Composition and Structure of the Atmosphere — Geography STD 9 — Question

The atmosphere is held by the gravitational force of the Earth and is densest near the Earth’s surface where gravity is maximum. There are concentric layers of air in the atmosphere. Each layer has a different density. The upper layers are continually pressing down on the lower ones. Therefore, the air of the lower layer is always much heavier or denser, while the upper layers are thinner or less dense. Thus atmosphere thins out at higher levels and about 90 percent of the mass of air lies within a height of about 20 km. from the surface of the earth.