The troposphere's the lowest portion of Earth's atmosphere. It contains approximately 75 percent of the atmosphere's mass and 99 percent of its water vapor and aerosols. The average depth of the troposphere's approximately in the middle latitudes. It's deeper in the tropical regions, up to, and shallower near the poles, at in summer, and indistinct in winter. The lowest part of the troposphere, where friction with the Earth's surface influences air flow,'s the planetary boundary layer. This la… (More on Troposphere)