Structure of Atmosphere: Troposphere The lowest layer of the structure of atmosphere is called troposphere. The upper limit of the troposphere is called the tropopause. The troposphere is most dense and extends up to a height of 18 km along the equator and about 8 km. along the poles. In this layer, the temperature of air decreases with height at an average rate of 1°C for every 165 meters. Stratosphere There is another layer above the Troposphere which is called Stratosphere. The thickness of stratosphere is 40-50 km. In this layer temperature remains constant and then decreases with height. This layer is free from clouds. The conditions in this layer are ideal good for flying of Jet aircraft. The ozone in this layer absorbs harmful ultra-violet radiation from the sun. Mesosphere The mesosphere extends above the stratosphere. It is a 30 km deep layer. Thermosphere This is the fourth layer of the atmosphere this layer extends from 80 km to the top layer. Its lower part is called ionosphere. Ionosphere contains ions. These particles reflect radio waves to the earth’s surface and enable wireless communication. The upper part of thermosphere is called exosphere. Exosphere starts 500 km above earth. There is no distinct upper limit to the exosphere and this gives way to interplanetary space. The whole atmosphere envelop weighs about 5000 million tons. The weight exerts a constant pressure of about 1 ton on the human body. Biosphere It means sphere of life. The idea of biosphere was given to us about a century ago by the Australian geologist. The distinguishing feature of the biosphere is that it supports life. It is estimated that the biosphere contains more than 3,50,000 species of plants including algae, fungi, moss and higher forms of plants and eleven million animal species ranging from unicellular protozoa to man. The biosphere supplies the essential requisites life for all these species, namely light, heat, food and living space or habitats. The biosphere and ecosystem is one and the same thing. The eco-system depend upon a whole set of delicately balanced cycles. These cycles are energy cycle, heat cycle, carbon cycle, oxygen cycle, nitrogen cycle and water cycle.