34.6 The Atmosphere

34.6.2 State the percentage of the two main gases in the atmosphere.

The two main gases in the Earth’s atmosphere are:

• Nitrogen — approximately 78%

• Oxygen — approximately 21%

Together, nitrogen and oxygen make up approximately 99% of the atmosphere.

34.6.4 Describe the variation of pressure as altitude increases.

As altitude increases, atmospheric pressure decreases.

Atmospheric pressure is caused by the weight of the air above a given point. As altitude increases, there is less air above, so the weight of the overlying atmosphere decreases and therefore the pressure decreases.

Near sea level, atmospheric pressure decreases by approximately 1 hPa for every 30 ft of altitude gained.

However, pressure does not decrease at a constant rate. As altitude increases and the air becomes less dense, the rate of pressure decrease becomes progressively smaller.

At approximately 18,000 ft, atmospheric pressure is about half the pressure at mean sea level.

Key point: Pressure decreases with increasing altitude, rapidly at lower levels and more gradually at higher levels.

34.6.6 Explain how the partial pressure of oxygen changes as altitude increases.