Composition and structure of the Atmosphere

·         The air is a mixture of several gases and it encompasses the earth from all sides.

·         The air surrounding the earth is called the atmosphere.

·         Nitrogen and oxygen are the two main gases in the atmosphere and 99 percentage of the atmosphere  is made up of these two gases.

Chemical Constituents of the Atmosphere:

1. Nitrogen (N2) - 78% (by volume).

2. Oxygen (O2) - 21%.

3. Argon (Ar) - 1%.

4. Carbon Dioxide (CO2) - 0.038% (376 ppm) and rising.

Structure of the Atmosphere:

Troposphere:

·         It is the lowermost layer.

·         Average height is 13 km with 16-18 km at the equator and 6-8 km at the poles.

·         75-80% of total mass.

·         Temperature generally decreases with height; Average lapse rate is 6.5°/km.

·         Referred to as the "weather layer".

·         Climate and weather changes occur here.

·         Zone separating troposphere from stratosphere is called tropopause.

Stratosphere:

•        It extends up to a height of 50 km.

•        Temperature is constant with height initially, then it increases with height, known as an inversion (temperature increases with height).

•        One important feature of stratosphere is that it contains a layer of ozone gas.

•        This layer is almost free from clouds and associated weather phenomenon,

making conditions most ideal for flying aeroplanes.

·         So aeroplanes fly in lower stratosphere, sometimes in upper troposphere where weather is calm.

·         Sometimes, cirrus clouds are present at lower levels in this layer.

Ozonosphere:

•        It lies at an altitude between 30 km and 60 km from the earth’s surface.

•        This layer reflects the harmful ultraviolet radiation.

•        The ozonosphere is also called chemosphere.

Mesosphere (from the Greek meso meaning middle):

•        It extends up to a height of 80 km.

•        The temperature gradually falls to -100°C at 80 km altitude.

•        Meteorites burn up in this layer on entering from the space.

Thermosphere (from the Greek therme meaning heat):

•        In thermosphere temperature rises very rapidly with increasing height.

•        It extends between 80-400 km.

•        Ionosphere is a part of this layer.

•        This layer helps in radio transmission.

•        The International Space Station and satellites orbit in this layer.

•        Aurora’s are observed in lower parts of this layer(Aurora Borealis and Aurora Australis).

•        Topped by exosphere.

Ionosphere:

•        This layer is located between 80 km and 400 km and is an electrically charged layer.

•        This layer is characterized by ionization of atoms.

•        Because of the electric charge, radio waves transmitted from the earth are reflected back to the earth by this layer.

•        Temperature again starts increasing with height because of radiation from the sun.

Exosphere:

•        Light gases like helium and hydrogen float into the space from here.

•        It is the outermost layer.

•        This layer coincides with space.

Vertical Structure of Composition:

1.Homosphere:(homo means same):

•        Well mixed, the chemical composition is constant.

•        This layer extended up to 80 km from the surface.

•        This part of the atmosphere continually circulates, so that the principal atmospheric gases are well mixed.

2.Heterosphere:(hetero means different):

•        Settling of gases according to weight above 80 km.

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