The troposphere is the lowest portion of Earth's atmosphere. It contains approximately 75% of the atmosphere 's mass and 99% of its water vapor and aerosols.The average depth of the troposphere is approximately 17 km (11 mi) in the middle latitudes.he chemical composition of the troposphere is essentially uniform, with the notable exception of water vapor. The source of water vapour is at the surface through the processes of evaporation and transpiration.