The troposphere is the lowest portion of Earth’s atmosphere. It contains approximately 80% of the atmosphere’s mass and 99% of its water vapor and aerosols. The average depth of the troposphere is approximately 17 km in the middle latitudes. It is deeper in the tropics, up to 20 km, and shallower near the polar regions, at 7 km in summer, and indistinct in winter. The lowest part of the troposphere, where friction with the Earth’s surface influences air flow, is the planetary boundary layer.