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World Geography / Climatology

Composition and Structure of Atmosphere


    Composition of atmosphere constitutes a very thin layer of various gases and aerosols surrounding the earth's surface. The structure of the atmosphere consists of various layers namely, Troposphere, Stratosphere, Mesosphere, Ionosphere and Exosphere. The atmosphere is held to the earth's surface by the gravitational pull. Atmosphere has contributed for the life on earth.

    Composition of Atmosphere

    The gaseous component of atmosphere comprises of 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.

    Greenhouse gases 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.

    Moisture in the Atmosphere

    • Moisture present in the atmosphere is around 0-4%. It is 0% when there is dry weather and 4% when there is humid weather. From Equator to Poles, there is decrease in moisture. Moisture of the atmosphere varies from time to time. Summers are humid and winters are generally dry.
    • The energy is stored in the atmosphere in terms of Latent Heat of Vapourisation. Moisture present means, latent heat of vapourisation is available. Moisture present in atmosphere makes the weather turbulent. Humid weather is always turbulent and dry weather is quiet and calm.
    • Moisture is concentrated around the dust particles which attract it. Moisture in this form turns into clouds. Dust particles act as nuclei of clouds. Moisture present in the cloud is released back to the surface of the earth in the form of precipitation, which can be snowfall or rainfall.
    • Clouds yield snowfall or rainfall when they are cooled and condensed ( Water cycle or Hydrological cycle ). During precipitation, the latent heat is released in the form of Latent heat of condensation, which is why the atmosphere becomes turbulent. If it is severe, it will become a cyclone.

    Structure of the Atmosphere

    The atmosphere consists of five different layers namely, Troposphere, Stratosphere, Mesosphere, Ionosphere and Exosphere from bottom to top. The junction between Stratosphere and Troposphere is Tropopause and the junction between Mesosphere and Stratosphere is Stratopause.

    Troposphere

    Troposphere is located up to 18 km from 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

    It is present up to 50 km 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 here. Stratosphere is quiet, calm and stable atmospheric layer and hence it is preferred for flying.

    Mesosphere

    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.

    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 layers are not always present but some layers present throughout the year.

    Up to D-layer in Ionosphere, the atmosphere is called Homosphere and above D-layer it is called 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

    It is at a height of more than 400 km. It is a very low density layer. Beyond 650 km of height, atmospheric mass is negligible. Within 400–650 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.