Barometric (or air) pressure, named after the instrument used to measure this pressure, is caused by the weight of the atmosphere pressing down on the land, ocean, and air below, with gravity creating the downward force. Because pressure is dependent on the amount of air above a certain point, pressures are greatest at the surface and less at higher altitudes. On the average, at sea level, the air has a pressure of 14.7 pounds per square inch (a one-inch square has 14.7 pounds of air pressure on each side).

A barometer is a type of gauge that measures atmospheric pressure, which makes it useful in detecting high and low pressure systems that can predict changes in the weather.

The United States National Weather Service does not measure pressure in pounds per square inch, but in terms of inches of mercury—or how high the pressure pushes mercury in a sealed tube. Air pressure aloft is reported in millibars (or hectopascals [hPa], a term most often used by scientists to measure air pressure).

Most of us are very familiar with air pressure as it changes with the weather. For example, the use of the terms “high pressure system” and “low pressure system” are often indicators of the types of weather fronts traveling through a region. In general, falling air pressure (seen on a barometer) means that clouds and precipitation are more likely; rising air pressure means that clear weather is more likely. In addition, many people experience “personal” changes in air pressure. For example, the discomfort or even pain felt in a person’s ears as they ascend or descend in an airplane, a large hill, or even in an elevator is evidence of changing air pressure.