What is a bomb cyclone?

We often hear that a storm system is undergoing 'bombogenesis' or, more simply put, is developing into a 'bomb cyclone.' But what exactly do these foreboding weather terms mean?

As a major storm moves in to blast the Northeast U.S. with high winds and heavy rain this week, many may be hearing the term "bomb cyclone." You may have heard or read about a bomb cyclone or a storm system undergoing "bombogenesis," but what exactly do these weather terms mean?

In simple terms, bombogenesis, a term used by meteorologists, is a large storm that undergoes rapid strengthening over a 24-hour period. The vast majority of such storms occur over the ocean. The storm can be tropical or non-tropical in nature.

The term bombogenesis comes from the merging of two words: bomb and cyclogenesis. All storms are cyclones, and genesis means creation or beginning. In this case, bomb refers to explosive development and altogether the term means explosive storm strengthening.

A cyclone is essentially a giant rising column of air that spins counterclockwise over the Northern Hemisphere. In the Southern Hemisphere, they spin clockwise.

When air rises, it produces a vacuum effect that results in lower atmospheric pressure. When a storm strengthens, the column of air rises at an increasing rate as the pressure within the storm lowers.

"Storms, of course, always have lower atmospheric pressure, but in this case the atmospheric pressure is lowering very quickly as the storm system intensifies," said AccuWeather Chief Meteorologist Jon Porter. "The barometric pressure will greatly reduce very quickly."

Average storms in the winter have a low barometric pressure reading of 29.53 inches of mercury. Some of the most intense storms may have a barometric pressure below 29.00 inches. But it's not the lowest pressure that defines bombogenesis but how quickly the pressure within the storm plummets.

When the barometric pressure falls at least 0.71 of an inch of mercury (24 millibars) in 24 hours, a storm has undergone bombogenesis.

For example, a weak storm that began with a barometric pressure of 29.98 inches and ended up with a barometric pressure of 29.27 inches in 24 hours underwent bombogenesis.

The Superstorm of 1993 (Storm of the Century) from March 12-13 is a prime example of a storm that underwent bombogenesis. The storm strengthened from 29.41 inches (996 mb) to 28.45 inches (963 mb), or nearly 1.00 inch (33 mb), in 24 hours. Much of this strengthening occurred over land.

Other examples of storms that underwent bombogenesis: Hurricane Charley in 2004 and Hurricane Wilma in 2005; the Blizzard of 2015 (Jan. 26-27); the Bering Sea storm of December 2015; Storm Dennis, which impacted the United Kingdom on Feb. 14, 2020; and the storms that hit the northeastern and western United States in late October 2021.

Storms that undergo bombogenesis are among the most violent weather systems that affect a broad area. This is because the rapidly ascending air near the center of the storm must be replaced by air surrounding the storm. As these winds move toward the center of the storm at high speed, property damage can occur, trees may fall and the power may go out.

The western North Atlantic is one favored area for storms to undergo bombogenesis. This is a region where cold air from North America collides with warm air over the Atlantic Ocean. Warm waters of the Gulf Stream may also provide a boost in a festering storm. As a result, some nor'easters undergo bombogenesis.

The intense winds often create massive seas and may cause significant beach erosion. Very heavy rain and/or snow may fall in the path of a storm undergoing bombogenesis. Precipitation rate is produced from the rising column of air. When air rises, it cools and moisture condenses to form clouds and rain or snow. The faster the air rises and cools, the heavier the precipitation.

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