Although it’s easy to forget, air molecules all weigh something, and their combined weight pressing down is what causes this pressure. At sea level, the column of air above you weighs about a ton.
As you gain altitude, the number of air molecules above you decreases, and therefore so does the air pressure.
Average air (or atmospheric) pressure at sea level is 1.03 kilograms per square centimetre (14.7 pounds per square inch).
The lowest atmospheric pressure on Earth can be found at the summit of Mount Everest, where it’s just 0.3 kilograms per square centimetre (4.4 pounds per square inch).
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