A barometer measures atmospheric pressure, and there are water based, mercury-based and aneroid barometer instruments. Scientists usually report barometric pressure in millimeters of mercury or in hectopascals (which is more commonly used by metorologists). 1 mm of mercury is equal to about 0.750 hectopascals. Atmospheric pressure moves the mercury up into the measuring tube to somewhere around 30 inches at the surface of our planet, and that pressure would be reported as a barometric pressure in hectopascals as 39.99-- or perhaps 40 if rounded up. The abbreviation or scientific notation for hectopascal is hPa.
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