It is the envelope of air which surrounds the earth. This envelope of air extends upto a considerable height from the surface of the earth. Since the atmosphere is not of the same density throughout and that atmospheric pressure decreases with height, it is a bit difficult to mark the outer limit of the atmosphere.
There is, however, definite evidence that the atmosphere extends beyond 160 km. Although almost all of the atmosphere (about 97%) lies within 30 km of the earth’s surface the upper limit of the atmosphere can be drawn approximately, at a height of 10,000 Km.
The atmosphere is held to the earth by the gravitational pull of the latter. The atmosphere is densest at sea level and thins rapidly upwards. It constitutes a very insignificant percentage of the mass of the earth.
Composition of Atmosphere
From the earth’s surface upwards to an altitude of about 80 km, the chemical composition of the atmosphere is highly uniform throughout in terms of the proportions of its component gases. In the uppermost reaches the atmosphere is charged with subatomic particles.
Thus, except for the water vapour present, the composition of the atmosphere near the surface of the earth is practically uniform throughout the globe. Pure, dry air has very nearly the following constitution:
Structure of the Atmosphere
The structure of the atmosphere is highly complex but its layering is now well understood. The atmosphere has been divided into several layers according to temperatures and zones of temperature change. Attitudinally arranged the atmosphere falls into five layers or divisions such as:
It is the lower most layer of the atmosphere. On an average, it extends up to a height of 12 km from the surface of the earth. At the equator, the thickness of the troposphere is the greatest i.e. about 18 km and about 8 kms thick over the poles.
The troposphere contains about three-fourths of the total mass of the atmosphere, thus it is the densest of all layers. It is the locale of all the vita] atmospheric processes which create the climatic and weather conditions on the earth’s surface. The troposphere is characterized by:
(i) varying moisture content,
(ii) mobility of the air masses, both vertical and horizontal, and
(iii) regular temperature decline with height. The temperature of air in the troposphere decreases at the rate of 1°C per 165 meters of height.
It is an undefined region lying between troposphere and stratosphere. Here the temperature remains constant. The height of tropopause varies with latitude.
It is the atmospheric zone extending from the upper boundary of the troposphere, to a height around 55 kms. In contrast to the troposphere, a steady rise in temperature with height is observed here. Here air is at rest.
It is an isothermal region and is free of clouds, dust and water vapour. The upper strata is rich in ozone. The ozone layer serves as a shield, protecting the troposphere and earth’s surface by absorbing most of the ultra-violet radiation found in the sun’s rays, thus, is of great importance for the existence of life on the earth’s surface.
Since the water vapour content of the stratosphere is negligible, weather creating processes are never generated here. In the upper layers of the stratosphere the temperature rises to 0°C and higher. A stabilization of temperature occurs at a height of around 55 kms and is called the STRATOPAUSE. Above it a fall in temperature is recorded.
Above the stratopause lies the mesosphere, which is a very cold region. This layer extends upward to about 80 kms from the surface of the earth. Within the mesosphere, at a height of about 60 kms, there occurs a layer called radio-waves absorbing layer.
At the end of the mesosphere, there is a transitional layer of minimum temperature of -80°C. An important feature of the mesosphere is its higher temperature in winter compared with summer, which is apparently due to a small ozone content.
The ionosphere extends from an altitude of about 80 km upward. Studies have shown that the ionosphere extends up to a height of 1000-2000 km from the earth’s surface. The part of the ionosphere lying between 80- 800 km is called thermosphere, which is characterized by a steady rise in temperature.
In the ionosphere, almost all the atoms are ionized i.e this zone is made up primarily of ions (charged atoms). The ionosphere is much rarefied and therefore has a very low total mass despite a huge volume. This layer protects us from falling meteorites as it burns most of them. Besides, it reflects the radio-waves making wireless communication between places possible.
Above the ionosphere lies the exosphere. It is the outermost zone of the atmosphere. It is also known as the diffusion zone, where the atmospheric gases diffuse into the open space.
It is prevented, in part, however, by the earth’s magnetic field, which retains the ionized particles within the magnetosphere. Much about the exosphere is yet to be known.