Stratosphere

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Atmosphere diagram showing stratosphere. The layers are not to scale: from Earth's surface to the top of the stratosphere (50km) is just under 1% of Earth's radius. (Click to enlarge)

The stratosphere is the second layer of Earth's atmosphere, just above the Troposphere, and below the Mesosphere. It is stratified in temperature, with warmer layers higher up and cooler layers farther down. This is in contrast to the troposphere near the Earth's surface, which is cooler higher up and warmer farther down. The stratosphere is situated between about 10 km and 50 km altitude above the surface at moderate latitudes, while at the poles it starts at about 8 km altitude.

The stratosphere is layered in temperature because it is heated from above by absorption of ultraviolet radiation from the Sun. Within this layer, temperature increases as altitude increases (see temperature inversion); the top of the stratosphere has a temperature of about 270 K, just slightly below the freezing point of water. This top is called the stratopause, above which temperature again decreases with height. The vertical stratification, with warmer layers above and cooler layers below, makes the stratosphere dynamically stable: there is no regular convection and associated turbulence in this part of the atmosphere. The heating is caused by an ozone layer that absorbs solar ultraviolet radiation, heating the upper layers of the stratosphere. The base of the stratosphere occurs where heating by conduction from above and heating by convection from below (through the troposphere) balance out; hence, the stratosphere begins at lower altitudes near the poles due to the lower ground temperature there.

Commercial airliners typically cruise at an altitude near 10 km in temperate latitudes, in the lower reaches of the stratosphere. This is to avoid atmospheric turbulence from the convection in the troposphere. Turbulence experienced in the cruise phase of flight is often caused by convective overshoot from the troposphere below. Similarly, most gliders soar on thermal plumes that rise through the troposphere above warm patches of ground; these plumes end at the base of the stratosphere, setting a limit to how high gliders can fly in most parts of the world. (Some gliders do fly higher, using ridge lift from mountain ranges to lift them into the stratosphere.)

The stratosphere is a region of intense interactions among radiative, dynamical, and chemical processes, in which horizontal mixing of gaseous components proceeds much more rapidly than vertical mixing. An interesting feature of stratospheric circulation is the quasi-Biennial Oscillation (QBO) in the tropical latitudes, which is driven by gravity waves that are convectively generated in the troposphere. The QBO induces a secondary circulation that is important for the global stratospheric transport of tracers such as ozone or water vapor.

In northern hemispheric winter, sudden stratospheric warmings can often be observed which are caused by the absorption of Rossby waves in the stratosphere.

Main article: Ozone depletion

The reported main cause of ozone depletion is the presence of chlorofluorocarbons (aka CFCs - CCl₂F₂, CCl₃F) in the Earth's stratosphere. Chloroflorocarbons are compounds of chlorine, fluorine, and carbon. Because CFCs are stable, inexpensive, non-toxic, non-flammable, and not corrosive, they are used as propellants, as refrigerants, as solvents, etc. However, it is this stability that causes these CFCs to persist within the environment. These molecules eventually find their way to the stratosphere, where they undergo a series of chain reactions which ultimately lead to the destruction of the ozone layer.

The U.S. government banned the use of CFCs for aerosol propellants in 1980. Worldwide efforts to reduce the use of CFCs began in September 1987 and by 1996 an international ban was put into effect preventing the industrial production and release of CFCs. These efforts have been drastically thwarted by black market operations in China and Russia where up to $500 million worth of illegal CFCs are manufactured. The amounts of CFCs in the stratosphere rose until early 2000, and are expected to reach acceptable levels by mid-century.