Composition and Structure of Atmosphere — Class 11 Geography

The atmosphere is the envelope of air that surrounds the earth; 99 per cent of its mass lies within 32 km of the surface. It is composed of gases, water vapour and dust particles. Among the gases, nitrogen and oxygen dominate, but carbon dioxide is meteorologically important because it is transparent to incoming solar radiation but opaque to outgoing terrestrial radiation, making it largely responsible for the greenhouse effect; its volume has been rising due to fossil-fuel burning. Ozone, found between 10 and 50 km, absorbs harmful ultraviolet rays. Water vapour (up to four per cent in the warm wet tropics) absorbs insolation and preserves the earth's radiated heat, acting like a blanket. Dust and salt particles act as hygroscopic nuclei around which water vapour condenses to form clouds. The atmosphere is divided into five layers by temperature: the troposphere (the lowermost, with all weather and biological activity, where temperature falls with height), the stratosphere (containing the ozone layer), the mesosphere (where temperature again falls), the thermosphere/ionosphere (which reflects radio waves), and the exosphere (the rarefied outermost layer merging with space). The troposphere and stratosphere are of greatest concern to geographers.