Climate Crisis

What is a ‘bomb cyclone’?

Weather event is defined by how rapidly pressure drops in low-pressure mass

A giant tree lies across the intersection of West Delavan Avenue and Bidwell Parkway in Buffalo, New York. Photograph: Derek Gee/The Buffalo News/AP
A giant tree lies across the intersection of West Delavan Avenue and Bidwell Parkway in Buffalo, New York. Photograph: Derek Gee/The Buffalo News/AP
Henry Fountain
Mon Dec 26 2022 - 09:23

The storm pummeling large swathes of the US and Canada is what forecasters call a “bomb cyclone.” While this kind of storm is not exceedingly rare, this one is very strong, with high winds that are bringing heavy snow or rain to many areas.

Storms can form when a mass of low-pressure air meets a high-pressure mass. The air flows from high pressure to low, creating winds. What defines a bomb cyclone is how rapidly the pressure drops in the low-pressure mass – by at least 24 millibars in 24 hours. This quickly increases the pressure difference, or gradient, between the two air masses, making the winds stronger. This process of rapid intensification has a name: bombogenesis.

As the winds blow, the rotation of the Earth creates a cyclonic effect. The direction is counterclockwise in the northern hemisphere (when viewed from above).

John Moore, a meteorologist and spokesman for the National Weather Service, said conditions for a bomb cyclone had been met over the Great Lakes, where frigid Arctic air from the meandering polar vortex met very warm air to the east.

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Air pressure dropped to at least 962 millibars, while elsewhere it was as high as 1,047 millibars.

“It’s a really sharp gradient,” he said.

As the area where the two air masses meet, called the Arctic front, moves northward and eastward, conditions for bombogenesis should continue moving as well, Mr Moore said.

But as the Arctic air spreads over most of the country it will eventually warm, reducing the pressure difference. The storm will dissipate. And forecasts predict for above-average temperatures across most of the country next week, he said. – This article originally appeared in The New York Times.

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