How low does the pressure have to drop for a storm to officially be a bomb cyclone?

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Dear Tom,
How low does the pressure have to drop for a storm to officially be a bomb cyclone?
 Liz Houser Oak Park
Dear Liz,
It’s not how low the barometric pressure falls, but how rapidly. Bombogenesis is a term applied to a rapidly intensifying low-pressure system in which the central pressure drops at the rate of one millibar an hour for 24 hours. In terms of inches of mercury, that would equate to pressure falling at least .03 inches an hour for 24 hours. This type of rapid intensification often occurs with cold-season storm systems off the Atlantic Seaboard dubbed nor’easters, which produce many of the region’s major snowstorms. While not as common, these storms do occur in the Midwest. On Dec. 14-15, 1987, low pressure moving northeast from the southern Plains “bombed out,” bringing 8-12 inches of heavy, wet thundersnow to the Chicago area.

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