Can you feel the pressure? It's around you…all the time…everywhere you go. What is it? Atmospheric pressure — often referred to simply as air pressure — is the constant force exerted on you by the weight of little particles of air.
These tiny air particles, called air molecules, can't be seen, but they are all around you. They have weight, which means they constantly “push" down on you. If you look straight up in the air, you can imagine a tall column of air above your head reaching all the way to the edge of the Earth's atmosphere.
The weight of that column of air is the amount of air pressure exerted on you. If you move to a higher elevation (climb a mountain, for example), the air pressure will be lower. Why? The length of that column of air above you has decreased by the amount of your increase in elevation.
As you move to a higher elevation, you may notice that your ears have to “pop." This balances the pressure between the inside and outside of your ear. Since there are fewer air molecules the higher you go, you will also probably need to breathe faster to breathe in more molecules to make up for the deficit.
Air molecules also take up space. Because there tends to be a lot of empty space between air molecules, air can either fill a big area or it can be compressed to fit into a smaller area. When it's compressed, air is said to be under high pressure.
Earth's atmosphere presses down on you with a force of almost 15 pounds per square inch. You may be wondering why it doesn't feel that heavy or why you're not crushed under the weight. Remember that thing you do called breathing?
The air inside your body balances out the pressure from air in the atmosphere, which prevents you from being squished by the pressure of the atmosphere. You don't sense air pressure as a constant force, because the air inside you balances outside pressure and you're used to that feeling.
Barometers measure atmospheric pressure using mercury, water or air. You'll usually hear forecasters give measurements in either inches of mercury or in millibars (mb). Forecasters use changes in air pressure measured with barometers to predict short-term changes in the weather.
Changes in air pressure signal the movement of high- or low-pressure areas of air, called fronts. Air molecules in high pressure areas tend to flow toward low pressure areas. We call this flow of air molecules wind. The larger the difference in pressure between areas, the stronger the winds will be.
As weather forecasters monitor air pressure, falling barometer measurements can signal that bad weather is on the way. In general, if a low pressure system is on its way, be prepared for warmer weather with storms and rain. If a high pressure system is coming, you can expect clear skies and cooler temperatures.