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barometric pressure


In the U.S., air pressure at the surface is reported in inches of mercury while air pressure aloft is reported in millibars. However, scientists generally use pressures in millibars. Millibars are a direct measure of pressure, like pounds per square inch, but in the metric system. Inches of mercury measure how high the pressure pushes the mercury in a barometer. To convert between inches of mercury and millibars, one millibar is equal to 0.03 inches of mercury.

The direct reading from a barometer is the station pressure. Two things affect this barometer reading, the high and low pressure caused by weather systems and the change in pressure caused by the station's elevation. No matter what the weather systems are doing, air pressure decreases with height. That is why it is important to examine a uniform pressure field by reducing it to sea level.

Sea-level pressure is defined as: "A pressure value obtained by the theoretical reduction of barometric pressure to sea level." Another kind of barometric reading is the altimeter setting, which aircraft use. It's defined as: "The pressure value to which an aircraft altimeter scale is set so that it will indicate the altitude above mean sea level of an aircraft on the ground at that location." A rule of thumb for the altimeter correction is that the pressure drops about 1 inch of mercury for each 1000 foot altitude gain. And if using millibars, the correction is 1 millibar for each 8 meters of altitude gain. These rules of thumb work pretty well for elevations or altitudes of less than a two or three thousand feet.

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Spokane Weather Forecast Office
2601 N. Rambo Rd.
Spokane, Washington 99224

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