Passengers in an airlinerIf you have flown in plane, you will know all too well that stuffy, plugged up feeling in your ears before they suddenly “pop” open again after you have swallowed. The reason for this is to do with atmospheric pressure: the higher up you are, the lower this is. A column of air above you—which goes all the way up to the top of the atmosphere—pushes down on top of you. This column of air becomes shorter the higher up you go, so the weight of it becomes less and less. You don’t even have to climb very high to experience this: your ears can pop when you ride in a car as it goes up a hill. When we go up in a plane, we may reach heights of more than 10 kilometres above sea level. Although the cabin inside a plane is pressurized, the air pressure is still much lower than normal.
Diagram of inside the earThe outside of the eardrumAt or near sea level, where most of us live, the air pressure inside your ear is normally the same as the air pressure outside it. As you ascend in a plane and the air pressure decreases, the pocket of air behind your eardrum where the tiny bones sit (called the "middle ear") now has a higher pressure than the air outside it. This makes your eardrum push outwards, causing discomfort in your ears.
At sea level, every square centimetre of surface area has about 1 kilo of air pushing down on it. You have the pressure equivalent to about three elephants pressing down on your body all the time. You don’t feel this pressure because you have about three elephants' worth of pressure inside your body pushing out at the same time.
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