Forces of Flight on this Page


Air is Stuff

Air is not just empty space—it has substance, or mass. Air has molecules that are constantly moving. Because air has mass, Earth’s gravity attracts it and gives it weight. Because it has weight, and the air molecules are constantly bumping into things, it exerts pressure. 

The Earth's atmosphere is an extremely thin layer of air. There is no exact place where the atmosphere ends; it just keeps getting thinner until it merges with space. The most commonly accepted boundary between Earth’s atmosphere and space is at about 100 kilometers (62 miles) above the Earth’s surface. This boundary is called the “Kármán Line.”  In this picture, taken from a spacecraft orbiting at 320 kilometers (200 miles), the atmosphere appears as a thin blue band between the Earth’s surface and the blackness of space. If the Earth were the size of a basketball, the thickness of the atmosphere could be modeled by a thin sheet of plastic wrapped around the ball.
Credit: NASA

Earth’s atmosphere is pressing against each square inch of you with a force equal to about 1 kilogram per square centimeter (14.7 pounds per square inch). The force on 1,000 square centimeters (a little larger than a square foot) is about a ton!

What Happens to Air Pressure As You Change Altitude?

As you go up, air pressure goes down, because the higher you go, the less air there is pressing down on you from above.

If you were to stand at sea level, at an altitude of 0 meters (0 feet), the air pressure would be 1 kilogram per square centimeter (14.7 pounds per square inch). If you were to move to Denver, Colorado, at an altitude of about 1,610 meters (5,280 feet), the air pressure would be 0.85 kilograms per square centimeter (12 pounds per square inch). And if you were to go to the top of Mt. Everest, at an altitude of about 8,848 meters (29,028 feet), the air pressure would only be 0.3 kilograms per square centimeter (4.5 pounds per square inch).

Why Doesn’t All This Pressure Squash You to a Pulp?

The pressure of the air in your lungs and in other spaces inside your body balances the pressure of the air around you.

If the average adult has about 2 square meters (21 square feet) of surface area, that adds up to about 20,000 kilograms (44,100 pounds) of pressure!

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Watch a Video

Air Pressure

See how Magdeburg hemispheres demonstrate the properties of air pressure.

Did You Know?

Air is a mixture of different gases – about 78% nitrogen, 21% oxygen, and about 1% other gases. These gases make air a fluid, like water. Fluids are states of matter that tend to flow around objects or take the shape of their container.

Hands-On Experiments

Pop Quiz

True or False: when you suck on a straw, you are pulling the liquid up the straw.

A) True
B) False

Suction is not a pull – it’s a push!  When you suck on a straw in a glass of liquid, you reduce the air pressure within the straw, so there is more pressure on the surface of the liquid in the glass than on the liquid in the straw. This higher pressure pushes the liquid up the straw.  The same basic mechanism works for vacuum cleaners.  A vacuum cleaner doesn’t pull the dirt off your floor, it pushes the dirt off the floor and into the vacuum cleaner.  A vacuum cleaner uses an air pump to decrease the air pressure in the vacuum cleaner below the air pressure of the surrounding room, This creates suction inside the vacuum cleaner, which allows the air in the room to push itself into the vacuum cleaner – along with all that dirt!