The Atmosphere

The principal gases which constitute the atmosphere.

The atmosphere is a mixture of gases that surrounds the Earth.

The 3 main gases are:

Nitrogen                    - 78%

Oxygen                      - 21%

Other                         - 1%      

(argon (0.9%), carbon dioxide (0.04%), neon, helium, ozone, methane, krypton, and hydrogen)

The variation of pressure as altitude increases.

As altitude increases, the atmospheric pressure decreases.

This means that the air pressure becomes lower the higher you go in the atmosphere.

The decrease in pressure is due to the decreasing density of the air molecules as you move further away from the Earth's surface. 

Even though as you move up in altitude, the air pressure decreases, the relative percentages of gases like nitrogen, oxygen stay the same.