Atmosphere and its Compositions
- The atmosphere is a gaseous envelope that surrounds the earth from all sides and is attached to the earth’s surface by gravitational force.
- The height of the atmosphere is estimated between 16 to 29 thousand km from the sea level.
- It is estimated that 97% of the effective atmosphere is up to the height of 29 km. The early atmosphere was ANOXYGENIC (without oxygen).
Origin of Atmosphere
- Initially, the Earth was a hot surface and the original atmosphere escaped into space and was lost. Today’s atmosphere is the secondary atmosphere/tertiary atmosphere.
- This atmosphere has come out from inside the earth by the process of degassing from volcanoes.
- Rain fell for millions of years after cloud formation & much of CO2 went into oceans & N2 dominated.
- O2 was added later after the evolution of green plants.
- Gradually atmosphere has become more habitable.
Composition of Atmosphere
- The atmosphere is composed of three major constituents.
- (a) Gases:
- Two categories of gaseso Constant/permanent gases-
- Nitrogen, oxygen & organ which constitutes 78%, 20.9% & 0.9% are major permanent gases by volume of the gaseous composition of the atmosphere.
- The proportion of constant gases remains constant in the lower atmosphere (up to 80 km from sea level).
- Variable/Minor gases-
- Include water vapour, carbon dioxide, ozone, hydrogen, helium, etc.
- (b) Aerosols:
- They are suspended particulate matter (SPM) in the atmosphere including solid particles of varying sizes and liquid droplets.
- Help in the selective scattering of shortwave electromagnetic solar radiation & act as hydroscope nuclei.
- Their presence in the atmosphere causes variations in weather conditions.
- (c) Water Vapour:
- Responsible for different types of condensation & precipitation.
- Considered as a primary greenhouse gas 90% of total atmospheric vapour is found up to the height of 5 km.
Layering/Structure of Atmosphere
- The atmosphere is a mechanical mixture of several gases, hence behaving as a unified gas and for much height, the atmosphere behaves like a single gas.
- But the atmosphere has layers & the layering of gases does not separate according to molecular weight. The atmosphere has however density layers.
- The layered structure of the atmosphere has been classified into two major considerations:
(A) On the basis of temperature:
- This is the lowermost layer of the atmosphere zone and is also called a turbulent zone because of the dominance of turbulent activities.
- The layer is also known as a convective layer because of the turbulence of eddies.
- Decrease in temperature at 6.50C/km as the atmosphere is heated from below mainly. The density of the atmosphere decreases with height & density of GHG also declines. The troposphere is most important because of all the weather phenomena that occur in this layer.
- Contains about 75% of the gaseous mass of the atmosphere, most of the water vapour, aerosols & pollutants.
- The average height of the troposphere is about 16km over the equator and 6 km over the poles. But there is seasonal variation in the height the of troposphere. (height increases during summer & decreases during winter).
- The troposphere forms the limit of all upward convection currents because beyond Tropopause temperature increases and buoyancy is not found. This is because the size of air parcels increases. This causes its temperature to decrease and hence limits its buoyancy.
- On average the upper limit of the stratosphere is 50 km, 15-35km called as ozonosphere.
- The temperature gradually rises upward as it becomes as high as 00C at the height of 50 km, the stratosphere’s upper limit known as stratopause.
- Increase of temperature because of the absorption of ultraviolet solar radiation by ozone and lesser density of air.
- Layer is characterized by the near absence of weather phenomena because of stable conditions.
- Sometimes cirrus clouds known as the mother of pearl clouds or nacreous clouds appear in the lower stratosphere.
- The lower part of the stratosphere is significant for life forms because of the concentration of ozone gas (O3) between the height of 15-35 km.
- The maximum O3 concentration is at the height of 22km, though ozone has been discovered up to 80 km.
- Extends between 50 km & 80 km. Temperature decreases with increasing height.
- Noctilucent clouds appeared during the summer season over polar areas. The lowest temperature is noticed at the mesosphere above which temperature increases with increasing height.
- Beyond the mesosphere, wherein temperature increases rapidly with increasing height but air pressure becomes extremely low due to very low atmospheric density.The thermosphere is divided into two layers –
- Extends from 80 km to 640 km. There are several ionic layers in this sphere where the lower ionosphere has greater ion concentration & upper ionosphere has a lower concentration of ions.
- The uppermost layer of the atmosphere extends beyond 640 km. height from the sea level.
- Temperature increases in this layer and the layer has a very low concentration of ions.
- The zone is characterized by Van Allen radiation belts.
B. On the basis of Chemical characteristics
Based on the chemical composition the atmosphere is divided into two broad zones:
- The lower portion of the atmosphere
- & extends up to the height of 90 km from sea level.
- Oxygen (20.9%) and nitrogen (78%) are the main constituent gases.
- The zone is called the homosphere because of the homogeneity of the proportion of variation gases.
- On the basis of thermal conditions, the homosphere has been divided into three layers.
- (a) Troposphere
- (b) Stratosphere
- (c) Mesosphere.
- Extends from 90 km to 10000 km. Different layers of this sphere vary in their chemical and physical properties.
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