The stratosphere (from LAT. Stratum: layer) is a layer of the atmosphere that is located at a height from 11 to 50 km. There is a little decrease of the temperature at the heights from 11 to 25 km (the lower part of the stratosphere) and an increase of it in the layer of 25-40 km from -56.5 to 0.8°C (the top layer of the stratosphere or the inversion field ). At the altitude of 40 km, the temperature value is about 273°K (almost 0° by Celsius), the temperature remains constant up to the height of about 55 km. This area of constant temperature is called the stratopause and is the boundary between the stratosphere and mesosphere. The density of air in the stratosphere is hundreds times less than at the sea level.
It is in the stratosphere, where the ozone layer is located (at the height from 15-20 km to 55-60 km) that specifies the upper limit of the life in the biosphere. Ozone (O3) is generated during photochemical reactions most intensely at the altitude of about 30 km. At the normal pressure, the total mass of O3 would be a layer with thickness of 1.7-4.0 mm, but that is enough to absorb ultraviolet radiation from the Sun harmful for the life form. Destruction of O3 occurs when it reacts with free radicals, NO, Halogen containing compounds (including freon).
In the stratosphere, most of the short-wave part of the ultraviolet radiation (180-200 nm) and short-wave energy transformation occurs. Under the influence of these rays, magnetic fields are changing and there is the molecule decay and ionization, de novo synthesis of gases and other chemical compounds. These processes can be observed in the form of Northern Lights and other glows.
In the stratosphere, there is also little water vapor.
Flying to the stratosphere began in the 1930’s. Widely known is the flight with the first high-altitude balloon (FNRS-1), made by Auguste Piccard and Paul Kipfer May 27, 1931, reaching the height of 16.2 km. The Picard’s flights aroused much interest, and in 1933-1934, high altitude balloons in a great quantity all over the Europe.
Modern combat and supersonic commercial aircraft fly in the stratosphere at altitudes of up to 20 km mainly (although their dynamic ceiling can be much higher). High-altitude weather balloons rise up to 40 km of altitude; the record for an unmanned balloon is 51.8 km.
Recently, in United States, the military paid much attention to flying in the stratosphere above 20 km, often called "near space." The unmanned aircrafts and planes using solar energy as a fuel (such as NASA Pathfinder) can remain for long periods of time at a height of about 30 km and provide supervision of a very large territory, while remaining unseen by air defense; such devices are much cheaper than satellites.
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