Atmosphere is the blanket of air that envelops the Earth. It contains many gases and particles of various materials. Among the gases of the atmosphere, nitrogen constitutes 78%, oxygen 21%, carbon dioxide and other gases 1%. The atmosphere also contains minute particles of water vapour, methane, nitrous oxide, carbon monoxide, hydrogen, ozone, helium, neon, krypton and xenon gases. In addition, sand particles, smoke, salt particles, volcanic ash particles, meteoric dust and pollen are also present in the atmosphere. The atmosphere is dense near the Earth’s surface, but becomes rarefied as one goes higher up. It is estimated that the atmosphere extends up to the exosphere, which is more than 500 kilometres above the Earth’s surface. It is made up of many layers. The pressure, density and temperature of the atmosphere vary with its height above the Earth. At a height of 6 kilometres, the air pressure is reduced to half of what it is at the Earth’s surface. Similarly, the temperature falls by 1°F for every 91 metres we ascend. On the basis of its physical properties, the atmosphere has been divided into the following five layers:- 1. Troposphere: Troposphere extends from the Earth’s surface to a height of about 17 kilometres. It accounts for 75% of the total weight of the atmosphere. It is the most important layer, as almost all living beings inhabit this part. As one goes higher, the temperature falls. Rain, clouds, storms and snow are formed in this part. 2. Stratosphere: Stratosphere extends up to a height of about 48 kilometres. Its upper portion contains ozone, which absorbs ultraviolet rays coming from the Sun. These rays are very dangerous for living organisms. There are neither strong winds nor varying temperatures in this layer. 3. Mesosphere: Mesosphere starts after a height of 50 kilometres. Here the temperature is significantly low and it is the minimum at a height of 85 kilometres. 4. Ionosphere: The atmospheric layer above the mesosphere and up to a height of about 500 kilometres is called ionosphere. It contains only charged particles, which reflect the radio waves towards the Earth and make radio communication possible. 5. Exosphere: It is the outermost layer of the atmosphere. Here the density of the atmosphere is very low. This part contains helium and hydrogen, and hence the temperature is very high in this layer. The atmosphere is extremely useful for life on Earth. Without it, no living being can survive. It protects us from the dangerous radiations of the Sun. Almost all meteors falling towards and on the Earth get destroyed; they burn up due to resistance and friction of the atmosphere.