Earth is surrounded by a thick blanket of air. The layer of air that envelopes the earth is called the atmosphere. This blanket of air extends up to several kilometres above the earth's surface. Air is a mixture of gases. It consists of nitrogen and oxygen in the          ratio about four to one –nitrogen 78 per cent, oxygen 21 per cent. Oxygen is essential for the survival of most organisms. The other gases like argon, carbon dioxide, neon, helium, krypton, hydrogen and xenon are present in smaller quantities. Air also holds      water vapour in various amounts depending on the temperature.
    On the basis of its composition, temperature and density, the atmosphere is divided into five concentric layers –Troposphere, Stratosphere. Mesosphere, Thermosphere and Exosphere
    Troposphere is the layer of the atmosphere closest to the earth's surface, reaching up to a height of about 10–15 km. It contains 75 per cent of the total air of the atmosphere. The temperature and the pressure drops as one goes higher up the troposphere.
    It is the layer where all weather changes occur. The stratosphere lies directly above the troposphere. Strong winds called jet streams blow here: This helps aircraft to fly.
    The next layer is the mesosphere. Temperature drops as low as –110°C here. Meteors or rock fragment burn up here.

    The ionosphere or thermosphere is a hot layer where temperature rises steadily to 1400°C. This layer contains electrically charged particles called ions which help in transmitting radio-waves back to the earth. Satellites, which help in transmitting signals              back to earth are located here.

   The exosphere is the outermost layer of the atmosphere and extends from 80 km above the earth's surface to the outer space.