Environmental Science 3(3+0)

(2) Stratosphere

• Stratosphere - 12 to 50 km - in the lower part of the stratosphere. The temperature remains fairly constant (-60 degrees Celsius). This layer contains the ozone layer. Ozone acts as a shield for in the earth's surface. It absorbs ultraviolet radiation from the sun. This causes a temperature increase in the upper part of the layer. It can be defined as the air mass extending from the tropopause (ie. upper most level of the troposphere), to the stratopause (ie ., upper most level of the stratosphere), about 50 km above the surface of the earth. The stratosphere exhibits several significant differences from the troposphere. Water vapour is virtually absent. The only clouds found in the stratosphere are very thin wispy clouds formed of tiny ice crystals. Ozone is present in significant quantities and it forms a well marked ozone layer called ozonosphere within the stratosphere.



• Ozone is formed from oxygen by a photochemical reaction in which energy from the sun splits apart the oxygen molecule to form atomic oxygen. The atomic oxygen then combines with molecular oxygen to form ozone.


• Ozone is in equilibrium with the rest of the air, which means that ozone is being produced from oxygen as fast as it is broken down to molecular oxygen. This is important because ozone absorbs UV radiation from the sun. In fact, the reason that the stratosphere bec omes warmer. With increasing distance from the earth is that the UV energy absorbed by the ozone is transformed into heat. Because of this heat, the ozonosphere also acts like a blanket that reduces the cooling rate of earth and thus adds to the effect of water vapour.