1. What is the "atmosphere"?

The earth's atmosphere and the setting sun, viewed from the Space Shuttle (Source: NASA)

The ATMOSPHERE is a layer consisting of a mixture of gases, commonly called AIR, that surrounds the earth. It can easily be seen as a faint layer in photographs taken from space, as in the picture shown above.

The atmosphere is not of uniform thickness, and the density of the air decreases with distance from the surface of the air. About 75% of the atmosphere lies below an average altitude of 11 km, and it is in this zone, known as the TROPOSPHERE, that most of the weather phenomena occur.

Arbitrarily, the atmosphere is considered to end at an altitude of 100 km, but it does in fact extend much further into space. The atmosphere supports some forms of life up to about 8000 m, but humans cannot survive for extended periods without oxygen supplies at that altitude.

It is well to bear in mind that life is not supported above the troposphere, which has an insignificant thickness (10 km) compared to the radius of the earth (6400 km).

2. Composition of the atmosphere

Dry air is a mixture of gases, with about 21% oxygen (O2) and 78% nitrogen (N2), the balance being largely made up of argon (Ar) and carbon dioxide(CO2). Water vapour is a variable constituent, normally averaging 1-4% by volume. When this vapour precipitates out of the atmosphere as liquid droplets or ice crystals, clouds are formed.

Human activity is increasingly producing large quantities of gases that are considered to be harmful POLLUTANTS, and which cause harmful effects on health and the environment.

Gas Percent by volume
Nitrogen (N2)
Oxygen (O2)
Argon (Ar)
Carbon dioxide (CO2)
Neon (Ne)
Source: Wikipedia

3. Additional questions