What are millibars?

Dear Tom,
Would you explain what millibars are? How is 923 millibars assigned in a hurricane?
Margaret Robertson, Chicago
Dear Margaret,
A millibar is a measure of pressure. The Earth’s atmosphere exerts a pressure of 14.7 pounds on every square inch at sea level, or 1,013.25 millibars (and progressively lesser pressures at higher elevations because less of the atmosphere lies above). Millibars are units of pressure in the centimeter-gram-second system of measurement in which centimeters are the measure of length, grams are the measure of weight and seconds the measure of time.
An atmospheric pressure of 923 millibars equals 27.26 inches of mercury — a very low air pressure reading that is found in only the most intense hurricanes. One example of such a hurricane is Hurricane Hugo, which had a central pressure of 27.11 inches on Sept. 15, 1985, in the Caribbean Sea.

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