Atmosphere

 

Atmosphere (वातावरण) means Layer of vapour & gases (atmos-vapor &sphere-layer of gas) that may surround a material body of sufficient mass, and that is held in place by the gravity of the body. An atmosphere may be retained for a longer duration, if the gravity is high and the atmosphere’s temperature is low. Some planets consist mainly of various gases, but only their outer layer is their atmosphere. Earth’s atmosphere, which contains  oxygen used by most organisms  for respiration and carbon dioxide used by plants,  algae and cyanobacteria  for  photosynthesis , also protects living organisms from genetic damage by solar  ultraviolet radiation.

The atmospheric composition on Earth is largely governed by the by-products of the very life that it sustains. Earth’s atmosphere contains 78.08% nitrogen, 20.95% oxygen, a variable amount water vapor, 0.93% argon, 0.038% carbon dioxide, and traces of hydrogen, helium, and other “noble” gases.

The Earth’s atmosphere consists, from the ground up, of the  Troposphere (which includes the planetary boundary layer as lowest layer) Stratosphere (which includes the ozone layer), Mesosphere, Thermosphere (which contains the ionosphere), Exosphere. Each of the layers has a different temperature range with height.

The atmosphere of Earth is a layer of gases surrounding the planet Earth that is retained by Earth’s gravity. The atmosphere protects life on Earth by absorbing ultraviolet solar radiation, warming the surface through heat retention (greenhouse effect), and reducing temperature extremes between day and night

Air is the name given to atmosphere used in breathing  and  photosynthesis. Air is mainly composed of nitrogen, oxygen, and argon, which together constitute the major gases of the atmosphere.. Many natural substances may be present in tiny amounts in an unfiltered air sample, including dust, pollen and spores, sea spray, and volcanic ash. Various industrial pollutants also may be present, such as chlorine (elementary or in compounds), fluorine compounds, elemental mercury, and sulfur compounds such as sulfur dioxide [SO₂].

In general, air pressure and density decrease in the atmosphere as height increases. Earth’s atmosphere can be divided into five main layers. From highest to lowest, these layers are:

1)Troposphere:-The troposphere begins at the surface and extends to between 9 km (30,000 ft) at the poles and 17 km (56,000 ft) at the equator(विषुववृत्त), with some variation due to weather. The troposphere is mostly heated by transfer of energy from the surface, so on average the lowest part of the troposphere is warmest and temperature decreases with altitude(उंची).

2)Stratosphere:-The stratosphere extends from the tropopause to about 51 km. Temperature increases with height due to increased absorption of ultraviolet radiation by the ozone layer. While the temperature may be −60 °C (−76 °F; 210 K) at the tropopause, the top of the stratosphere is much warmer, and may be near freezing^. The stratopause, which is the boundary between the stratosphere and mesosphere, typically is at 50 to 55 km

3)Mesosphere:-The mesosphere extends from the stratopause to 80–85 km . It is the layer where most meteors burn up upon entering the atmosphere. Temperature decreases with height in the mesosphere. The mesopause, the temperature minimum that marks the top of the mesosphere, is the coldest place on Earth and has an average temperature around −85 °C (−120 °F; 190 K). At the mesopause, temperatures may drop to −100 °C (−150 °F; 170 K). Due to the cold temperature of the mesosphere, water vapor is frozen, forming ice clouds . A type of lightning referred to as either sprites or ELVES, form many miles above thunderclouds in the troposphere.

4)Thermosphere:-Temperature increases with height in the thermosphere from the mesopause up to the thermopause, then is constant with height. Unlike in the stratosphere, where the inversion is caused by absorption of radiation by ozone, in the thermosphere the inversion is a result of the extremely low density of molecules. The temperature of this layer can rise to 1,500 °C (2,700 °F), though the gas molecules are so far apart that temperature in the usual sense is not well defined. The point dividing these two regions is known as the turbopause. The top of the thermosphere is the bottom of the exosphere, called the exobase.

5)Exosphere:-The outermost layer of Earth’s atmosphere. It is mainly composed of hydrogen and helium. The particles are so far apart that they can travel hundreds of kilometers without colliding with one another.