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Heat & Air · Question 10 of 20

Air pressure

You live at the bottom of an ocean of air, carrying a kilogram of it on every square centimetre of your body — and you can't feel a thing.

Plate XIII — The weight of the sky ~101 kPa · collisions per second
Pump the air out and watch the atmosphere fold the can.
outside: the whole sky, pressinginside the can
FIG. XIII — THE WEIGHT OF THE SKY
Air pumped out 0%
The outside never pushes harder — the inside just stops pushing back.
Pressure outside
101kPa, always
Pressure inside
101kPa
The red dots are drumming on the walls from inside; the sky's molecules drum from outside. Normally it's a perfect tie. Pump dots out and the outside drumming wins — the can crumples. Nothing sucked it in; the sky pushed.
The short answer

Air is real stuff made of zillions of tiny molecules, and they're constantly bumping into everything. That steady drumming of hits is air pressure.

What's actually happening

Air feels like nothing, but it is made of molecules with mass — about a kilogram of them stacked above every square centimetre of ground, in a column reaching to the edge of space. All that weight presses down, and the air at the bottom passes the squeeze along in every direction through collisions: roughly ten trillion trillion molecular impacts per square centimetre per second. That relentless drumming is air pressure: about 101,000 newtons on every square metre at sea level.

Why aren't you crushed? Because you are not a hollow shell holding back a vacuum. The air inside your lungs, ears, and tissues pushes outward exactly as hard as the atmosphere pushes in. Pressure only does dramatic, visible things when the two sides of a surface stop matching — and then it does them instantly. Pump the air out of a steel drum and the unopposed atmosphere flattens it like a stepped-on can.

Most "suction" in your life is actually this push in disguise. A drinking straw doesn't pull liquid up — you lower the pressure in your mouth and the atmosphere pushes the drink up the straw. Suction cups, vacuum cleaners, plungers: none of them pull. They all just remove air from one side and let the weight of the sky do the work.

Try it at home Let the sky hold water for you
  1. 1Fill a glass right to the brim with water and lay a postcard or stiff card flat across the top.
  2. 2Hold the card in place, flip the glass upside down over a sink, and let go of the card.
  3. 3The card stays, holding back the water. The atmosphere pushing up on the card is stronger than the weight of the water pushing down — by a comfortable margin.