The vast expanse of air which envelopes the earth all round is called the atmosphere. It contains life-giving gases like Oxygen (O2) for man and animal, and carbon dioxide for plants. It acts like a greenhouse by trapping the heat. Like the glass in a greenhouse, it allows short wave radiation to enter it and reach the earth's surface. But it is nearly opaque to long wave terrestrial radiation and thus keeps the earth's average temperature 35 degree centigrade warmer than it would otherwise be. It protects the earth from the harmful radiation from the sun. It also serves as a storehouse for water vapour which leads to precipitation fairly distributed over land and sea. It is an integral part of earth like lithosphere (land), and hydrosphere (water) it is inseparable from the earth due to the force of earth gravity.