‘Bomb cyclones’ really not all that rare

A powerful winter storm has brought snow to the South and fears of whiteout conditions in the Northeast.

When discussing the storm, some weather forecasters have referred to a “bomb cyclone.” Calling it a “bomb” sounds dire, but those kinds of storms are not exceedingly rare — there was one in New England recently.

What makes a storm a “bomb” is how fast the atmospheric pressure falls; falling atmospheric pressure is a characteristic of all storms. By definition, the barometric pressure must drop by at least 24 millibars in 24 hours for a storm to be called a bomb cyclone.

Here is how it works: Deep drops in barometric pressure occur when a region of warm air meets one of cold air. The air starts to move, and the rotation of the Earth creates a cyclonic effect. The direction is counterclockwise in the Northern Hemisphere (when viewed from above), leading to winds that come out of the northeast — a nor’easter.

That’s what happened at the end of October, when warm air from the remnants of a tropical cyclone over the Atlantic collided with a cold front coming from the Midwest. Among other impacts then, more than 80,000 electric customers in Maine lost power as high winds toppled trees.

A similar effect is expected late Wednesday, as warm air over the ocean meets extremely cold polar air that has descended over the East. Pressure is expected to fall quickly from Florida northward.