Atmospheric stratification is the division of the atmosphere into distinct layers, each with specific characteristics such as temperature or composition.
In general, air pressure and density decrease in the atmosphere as height increases. However, temperature has a more complicated profile with altitude. Because the general pattern of this profile is constant and recognizable through means such as balloon soundings, temperature provides a useful metric to distinguish between atmospheric layers.
The lowest layer is the troposphere, in which temperature decreases at what is known as the atmospheric lapse rate. The farther that air is from Earth's surface, the less heat it receives radiated from the ground. This continues until the tropopause is reached. The tropopause ranges in altitude depending on latitude and weather patters, but is typically around 9 km at the poles, and 17 km at the equator.[1]
Above the tropopause, the temperature begins to rise due to increased absorption of ultraviolet radiation by the ozone layer. This region is known as the stratosphere. While the temperature may be -60° Celsius at the troposphere, the top of the stratosphere is much warmer, and may be near freezing temperature. A temperature gradient that warms with altitude, as in the stratosphere, is known as an inversion.
The stratosphere is capped by the stratopause, above which is the mesosphere. The temperature again falls with height in the mesosphere, and may drop to -100° Celsius at the top of this layer in the mesopause.[2]
The next layer is the thermosphere, where there is once again a temperature inversion. 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 absorption of solar radiation by such a thin atmosphere causes the temperature to rise tremendously, to an average of 80°C at 120 km altitude. The air is so rarified, that an individual molecule (of oxygen, for example) travels an average of 1 kilometer between collisions with other molecules.[3] Because of the relative infrequency of collisions, air above the mesopause is poorly mixed compared to air below. While the composition from the troposhpere to the mesosphere is fairly constant, above a certain point, air is poorly mixed and becomes compositionally stratified. The point dividing these two regions is known as the turbopause. The region below is the homosphere, and the region above is the heterosphere.
The last layer is termed the exosphere, and it is here that a quickly moving particle can easily escape Earth's gravity and leave the atmosphere without colliding with another molecule.
References
- ^ http://www-das.uwyo.edu/~geerts/cwx/notes/chap01/tropo.html
- ^ http://www.ace.mmu.ac.uk/eae/Atmosphere/Older/Mesosphere.html
- ^ Ahrens, C. Donald. Essentials of Meteorology. Published by Thomson Brooks/Cole, 2005.
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