By Smithsonian's National Air and Space Museum
Atmospheric pressure, also known as barometric pressure, is the pressure within the atmosphere of Earth. It’s caused by air molecules colliding with a surface.
Atmospheric pressure decreases as altitude increases, because there are fewer air molecules pressing against you the higher up you go. The highest air pressure is at sea level where the density of air molecules is the greatest.
It is commonly measured with a barometer, a column of mercury in a glass tube that rises or falls as the weight of the atmosphere changes. Meteorologists describe the atmospheric pressure by how high the mercury rises.
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 millibars, or 760 millimeters (29.92 inches) of mercury.
As the pressure decreases, the amount of oxygen available to breathe also decreases so mountain climbers may need to use bottled oxygen when they ascend very high peaks and airplanes need to create artificial pressure in the cabin so passengers remain comfortable while flying.
Atmospheric pressure is also 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.
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