Atmospheric Pressure

Core Idea

Atmospheric pressure is the force exerted by the weight of air above us. At sea level, it is about 101,325 Pa (1 atmosphere). This pressure acts in all directions and decreases with altitude because there is less air above. We do not feel it because the pressure inside our bodies balances the external pressure. Atmospheric pressure can support a column of mercury about 760 mm tall, which is how barometers measure it.

How It's Best Learned

Perform the collapsing can experiment: heat a small amount of water in a can, flip it upside down into cold water, and watch atmospheric pressure crush it. Use a suction cup to feel how atmospheric pressure holds it to a smooth surface. Track barometer readings over several days and correlate changes with weather.

Common Misconceptions

• We are not affected by atmospheric pressure. (Atmospheric pressure pushes on us with about 10 N per square centimeter, but we do not feel it because the pressure inside our body pushes back equally.)
• Suction cups work by creating a vacuum that "sucks" onto surfaces. (Suction cups work because atmospheric pressure pushes them onto the surface after the air is squeezed out from underneath.)
• Air is weightless. (Air has mass and weight. A room-sized volume of air can weigh hundreds of pounds, and its total weight above us creates atmospheric pressure.)
• Atmospheric pressure is the same everywhere on Earth. (It varies with altitude, temperature, and weather. Mountain tops have lower pressure than sea level, and weather systems cause pressure to fluctuate.)

Explainer

Right now, a column of air stretching from the ground all the way to the edge of space is sitting on top of you. That column has mass, and gravity pulls it downward, so it exerts a force on everything beneath it. This force per unit area is atmospheric pressure, and at sea level, it amounts to about 101,325 Pa — roughly 10 newtons pushing on every square centimeter of your body.

To put that in perspective, the total force on a person with about 1.7 m² of body surface area is around 170,000 newtons — about 17 tons of force. So why are you not crushed? Because the air pressure inside your body — in your lungs, bloodstream, and tissues — pushes outward with equal force. The internal and external pressures balance, and you feel nothing. You only notice changes in atmospheric pressure, like the uncomfortable ear-popping feeling when an airplane climbs or descends.

Altitude has a dramatic effect on atmospheric pressure. At sea level, the full atmosphere is above you. At the top of Mount Everest (about 8,800 m), roughly two-thirds of the atmosphere is below you, and the pressure is only about one-third of sea level pressure. This is why climbers need supplemental oxygen — lower pressure means fewer air molecules per breath. In space, just above the atmosphere, the pressure drops to essentially zero.

Scientists measure atmospheric pressure with a barometer. The first barometer, invented by Evangelista Torricelli in 1643, was a tube of mercury inverted in a bowl. Atmospheric pressure pushes down on the mercury in the bowl, forcing it up the tube. At sea level, the mercury rises about 760 mm — this is why pressure is sometimes given in "millimeters of mercury" (mmHg). Weather forecasters use barometers because changes in atmospheric pressure indicate changing weather: falling pressure often signals approaching storms, while rising pressure suggests fair weather.

Atmospheric pressure has countless practical consequences. Drinking through a straw works because you reduce the air pressure inside the straw by sucking, and the greater atmospheric pressure on the drink's surface pushes liquid up into the lower-pressure straw. Vacuum cleaners work on the same principle — the motor reduces internal pressure, and atmospheric pressure pushes air (carrying dirt) into the machine. Understanding atmospheric pressure reveals that many things we attribute to "suction" are actually the atmosphere doing the pushing.