Atmospheric Pressure

The air around you has weight, and it presses against everything it touches. That pressure is called atmospheric pressure, or air pressure. It is the force exerted on a surface by the air above it as gravity pulls it to Earth.

Atmospheric pressure is commonly measured with a barometer. In a barometer, a column of mercury in a glass tube rises or falls as the weight of the atmosphere changes. Meteorologists describe the atmospheric pressure by how high the mercury rises.

Atmospheric pressure is expressed in several different systems of units: millimetres of mercury, or inches of mercury, pounds per square inch (psi), dynes per square centimetre, millibars (mbar), standard atmospheres, or kilopascals. An atmosphere (atm) is a unit of measurement equal to the average air pressure at sea level at a temperature of 15°C (59°F). One atmosphere is 1,013 mbar, 760 mmHg, or 29.92 inHg.

Atmospheric pressure drops as altitude increases. As the pressure decreases, the amount of oxygen available to breathe also decreases.

While in the atmosphere, atmospheric pressure and ambient pressure are the same. But they differ underwater. When underwater, ambient pressure is a combination of atmospheric pressure and hydrostatic pressure.

Atmospheric pressure is an indicator of weather. When a low-pressure system moves into an area, it usually leads to cloudiness, wind, and precipitation. High-pressure systems usually lead to fair, calm weather.