Troposphere
![Space Shuttle Endeavour silhouetted against the atmosphere. The orange layer is the troposphere, the white layer is the Stratosphere and the blue layer is the mesosphere.[1] (The shuttle is actually orbiting at an altitude of more than 320 km (200 mi), far above all three layers.)](/uploads/202502/17/Endeavour_silhouette_STS-1302701.jpg)
The troposphere is the lowest portion of Earth's atmosphere, and is also where all weather takes place. It contains approximately 75% of the atmosphere's mass and 99% of its water vapour and aerosols. The average depths of the troposphere are 20 km (12 mi) in the tropics, 17 km (11 mi) in the mid latitudes, and 7 km (4.3 mi) in the polar regions in winter. The lowest part of the troposphere, where friction with the Earth's surface influences air flow, is the planetary boundary layer. This layer is typically a few hundred meters to 2 km (1.2 mi) deep depending on the landform and time of day. Atop the troposphere is the tropopause, which is border between the troposphere and stratosphere. The tropopause is an inversion layer, where the air temperature ceases to decline with height and remains constant through its thickness.