Air has mass and weight. The whole column of air above you, all the way up through the atmosphere, presses down on everything below it. That push spread over an area is air pressure. At sea level it is about 1,013 hectopascals (hPa), or about 10 newtons on every square centimeter: like the weight of a 1 kg bag on each fingernail-sized patch.
Key idea
Air pressure is the weight of the air above, spread over an area. It is highest at sea level and drops with height, falling to about half for every 5.5 km you go up. It pushes in every direction.
Why it works
Air is a gas, and gases can be squeezed. The air near the ground holds up all the air above it, so it is packed tightest, and its density is highest there. Climb higher and there is less air above you pushing down, so the pressure and density both drop. Because air molecules zoom around in all directions, they push on surfaces sideways and upward too, not only down.
Worked example: Halving with height
Sea-level pressure is about 1,000 hPa. About what is the pressure 11 km up?
- 11 km is two steps of 5.5 km.
- Halve twice: $1{,}000 \to 500 \to 250$ hPa.
Answer: About 250 hPa
Worked example: The push on a book
Air pushes about 10 N on each square centimeter. A book cover is 20 cm by 25 cm. How hard does the air push on it?
- Area: $20 \times 25 = 500$ cm².
- Force: $500 \times 10 = 5{,}000$ N, as much as the weight of about 500 kg. The air under the book pushes up just as hard, so it doesn't move.
Answer: About 5,000 N
Common mistake
Thinking air weighs nothing, or that air pressure only pushes down. Air pushes in every direction, and the air and fluids inside our bodies push back out just as hard, which is why we don't feel crushed. Check: a sealed bag of chips puffs up on a mountain, because the air pressure outside it is lower there.
See pressure come from molecules hitting the walls in the Gas Lab.