The troposphere is the lowermost portion of Earth's atmosphere and the one in which clouds and most other weather phenomena occur. This layer extends to an altitude of 16-18 km over tropical regions, decreasing to less than 10 km over the poles, and contains approximately 80% of the total air mass. Generally, jets fly near the top of this layer. The troposphere is directly below the stratosphere.

The word troposphere stems from the Greek "tropos" for "turning" or "mixing". This region, constantly in motion, is the densest layer. Nitrogen and oxygen are the primary gases within this region. The change of temperature with height is larger than in other layers, the temperature decreasing with altitude.

The tropopause marks the limit of the troposphere and the beginning of the stratosphere. The temperature above the tropopause increases slowly with height up to about 50 km.