The Troposphere is the lowermost portion of Earth's atmosphere. It is the densest layer of the atmosphere and contains approximately 75% of the mass of the atmosphere and almost all the water vapour and aerosol.

The troposphere extends from the Earth's surface up to the tropopause where the stratosphere begins. The depth of the troposphere is greatest in the tropics (about 16km) and smallest at the poles (about 8km).

The word troposphere stems from the Greek "tropos" for "turning" or "mixing". The troposphere is the most turbulent part of the atmosphere and is the part of the atmosphere in which most weather phenomena are seen. Generally, jet aircraft fly near the top of the troposphere to avoid turbulence by flying above it.