What is a "bomb cyclone"?

The storm hitting much of the eastern half of the United States is what forecasters call a "bomb cyclone." Although this type of storm is not extremely rare, this one is very strong, with high winds bringing snow or heavy rain to many areas.

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

When the winds blow, the rotation of the earth creates a cyclonic effect. The direction is counter-clockwise in the northern hemisphere (seen from above).

John Moore, meteorologist and spokesman for the National Weather Service said conditions for a bomb cyclone had been encountered over the Great Lakes, where frigid Arctic air from the winding polar vortex met very warm air at the is. it was as high as 1047 millibars. "It's a very steep gradient," he said.

While the area where the two air masses meet, called the Arctic Front, moves north and east, bombogenesis conditions are expected to continue moving as well, Moore said.

But as air arctic will spread over most of the country, it will eventually warm up, reducing the pressure difference. The storm will dissipate. And forecasts call for above-average temperatures across most of the country next week, he said.