Now it is clear that atmosphere starts from Earth surface but scientists have different views about its maximum height. On the basis of information provided by radars, balloons, rockets, satellites at present large number of scientists agree on a point that the height of atmosphere is 29000 kilometer and layer upto 800 kilometer is considered as important part of atmosphere.
On the bases of air pressure, temperature and weather phenomena, out of the total amount of major gases in atmosphere 97% are found upto 29 km of height from Earth while 50% are found upto 5.6 kilometers only.
In 20th Century with the help of advanced technical devices, scientists have succeeded in solving major mysteries about atmosphere. Teisserence de Bort, S. Petteisun, Sir Napier shaw, Picardy, Kennelly Heaviside, Ferrel etc. have played an important role in this field. Atmosphere may be divided into layers or spheres on the basis of height, every layer having its own importance. Boundaries of these layers are also arbitrarily established as other natural boundaries.
Structure of atmosphere can be divided in two ways:
i. On the basis of temperature and
ii. On basis of Chemical Composition.
On the basis of temperature S. Peterson has divided atmosphere into five layers vertically:
This is the lowest layer of atmosphere and it is very important for all the living organisms because all the activities and aspects related with weather e.g. Evaporations, condensation, precipitation, storm, lightening etc. originate in this sphere. Around 75% mass of gases and nearly all water vapours and aerosols are present in this layer.
Teissesence de Bort was the first scientist who used this name ‘…T..ro..po…. sphere’ it has been derived from Greek word ‘Tropos’ which means mixing or Turbulence, overall we can say ‘region of mixing’.
One of the important feature of this sphere is that in this part of atmosphere, temperature decreases with increase in height. In normal conditions the rate of decrease is 6.5 degree Celsius per kilometer or 3.6 degree F per 1000 feet. This is also known as mean temperature fall rate. This happens because of decrease in gases with increase in height, air pressure, decrease in aero sides with increasing height. However, due to local reasons, at some places this phenomenon reverses also.
The height of Troposphere changes with change of weather. The change is registered lesser towards poles as compared to tropical regions and in winters as compared to summers. According to scientists the height of this sphere on Equator is 16 km and on poles it is 8 km. Interesting fact is that the temperature decreases at the upper part of this sphere at Equator not on poles. At average its height is upto 12 kilometer from ground.
The region between Troposphere and Stratosphere which is 1.5 kilometer high, known as boundary of troposphere. The fall in temperature comes to an end in this region named as tropopause. Turbulent mixing of gases, winds, and radiation etc. none of the weather activities take place in this region. The minimum temperature in winters, at Equator (At the height of 17 kilometer) is 70° C and in summers over the poles it is – 45° C (At the height of 9 kilometer). Air pressure in this region is also between 100 millibar over Equator and 250 millibar over poles.
This sphere starts from 12 to16 kilometers above the Earth surface and extend upto 50 kilometers known for isothermal characteristics also. Although scientists have different views about the height, width and temperature but they all agree on one point that at the lower layer of this sphere temperature does not change with increase in height but at the height of 20 KM in upper stratosphere temperature starts increasing because of absorption of ultra-violet rays by ozone gases. At the height of 50 KM it becomes 0° Celsius (32°F).
Region where ozone gas is present in abundance is also known as ozone layer. This gas is present in abundance between 15 to 35 kilometers above the Earth surface, which originates when oxygen molecule disintegrates into two atoms of oxygen and then integrating back into a molecule. Molecules of ozone layer are not stable, their origin and disintegration is a gradual and continuous process.
It has been proven that the ozone layer is depleting day by day because of human activities like spray and synthetic chemicals used for air conditioning which are stable compounds of clorine, florine and carbon on Earth but when these reach upto outer layers of atmosphere by vertical currents of air they affect the ozone layer.
Because of depletion of ozone ultra-violet rays are increasing the temperature of Earth. Moreover they cause skin problems, blindness and also kill sensitive organisms. Apart from all this, these rays also decrease the process of photosynthesis, supersonic jets fly at the height of 20 km above the land surface and they emit nitrogen oxide which also affects the ozone layer.
Above the upper boundary of stratosphere, which extends from 50 kms. above surface of Earth, is mesosphere, upto 80 kms. In this layer temperature again starts decreasing with the increase in height and at the upper boundary of this layer it touches down to -80°C point. Air pressure is very low in this layer. At the height of 50 kms it is 1 mb and at height of 80 kms it drops to 0.01 mb.
The height of this sphere extends from 80 to 640 kms above surface of Earth. Credit of discovering this layer goes to scientists Kennelly and Heaviside who, with the help of radio waves proved that the atoms and molecules of the gases present in this layer get electrically changed by the addition and removal of electrons due to the effect of ultraviolet, Xray and Gama rays.
Short wavelength rays of solar energy (UV, X, L) get absorbed by molecules and atoms of nitrogen and oxygen because of which temperature increases upto 1000°C in this scanty pressure layer of atmosphere. Because of scanty air pressure, temperature here is quite different as compared to the temperature felt at the surface of Earth.
On the basis of height ionosphere may be further divided into following layers:
This is the outermost layer of atmosphere and we do not have much information about this layer. It lies above the height of 640 kms. Only Helium and hydrogen are found in this layer. Moreover the density of this layer is very low.