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From Citizendium, the Citizens' Compendium

The troposphere is the lowest of the main layers of the Earth's atmosphere. The troposphere extends from Earth's surface to an average height of about 12 km.

The name "troposphere" is derived from the Greek tropos, meaning "turning." This reflects the fact that air within the troposphere commonly "overturns" or mixes. A vivid example of this overturning is the motion within a deep thundercloud, where air parcels near the Earth's surface often ascend to the top of the troposphere (called the tropopause).

The troposphere contains about 80% of the mass of Earth's atmosphere and over 99% of its water vapor.[1] Almost all the phenomena that we associate with day-to-day weather occur within the troposphere.