平流层（英语：Stratosphere），亦称同温层，位于对流层的上方和中间层的下方。其下界在中纬度地区位于距离地表10km处，在极地则在8km左右，其上界则约在离地50km的高度。平流层的温度上热下冷，随着高度的增加，平流层的气温在起初基本不变，然后迅速上升。在平流层里大气主要以水平方向流动，垂直方向上的运动较弱，因此气流平稳，基本没有上下对流。

由于含有大量臭氧，平流层的上半部分能吸收大量的紫外线，这层使特殊气体形成的区域也被称为臭氧层。

The stratosphere (/ˈstrætəˌsfɪər, -toʊ-/) is the second major layer of Earth's atmosphere, just above the troposphere, and below the mesosphere. About 20% of the atmosphere's mass is contained in the stratosphere. The stratosphere is stratified in temperature, with warmer layers higher and cooler layers farther down. This is in contrast to the troposphere, near the Earth's surface, which is cooler higher and warmer farther down. The border between the troposphere and stratosphere, the tropopause, marks where this temperature inversion begins. Near the equator, the stratosphere starts at 18 km (59,000 ft; 11 mi); at mid latitudes, it starts at 10–13 km (33,000–43,000 ft; 6.2–8.1 mi) and ends at 50 km (160,000 ft; 31 mi); at the poles, it starts at about 8 km (26,000 ft; 5.0 mi). Temperatures vary within the stratosphere with the seasons, in particular with the polar night. The greatest variation takes place over the poles in the lower stratosphere, but are largely steady at lower latitudes and higher altitudes.