The layer of air directly above Earth’s surface where temperature usually decreases with height is the troposphere. It is the lowest major layer of the atmosphere and contains nearly all atmospheric water vapour and most clouds.
The troposphere extends from the surface to the tropopause. Its average depth is about 8 kilometres near the poles and about 18 kilometres near the equator, because stronger heating makes air expand and the tropical troposphere rise higher.
Weather occurs mainly in the troposphere because it contains moisture and is strongly mixed by convection. Above it lies the stratosphere, where temperature generally increases with height because ozone absorbs ultraviolet radiation. The boundary between the two layers is the tropopause, whose altitude changes with latitude and weather conditions.