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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 level the pressure drops to 14.1 pounds per square inch. And if you travel four miles above the ground—the point at which half of the atmosphere's mass (measure of the total amount of matter in an object) is above and half is below you—the air pressure is about 7.3 pounds per square inch.
The barometric pressure at any given location is constantly changing. Those changes produce the winds, bring in clouds, or clear the way for sunny skies. Air pressure readings are important in weather forecasting. For instance, rising barometric pressure often coincides with clearing skies and fair weather; falling pressure indicates that wet or stormy weather may be on the way. Areas of very low pressure are associated with severe storms, such as hurricanes.
Sources: Engelbert, Phillis. The Complete Weather Resource, vol. 1, pp. 14-18, vol. 3, pp. 393-4; Lee, Sally. Predicting Violent Storms, pp. 16-17; Tufty, Barbara. 1001 Questions Answered About Hurricanes, Tornados, and Other Natural Air Disasters, p. 316; Williams, Jack. The Weather Book, p. 30.
Barometric pressure is another name for atmospheric pressure. It is the force per unit area exerted on a surface by the weight of air above that surface. In other words, it is the pressure exerted by the weight of the atmosphere. Barometric measure changes constantly. It also changes with how above the surface you are. For example, the barometric pressure at sea level is different than the barometric pressure on a mountain. The Coriolis Force can also produce what we know as high and low pressure systems. To measure barometric pressure, a barometer is used.
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