Stratosphere
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Description
The stratosphere is the layer of the atmosphere immediately above the tropopause and under the stratopause. Its altitude is therefore 8-20 km at its lower limit to about 50 km, typically with pressures from 100 hPa to 1 hPa. It is characterised by the temperature rising with altitude from about -55 °C at the tropopause to about 0 °C at the stratopause (with seasonal and latitudinal variations).
Detailed description
The stratosphere contains about 20 per cent by mass of the earth's atmosphere. Its chemistry is largely governed by the photolysis of oxygen forming the ozone layer. The essential absorption of UV energy from solar radiation by ozone is the factor explaining the temperature rise of the stratosphere. This occurs most close to the stratopause, but the highest density of ozone is at lower altitudes, reaching a maximum at about 20-25 km altitude. Because of the temperature inversion within the stratosphere, weather is generally calmer than in the troposphere. The polar conditions are more variable. A cycling of winds occurs in the lower tropical stratosphere, lasting about 52 months. For half this period the wind is westerly from the tropopause up to about 30 km and then it changes to easterly. The change is slow and starts at the high altitudes, taking about a year to reach the low altitudes. This may have effects on the tropospheric weather, but it is not yet fully understood.
