World Geography / Climatology
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The air that is surrounding the earth is known as the atmosphere. The structure of atmosphere consists of different layers of gases and its composition comprises of gases like Oxygen, Nitrogen, Carbondioxide, etc. and aerosols surrounding the earth's surface. The composition and structure of atmosphere has contributed for the life on earth by maintaining suitable temperature. It helps in obstructing the ultraviolet (UV) rays from the sun that are harmful to the life. The atmosphere is held to the earth's surface by the gravitational pull.
The structure of atmosphere consists of five different layers namely, Troposphere, Stratosphere, Mesosphere, Thermosphere and Exosphere from bottom to top. The junction between Stratosphere and Troposphere is called Tropopause and the junction between Mesosphere and Stratosphere is called Stratopause.
Troposphere is located up to 18 km from the Equator and up to 9 km from the Poles. It is the most turbulent layer of the atmosphere due to the presence of lot of moisture (99% of moisture is in Troposphere) and hence clouds are concentrated and latent heat is present here.
All the planetary winds, local winds, cyclones, etc.
are happening in Troposphere, which is the densest layer of the atmosphere (90% atmospheric mass is concentrated here).
In this layer,
there is decline in temperature with height (6.50C lower temperature for every 1 km of ascent) which is also called
Normal Lapse Rate.
Stratosphere is present up to 50 km from the upper Troposphere boundary and the atmosphere here is uniform throughout the globe. It is divided into two parts by Ozonosphere at a height of 25 km
(dividing it into Lower and Upper Stratosphere).
The Ozone layer shields us from the potentially harmful ultraviolet (UV) radiation from the sun.
In the Lower Stratosphere, temperature is constant with height and some scattered clouds are present here which are not dense or massive.
In Upper
Stratosphere, temperature increases with height and there are no clouds present here. Stratosphere is a quiet, calm and stable atmospheric layer and hence it is
preferred for flying.
It is extended from 50-80 km of height. It is chemically very active layer and hence sometimes called Chemosphere. There is decline in temperature
with height in this layer.
It is known for meteoritical activity. Meteorites burn in the Mesosphere when they enter the atmosphere.
Thermosphere is located from around 80 km to around 500 kilometers above Earth's surface. The temperatures of this layer inceases with ultitude and reaches up to 2,000 °C as this layer is much closer to the sun. This layer is free from both water-vapor and clouds. The lower part of Thermosphere is called Ionosphere.
It is extended from 80-400 km of height. In this layer, gases are in ionized state. It is electrically charged layer and hence it is very much useful for
long distance terrestrial radio communication.
Sometimes, it is regarded as communication layer. Waves are reflected back here because of the charged layer.
Within Ionosphere, Kennelly–Heaviside layer is used for terrestrial radio communication (at a height of 90-150 km). Within this layer, we
have D, E1, E2, F1, F2, G layers.
All these 6 layers are not always present in the structure of atmosphere but some layers are present
throughout the year. Up to D-layer in Ionosphere, the atmosphere is called Homosphere and above D-layer it is called Heterosphere. That is Troposphere,
Stratosphere, Mesosphere and part of Thermosphere comes under Homosphere. The remaining part of Thermosphere and Exosphere comes under Heterosphere.
In Homosphere, the composition of gases is uniform and in Heterosphere,
different types of gases are concentrated in series of shells like Nitrogen layer, Oxygen layer, Hydrogen layer, etc.
Exosphere starts from around 500 km and extends up to 10000 km. It is a very low density layer. Beyond 650 km of height, atmospheric mass is negligible. Within 500–650 km, the
atmosphere is in rarefied state in which the bonding forces are very weak.
Normal gaseous laws are not applicable to this layer. Heat present here is not in
sensible form (we cannot feel) though temperature is high. Astronauts perform space walk in this layer.
The composition of atmosphere comprises of both air and the moisture. The air contains Nitrogen (78%), Oxygen (21%), numerous inert gases
and greenhouse gases (Carbon Dioxide, Methane, Nitrogen Oxides, etc.).
Nitrogen in the
atmosphere dilutes the combustible nature of Oxygen. Oxygen is life sustaining gas and helps in metabolism through combustion.
The greenhouse gaseous components of atmosphere keep the globe warm (regulate the temperature) as they are transparent to short wave insolation (incoming solar radiation) and are somewhat opaque in nature to the long wave terrestrial radiation. They obstruct only part of the long wave terrestrial radiation, which will be stored in the atmosphere.