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The troposphere (from Greek: tropein - to change, circulate or mix) is the lowermost layer of the Earth's atmosphere. Most of the weather phenomena, systems, convection, turbulence and clouds occur in this layer, although some may extend into the lower portion of the stratosphere.

The troposphere contains almost all the atmospheric water vapour, in fact it contains about 70 to 80 per cent of the total mass of the Earth's atmosphere and 99 per cent of the water vapor. Temperature and water vapor content in the troposphere decrease rapidly with altitude and thus most of the water vapour in the troposphere is concentrated in the lower, warmer zone. Water vapor concentrations vary with latitude. They are greatest above the tropics and decrease toward the polar regions.

In the troposphere air temperature on average decreases with height at an overall positive lapse rate of about 6.5°C/km , until the tropopause, the boundary between the troposphere and stratosphere, is reached. The tropopause, extending from 11 to 20 km, is an isothermal layer in the atmosphere where temperature remains constant over a distance of about 9 km. Troposphere and tropopause are also known as the lower atmosphere. It is also the layer in the atmosphere where the winds increase with height and jet streams usually occur in the upper troposphere, just below the tropopause.

However, lapse rate variations that sometimes occur within the troposphere include inversions (temperature increase with height within some limited layer). In the upper troposphere temperature falls below about -50°C and only little moisture is present or condensing out as ice crystals.

The thickness of the troposphere varies from about 7 to 8 km (5 mi) at the poles to about 16 to 18 km (10 to 11 mi) at the Equator. In addition, it varies in height according to season, being thinner in winter when the air is densest. This seasonal effect is strongest at the mid-latitudes, where it varies around 11 km (7 mi). Increasingly, it is understood that air movements in the upper troposphere greatly influence weather systems in the lower troposphere.

The term troposphere was first used in 1902 by Léon Philippe Teisserenc de Bort, a french meteorologist who was a pioneer in the use of meteorological balloons.