Why Does the Atmosphere Exert Pressure on the Earth?

The Reason Why Air Exerts Pressure

Air has pressure because molecules have energy to interact and because gravity holds the gases together near the Earth.
Air has pressure because molecules have energy to interact and because gravity holds the gases together near the Earth. John Lund, Getty Images

Except when the wind is blowing, you're probably unaware that air has mass and exerts pressure. Yet, if there were suddenly no pressure, your blood would boil and the air in your lungs would expand to pop your body like a balloon. Yet, why does air have pressure? It's a gas, so you might think it would expand out into space. Why does any gas have pressure? In a nutshell, it's because molecules in the atmosphere have energy, so they interact and bounce off each other, and because they are bound by gravity to stay near each other. Take a closer look:

How Air Pressure Works

Air consists of a mixture of gases. The molecules of the gas have mass (although not much) and temperature. You could use the ideal gas law as one way to visualize pressure:

PV = nRT

where P is pressure, V is volume, n is the number of moles (related to mass), R is a constant, and T is temperature. The volume is not infinite because the Earth's gravity has enough "pull" on the molecules to hold them close to the planet. Some gases escape, like helium, but heavier gases like nitrogen, oxygen, water vapor, and carbon dioxide are bound more tightly. Yes, some of these larger molecules still bleed off into space, but terrestrial processes both absorb gases (like the carbon cycle) and generate them (like evaporation of water from the oceans).

Because there is a measurable temperature, the molecules of the atmosphere have energy. They vibrate and move around, bumping into other gas molecules. These collisions are mostly elastic, meaning the molecules bounce away more than they stick together. The "bounce" is a force. When it is applied over an area, like your skin or the Earth's surface, it becomes pressure.

How Much Is Atmospheric Pressure?

Pressure depends on altitude, temperature, and weather (largely the amount of water vapor), so it's not a constant. However, the average pressure of air under ordinary conditions at sea level is 14.7 lbs per square inch, 29.92 inches of mercury, or 1.01 × 105 pascals. Atmospheric pressure is only about half as much at 5 km altitude (about 3.1 miles).

Why is pressure so much higher close to the Earth's surface? It's because it's really a measure of the weight of all the air pressing down at that point. If you are high in the atmosphere, there isn't much air above you to press down. At the Earth's surface, the entire atmosphere is stacked above you. Even though gas molecules are very light and far apart, there are a lot of them!