Atmospheric pressure

1An ocean of air

Air has mass, so the atmosphere above you has weight. At sea level it presses with about 100 000 Pa: like a 10-tonne weight on every square metre.

atmospheric pressure

2Particles pushing

Air pressure comes from air particles hitting surfaces. Huge numbers of tiny pushes every second add up, and they push in every direction.

3Higher means lower pressure

The higher you go, the less air there is above you, so the air pressure is lower. At the top of Everest it is only about a third of that at sea level.

altitude

4Thinner air

High up, the air particles are more spread out, so there are fewer hits each second. Each breath holds less oxygen, so climbers on Everest often carry oxygen.

5Crushing a can

Steam fills a can, which is then sealed and cooled. The steam turns back to water, the pressure inside drops, and the air outside crushes the can.

6Straws and suction cups

Sucking on a straw lowers the pressure inside it, and the air pressing on the drink pushes it up. Air pressure holds a suction cup on in the same way.

7Pressure and planes

Airliners fly where the air pressure is only about a quarter of that at sea level, so their cabins are pressurised. When the pressure changes quickly, your ears pop.

pressurised