The troposphere is the lowest portion of Earth's atmosphere. It contains approximately 75% of the atmosphere's mass and 99% of its water vapour and aerosols. The average depth of the troposphere is approximately in the middle latitudes. It is deeper in the tropics, up to , and shallower near the polar regions, approximately in winter. 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 deep depending on the landform and time of day. The border between the troposphere and stratosphere, called the tropopause, is a temperature inversion.