Atmospheric pressure

The atmosphere is the layer of air around the Earth. Key features of the atmosphere are:

  • it is thin compared to the radius of the Earth
  • it becomes less dense as the altitude increases

Pressure

Air molecules colliding with a surface cause atmospheric pressure. Atmospheric pressure at sea level is about 100,000 Pa (100 kPa) but just 27,000 Pa (27 kPa) at the cruising altitude of a passenger plane.

Atmospheric pressure decreases as the height of a surface above ground level increases. This is because, as the altitude increases:

  • the number of air molecules decreases
  • the weight of the air decreases
  • there is less air above a surface

This is why aircraft that fly at high altitudes must be pressurised. If the air pressure is too low, humans cannot take in oxygen quickly enough to meet their bodies’ needs.

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