Vertical Structure of Atmosphere
The vertical extent of the atmosphere is difficult to ascertain, for it has no sharp boundary with extraterrestrial space. Atmospheric phenomena associated with the earths' magnitude and gravitational fields extend outward for several thousand kilometers to a vague zone of nebulous gases and radiation praticles that become rarer and rarer until at last terrestrial characteristics of the atmosphere cease.
The atmosphere exhibits vertical temperature if four major layers, or shells are present
1- Troposphere:- The troposphere is the lower portion of the atmosphere, extending upto about 8 km at the poles and 16 km at the equator. It contains three-fourths of the atmospheric mass and is the realm of clouds, storms and convective motion. Thermal convection, being better developed in the low latitudes, is responsible for the greater vertical extent of the troposphere near the equator. Convective activity also explains the somewhat greater height of the troposphere in summer than in winter at a given latitude. The outstanding characteristics of the troposphere is the fairly uniform decrease in temperature with increase in altitude to minimum of -50 C or -60 C.
2- Stratosphere:- The zone marking the end of this temperature decrease is the tropopause. Above the tropopause lies the stratosphere, where temperature is nearly constant upward to about 20 km and then increases, owing to absorption of ultraviolet radiation by ozone. Sulphate particles are concentrated at the inner margin of the stratosphere. Near its outer limit, the stratoparse, concentration of ozone decrease. The height of this zone is about 50 to 60 km.
3- Mesosphere:- Beyond the stratopause, which has a mean altitude of about 50 km, is the mesosphere. Temperatures change slowly with altitude the lower mesosphere but then decease to a minimum near the mesopause, at 80 km. Most meteorites burn and disintegrate as they experience increasing friction in this layer.
4- Thermosphere:- In the thermosphere the temperature again rises to values which vary with solar activity and at times approach 2000 C at about 500 km. Such temperatures are not strictly with those registered by thermometers at the earth's surface. Although the gas molecules exhibit high kinetic energy, and therefore have high temperatures, they are too sparse to transfer significant quantities of energy to an ordinary thermosphere. The exposed hand of an astronaut would not feel hot in the thermosphere.
5- Ionosphere:- Coinciding with the lower portion of the thermosphere in the ionosphere, and atmospheric layer at 60 to 500 km delimited on the basis of ionized particles and their effects on the propagation of radio waves. Little was known of the ionosphere until it was found that radio waves are reflected by ionized layers at great heights.
6- Exosphere:- The height of this one is about 500-1000 km. In exosphere generally Hydrogen and Helium gasses are predominantly found.
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