| Troposphere | Structure of the Earth's Atmosphere : Troposphere |
Troposphere The troposphere is the atmospheric layer closest to the planet and contains the largest percentage of the mass of the total atmosphere. It is characterized by the density of its air and an average temperature decrease with height ( lapse rate ) of 6 oC per kilometer.
Temperature and water vapour content in the troposphere decrease rapidly with altitude. Water vapour plays a major role in regulating air temperature because it absorbs solar energy and thermal radiation from the planet's surface. The troposphere contains 99 % of the water vapour in the atmosphere. Water vapour concentrations vary with latitudinal position(i.e. North to South). They are greatest above the tropics, where they may be as high as 3 %, and decrease toward the polar regions.
All weather phenomena occur within the troposphere, although turbulence may extend into the lower portion of the stratosphere. Troposphere means "region of mixing" and is so named because of vigorous convective air currents within the layer.
The boundary between the troposphere, and the stratosphere is called the tropopause. The height of the tropopause from the ground ranges from 8 km in high latitudes, to 18 km above the equator. Its height also varies with the seasons; highest in the summer and lowest in the winter. Air temperature within the tropopause remains constant with increasing altitude. The tropopause is a boundary layer defined by a sudden change in lapse rate.
| Stratosphere | The structure of the Earth's atmosphere: Stratosphere |
Stratosphere The stratosphere is the second major stratus (layer) of air in the atmosphere. It resides between 10 and 50 km above the planet's surface. The air temperature in the stratosphere remains relatively constant up to an altitude of 25 km. Then it increases gradually to 470-490 oC at the boundary with mesosphere, the stratopause ( which is at a height of ~50 km). The boundary is marked by a decrease in temperature. Because the air temperature in the stratosphere increases with altitude, it does not cause convection and has a stabilising effect on atmospheric conditions in the region.
Ozone plays the major role in regulating the thermal regime of the stratosphere, since water vapor content within the layer is very low. The ozone absorbs the sun's ultraviolet radiation and warms up, therefore temperature increases with ozone concentration.
| Mesosphere | The structure of the Earth's atmosphere: Mesosphere |
Mesosphere The temperature decreases once more as we go through the mesosphere, which is the layer between 50 and 80 km. The temperatures in this layer reach 190-180 K (-83 to -93 0C)at an altitude of 80 km. In this region, concentrations of ozone and water vapor are negligible. Hence the temperature is lower than that of the troposphere or stratosphere. With increasing distance from Earth's surface the chemical composition of air becomes strongly dependent on altitude and the atmosphere becomes enriched with lighter gases. At very high altitudes, the gases begin to form into layers according to molecular mass (weight), because the force of gravity is greater on the heavier molecules. It is in this layer that foreign bodies (such as meteors and spacecraft) entering the atmosphere start to warm up.
| Thermosphere | The structure of the Earth's atmosphere: Thermosphere |
Thermosphere The thermosphere is located above the mesosphere and is separated from it by the mesopause transition layer. The temperature in the thermosphere generally increases with altitude up to 1000-1500 K. This increase in temperature is due to the absorption of intense solar radiation by the limited amount of remaining molecular oxygen. At an altitude of 100-200 km, the major atmospheric components are still nitrogen and oxygen. At this extreme altitude gas molecules are widely separated. It is within the thermosphere that the aurora phenomena may be observed.
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