6.3 Air Pressure
Liquids press in all directions — but what about the air, which we cannot even see? The atmosphere is the blanket of air surrounding the Earth (mostly nitrogen and oxygen, with argon, carbon dioxide and other gases), reaching many kilometres up. Does all that air press on us?
Activity 6.3 — The heavier sheet to lift Stand a stick on an inverted paper plate and cover the plate with a folded chart-paper sheet — lift it by the stick and note the effort. Now cover it with the same sheet unfolded (much larger area, but the same weight) and lift again. It is harder to lift the unfolded sheet! Since the weight didn’t change, the extra effort must come from air pushing down on the sheet — and that push grows with the area. Force over area is pressure, so air exerts pressure.
Atmospheric Pressure The pressure exerted by the air around us is called atmospheric pressure. Air, like a liquid, pushes in all directions — which is why a balloon you blow up expands evenly in every direction (the air inside presses outward on its walls). Air always flows out of an open balloon because the pressure inside is higher than outside.
How strong is it?
Activity 6.4 — The clinging sucker Press a rubber sucker onto a smooth surface. It clings — and is surprisingly hard to pull off. Pressing it squeezes most of the air out from underneath, lowering the pressure inside. Now the higher atmospheric pressure outside holds it firmly against the surface. To pull it off, you must overcome that pressure difference. (On a rough surface, air leaks back in, the pressures equalise, and the sucker won’t stick.)
Why Aren’t We Crushed? Atmospheric pressure is enormous: the air column over an area of just 15 cm × 15 cm pushes down with a force roughly equal to the weight of a 225 kg mass (about 2250 N)! So why doesn’t it crush us? Because the pressure of the fluids and gases inside our bodies pushes outward with equal pressure, balancing the atmosphere pushing in. The forces cancel, and we feel nothing.
A Step Further: Weather Units The SI unit of pressure is the pascal (Pa), but weather reports use the millibar (mb) and hectopascal (hPa) — and conveniently 1 mb = 1 hPa = 100 Pa. When a forecaster mentions a “low of 996 hPa,” they’re describing a region of low air pressure — the very thing that drives the winds and storms in the rest of this chapter.