir Edward Appleton. In 1924 he located and measured the reflecting layer, the Kennelly-Heaviside layer, about 65 miles high. He named that part of the atmosphere the ionosphere because of the high number of ions present there.  The temperature of this region varies from —93°C to 427°C. Ionosphere plays a fundamental role in the propagation of radio waves round the curved surface of the earth. But for the ionosphere long distance communication would have been an impossibility before the launching of satellites.

 

Note 1: The different regions of our atmosphere affect us differently. For troposphere temperature decreases with height. For stratosphere, mesosphere and ionosphere temperature increases with height.

 

Note 2: Ozonosphere: The upper region of stratosphere contains ozone (O3) in abundance. This region is called ozone layer. Ozone in this region absorbs ultraviolet radiations coming from the sun and hence the humanity is saved from the ill effects of the ultraviolet rays from the sun. Absorption of ozone in the visible region is extremely small.

 

The atmosphere permits the life giving radiation to pass through it, but provides a protecting cover from the harmful radiations such as X-rays, ultraviolet rays etc. These radiations are absorbed by the ozone layer. Energy from the sun heats the earth which itself radiates infrared radiation. However earth being at a lower temperature emits infrared radiation of longer wavelength. These are unable to cross the lower atmosphere as it reflects them back. On this account earth's atmosphere is richer in infra red radiation or thermal radiation. Low lying clouds too prevent escape of infra red radiation thus keeping the earth warm. This phenomenon is called green house effect.

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