3. Structure of the Atmosphere

The troposphere is the innermost layer of the atmosphere extending from the surface of the Earth up to \(8\ km\) at the poles and up to \(18\ km\) at the Equator. The word troposphere is derived from the Greek word ‘tropos’ means ‘turn’ or 'change'. Here the temperature decreases with an increase in height. Most of the weather phenomena take place in this layer. Hence, it is called weather making layer. The upper limit of the troposphere is called tropopause. Most of the mass (about \(75-80%\)) of the atmosphere is in the troposphere.

This is the second layer of the atmosphere. It extends from the tropopause to about 50 km above the Earth surface. Here, temperature increases with an increase in height, due to the absorption of the ultraviolet radiation of the Sun by ozone present in this layer. With the concentration of ozone molecules, it is also referred to as the ozonosphere. Large jet planes normally fly here. The upper limit of the stratosphere is called as stratopause.

Above the stratosphere lies the mesosphere. The mesosphere extends between 50km and 80km. Here, the temperature decreases with increasing height, falling as low as \(-90 °C\). Radio waves transmitted from Earth are reflected back to Earth from this layer. Most of the meteors nearing Earth's horizon get burned here. The uppermost limit of the mesosphere is mesopause.

 

The thermosphere lies above the mesosphere. This layer extends to a height of about \(600\ \)km. In this layer, the temperature rises dramatically, reaching up to \(1480 °C\). The rise in temperature is mainly due to the fact that the gas molecules in this layer absorb the X-rays and ultraviolet radiation of the Sun. This results in the break up of the gas molecules into positively and negatively charged particles or ions. This layer of thermosphere that contains Ions and free electrons is called as Ionosphere.

 Having more or less the same composition of gases lower thermosphere is also called "Homosphere". Whereas the upper portion of the thermosphere having an uneven composition of gases is referred to as the “Heterosphere”. Here the temperature increases with increasing height.

The uppermost layer of the atmosphere is called the exosphere. It extends beyond the thermosphere up to \(960\ \)\(km\). This layer is extremely rarefied with gases and gradually merges with the interplanetary space. This zone is characterized by aurora Australis and aurora borealis.

 

The Magnetosphere, which lies beyond the exosphere, is the Earth's magnetic belt, where protons and electrons coming out from the Sun are trapped. The magnetic field of Earth extends to around \(64,000\ \)km above the Earth.