Dear Tom,
I recently purchased a weather instrument that measures barometric pressure in two different units–“mb” and “inHg.” What do these units mean?
Claire E. Carr
Dear Claire,
Many different units are used to measure air pressure, among them hectopascals, millibars, and millimeters and inches of mercury. While the unit of inches of mercury is most commonly used in the U.S., most meteorologists prefer millibars. Your barometer’s pressure scales are calibrated in millibars and inches of mercury. One millibar (mb) is equal to about .03″ inches of mercury and a convenient reference point relating the two is 1000 mb which equates to 29.53″. An intense hurricane with a central pressure of 915 mb would convert to a pressure of 26.98″. In winter, a bitterly cold high pressure system might sport a central pressure of 1050 mb which would be equivalent to about 31.03?.
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Tom Skilling is chief meteorologist at WGN-TV. His forecasts can be seen Monday through Friday on WGN-TV News at noon and 9 p.m.
WGN-TV meteorologists Steve Kahn, Richard Koeneman and Paul Dailey plus weather producer Bill Snyder contribute to this page.
