Why Does the Atmosphere Exert Pressure on the Earth

Except when the wind is blowing, youre 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? Its a gas, so you might think it would expand out into space. Why does any gas have pressure? In a nutshell, its 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 Earths 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 Earths surface, it becomes pressure. How Much Is Atmospheric Pressure? Pressure depends on altitude, temperature, and weather (largely the amount of water vapor), so its 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 Ãâ€" 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 Earths surface? Its because its really a measure of the weight of all the air pressing down at that point. If you are high in the atmosphere, there isnt much air above you to press down. At the Earths surface, the entire atmosphere is stacked above you. Even though gas molecules are very light and far apart, there are a lot of them!