Air pressure

Air pressure is the force that air molecules exert on a surface due to their weight and movement. 

Normal value at sea level: 1013.25 hPa (hectopascal) = 1 atm = 760 mmHg. 

It decreases with increasing altitude (approximately 1 hPa per 8–10 meters of rise near the ground). 

This is caused by the weight of the air above (gravity). 

Measure: with a barometer (mercury or aneroid barometer). 

Influence on weather: High pressure (H) → often fair, stable weather. 

Low pressure (L) → often cloudy, rain, wind Formula (approximate): P = P₀ × e^(-h/H) (P = pressure at altitude h, P₀ = pressure at sea level, H ≈ 8400 m) 

In short: the more air above you, the higher the pressure. 

That's why you feel your ears popping on an airplane or in the mountains.

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