The atmosphere has a mass of about 5.15×1018 kg, three quarters of which is within about 11 km of the surface. The atmosphere becomes thinner and thinner with increasing altitude, with no definite boundary between the atmosphere and outer space.
Life on Earth would not be possible without air. We need oxygen from the air to breathe. Plants need carbon dioxide from the air to prepare food through photosynthesis. Humans get energy due to the combustion of food inside the body, which is supported by air.
The region of air present around the Earth is called atmosphere. Most of the Earth's atmosphere is contained within 40 km of the planet's surface. With increase in altitude, air becomes thinner. Above 32 km, the air becomes very thin. Air is found in traces even above 100 km. This 40 km thickness is ultra thin when compared with the size of Earth (diameter about 13,000 km).
If the Earth were the size of an apple, its atmosphere would be about as thick as the skin of the apple!
The density of air is highest near the sea level and as we go higher, the density of air goes on decreasing. The composition of air also changes with the change in altitude. Air molecules would escape into outer space and be lost from Earth if there were no gravity. The air molecules surrounding Earth would settle to the ground if there were no sunlight. Solar–heated air molecules move at about 1500 km per hour and a few make their way up to more than 50 km in altitude. Earth’s gravity combined with the heat from the Sun, provide a layer of air more than 40 km thick that we call the atmosphere. This atmosphere provides oxygen, nitrogen, carbon dioxide, and other gases needed by living organisms. It protects the Earth’s inhabitants by absorbing and scattering cosmic radiation.