Atmospheric pressure (or air pressure) is the weight of the Earth's atmosphere on the surface at a given location. It is generated by the downward force of Earth's gravity. Atmospheric pressure depends on the amount of air above the location where the measurement is taken, consequently the pressure drops as you go higher. Air pressure decreases by about 1 inch of mercury for each 1,000 feet of elevation (or 1 hectopascal for each 8 meters) above sea level. This knowledge led to the development of water barometers in 1644. Atmospheric pressure is also known as barometric pressure because barometers are used to measure it.

The National Weather Service reports air pressure in both inches of Mercury (inHg) and hectoPascals (hPa), also called millibars (mb). Air pressure measurement is considered to be static (little or no air movement). Inches of mercury refers to how high the air pressure pushes the mercury in barometers. In the metric system, the unit of pressure is the Pascal, named after the 17th century scientist Blaise Pascal. Pascals are a direct measurement of atmospheric pressure, like pounds per square inch. In order to avoid large numbers, air pressure is reported in hectoPascals. For example, a reading of 100,920 Pascals is equal to 1009.20 hectoPascals.

Barometric pressure changes as low and high pressure systems move across the Earth's surface. It is this fact that makes barometers one of the most important instruments used to predict future weather conditions. Today, surface pressure measurements are used along with temperature, humidity, and wind observations to locate and forecast synoptic macro-, meso-, and micro-scale pressure systems. With a vast network of calibrated surface pressure stations, meteorologists are able to develop a complete picture of the location and movement of weather systems. Severe thunderstorms can even be identified and tracked, helping to assess the risk of micro-bursts and tornado-producing thunderstorms.

The electronic aneroid barometer found in home weather stations are typically housed in the indoor display console. This solid state device uses a quartz membrane secured over an evacuated chamber, which flexes with changes in air pressure. These flexes are translated into voltage fluctuations that are converted to digital signals and sent to the microprocessor.

 

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