Troposphere
Troposphere is the lowest layer of Earth's atmosphere. It extends from Earth's surface to an average height of about 12 km, although this altitude varies from about 9 km at the geographic poles to 17 km at the equator, with some variation due to weather. The troposphere is bounded above by the tropopause, a boundary marked in most places by a temperature inversion (i.e., a layer of relatively warm air above a colder one), and in others by a zone which is isothermal with height.
Almost all weather conditions take place within the troposphere. Its chemical composition is primarily nitrogen (78%) and oxygen (21%), with only small concentrations of other trace gases. Nearly all atmospheric water vapor or moisture is found in the troposphere, so it is the layer where most of Earth's weather occurs. It has a great deal of vertical mixing due to solar heating at the surface. This promotes vertical mixing (unlike the stratosphere, which has stratified layers of temperature).
The word troposphere is derived from the Greek tropos (meaning "turn, turn toward, change") and sphere (as in the Earth), reflecting the fact that rotational turbulent mixing plays an important role in the troposphere's structure and behavior. The lowest part of the troposphere, where friction with the Earth's surface influences air flow, is the planetary boundary layer. This layer is typically a few hundred meters to 2 km (1.2 mi) deep depending on the landform and time of day.
Above this layer is the free troposphere, where the air is less influenced by the drag of the surface and the flow is more geostrophic. The changes in height of the planetary boundary layer can be triggered by changes in surface radiative heating and cooling, such as occurs at daybreak and sunset, which respectively increase and decrease the thickness of the planetary boundary layer. This can be as high as the tropopause if these top and bottom boundaries become indistinguishable, such as occurs during the high solar insolation around noon.
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Contributors: Prab R. Tumpati, MD
