Atmosphere’s total mass , 99 per cent is within the height of 32 km from the earth’s surface and is held to it by the force of gravity. The atmosphere is energised by the Sun.
ORIGIN OF THE ATMOSPHERE:
Today’s atmosphere is the result of very gradual change starting about 5 billion years ago by accretion of cold parti- cles chiefly of iron and magnesium silicate, iron and graphite. Then, the earth was too small to retain the primordial atmosphere of light gasses.
Gravitational collapse and radioactive decay caused the earth to heat up and material differentiated giving central solid nickel-iron core, liquid iron silicate shell, a mantle and lithosphere, In this process, degassing took place, forming a hew atmosphere and hydrosphere.This atmosphere was devoid of free oxygen but contained methane, ammonia (10-68 per cent), carbon dioxide (10-15 per cent) and water vapour (60-70 per cent).
ATMOSPHERE COMPOSITION:
The composition of the atmosphere in terms of three major gases – nitrogen, oxygen and argon. There are in addition rare gases like neon, krypton, and xenon, also called noble gases. This layer is generally, called atmosphere. Above 90 km, the composition begins to change with progressive increase in the lighter gases. This layer is known as heterosphere.
The oxygen and nitrogen, make up about 99 per cent of, the clean dry air of the homosphere. In addition, it contains small amount of carbon dioxide, water vapour, ozone, inert gases like xenon, krypton, neon and argon (which don’t take part in chemical reaction) and huge amount of solid and liquid particles, collectively called aerosols.
There has been pronounced an increase (10 times) in the content of carbon dioxide in the atmosphere from 1890 to 1970, Ozone is found in very small fraction in the strat- osphere between 20 and 25 km from the earth’s surface. It is, however, very useful as it absorbs ultra-violet ray sand thus, protects life from these harmful rays.
The water vapour and dust particles are the important variables of weather and climate. They are the sources of all forms of condensation and principal absorbers of heat received from the sun or radiated from the earth. Besides, they affect the stability of the atmosphere.
Water vapour may not exceed 3 per cent to 4 per cent of total volume of air. The amount of water vapour in the atmosphere decreases from the equator towards the poles. Nearly 90 per cent of it lies below 6 km of the atmosphere.
It is in this layer of the atmosphere where solid particles of dust, salt, pollens etc. are held in suspension. They act as hygroscopic nucleus with positive charge and entrap the negatively charged water particles to produce clouds. In the upper layer of the atmosphere microscopic dust particles scatter incoming solar rays and absorb all colours except blue, giv- ing blue colour to the sky. The larger size particles on the other hand, are responsible for red and orange colours at sunrise and sunset. Several Posts and Blogs are available in Internet for further information.
COMPOSITION OF THE ATMOSPHERE:
Component Per Cent
by Volume
Nitrogen (N2) 78.08
Oxygen (O2) 20.94
Argon (Ar) 0.93
Carbon dioxide (CO2) 0.03
Neon (Ne) 0.0018
Helium (He) 0.0005
Ozone (Og) 0.00006
Hydrogen (H) 0.00005
Methane (CH) Trace
Krypton (Kr) Trace
Xenon (Xe) Trace
STRUCTURE OF ATMOSPHERE ACCORDING TO TEMPERATURE:
Troposphere; stratosphere; Mesosphere; Ionosphere; Thermosphere; Exosphere
TROPOSPHERE: It is the most important zone for weather phenomenon, because of:
Gradual decrease of temperature with height i.e. 6.5 degree C per km. Temperature decreases except winter pole. The lowest part of troposphere up to 1.5-2 km. is called ‘Friction Layer’, where topography greatly influences wind speed and circulation It contains all the major atmosphere pollutants.
STRATOSPHERE: From Tropopause to about 50 Km; It is an Isothermal region and extremely dry free with clouds, water vapour and dust; here air is at rest and movement is almost horizontal
Some clouds found are called Mother of Pearls or Nacreous. Contains much of Ozone (03); therefore called Ozonosphere, especially between 15 km to 35 km from the sea level. The combining of atmosphere oxygen 02 with individual oxygen results in the creation of ozone.
CHEMOSPHERE: Chemosphere extends from tropo- sphere to an altitude of 50 kms. Overlapping both homosphere and heterosphere.In this air glove occurs at night especially geen and red. It is a part of Stratosphere.In this air glow occurs at night, especially green and red.
MESOSPHERE: Height from 50 km to 80 km. The temperature decreases fairly with height with the minimum temperature of about -9°C.
MESOPAUSE: (the top of the layer); above Mesopause temperature increases with increasing heightThe presence is because of meteoric dust particles.
THERMOSPHERE: The part of the atmosphere beyond Mesopause is known as termosphere wherein temperature increases rapidly with increasing height.It is above 200 km, and NO, and O, are found.Its lower portion is composed mainly of nitrogen and oxygen in molecular and atomic forms. Rapid temperature increase approaching 1700 degree C, at 350 kms.
Thermosphere is divided into two layers: (1) Ionosphere and (2) Exosphere
IONOSPHERE: 80 kms, to 640 kms, and above Radio waves found; it is a region of electrically charged or ionized air lying next to Mesosphere.High electron density.150 km.-380 km. known as Appleton. Absorbs deadly X-rays. The northern lights or aurora borealis are found.
This layer is called Kennelly Heaviside Layer (99- 130 kms) here interaction takes place between solar- ultraviolet photons with nitrogen. Sporadic Layer is associated with high velocitywinds. . The bulk of the atmosphere consists of electrically neutral atoms and molecules.
At high altitudes, however, a significant fraction of the atmosphere is electrically charged. This region is generally called the Ionosphere.
EXOSPHERE: 640 kms and above.The atoms of oxygen, hydrogen and helium form the tenuous atmosphere.The density becomes extremely low and the arms phere resembles a nebula because it is highly refied. Aurora Australis and Aurora Borealis are produce magnetic storms on the sun discharge electrified particles in the space. The earth’s magnetic poles attract these particles. Aurora Australis (the southern dawn). Autor Borealis (the northern dawn).