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