Barometric pressure

Barometric pressure, also called air pressure or atmospheric pressure, is the pressure exerted by the weight of air over a given area of Earth's surface. This value is a function of how many molecules of air there are in a specific area, how fast those molecules are moving, and how often they collide. (Molecules are particles made by the chemical combination of two or more atoms.) Barometric pressure is measured by an instrument called a barometer.

At sea level (the level of the ocean's surface used as a standard in determining land elevation and sea depths), where gravity is strongest and attracts the greatest number of molecules, air pressure is greatest. Because gravity weakens as you go up, air pressure is lower at higher altitudes. So while the average air pressure at sea level is 14.7 pounds per square inch, at 1,000 feet (304 meters) above sea