Stratosphere
The stratosphere is the atmospheric layer above the troposphere, extending to about 50 kilometers, where temperature rises with altitude.
Definition
The stratosphere is the calm, dry, thin layer of atmosphere above the troposphere, extending from the tropopause up to about $50\ \text{km}$, and unlike the layer below it, its temperature rises with altitude because ozone there absorbs ultraviolet radiation from the Sun and converts it to heat. This warm-over-cold structure makes the stratosphere very stable, with almost no vertical mixing, clouds, or weather, and the ozone layer, concentrated between about $15$ and $35\ \text{km}$, sits within it, produced and destroyed by the Chapman cycle in which oxygen molecules split under UV light and recombine into ozone. The Brewer-Dobson circulation slowly lifts air in the tropics and sinks it at high latitudes over years, and features such as the polar vortex govern much of the layer's behavior.
Example
Weather balloons and some high-altitude spy planes fly in the stratosphere, well above the clouds and storms below, and volcanic ash and sulfur from Mount Pinatubo's 1991 eruption reached the stratosphere and lingered for about two years, cooling the whole planet slightly. Chlorofluorocarbons released at ground level are stable enough to survive the trip up, where UV light frees chlorine atoms that destroy ozone catalytically, opening the Antarctic ozone hole each spring, though stratospheric chlorine has fallen and the layer is expected to recover to 1980 levels around 2066 following the 1987 Montreal Protocol.
Key Insight
The name comes from the way the stratosphere is layered, with air sliding sideways easily but almost never moving up or down, so anything that gets into it, whether ozone-destroying chemicals or volcanic aerosols, tends to stay for a long time. It is also a rare case where a global environmental agreement has clearly worked, since ozone levels have been recovering steadily since the Montreal Protocol phased out the chemicals responsible.