What is a bomb cyclone?

A powerful bomb cyclone is soaking California Wednesday and is expected to continue into Thursday, dropping several inches of rain and creating a widespread risk of flooding, mudslides and power outages in what some forecasters are suggesting will become one of the most impactful storms to strike the state in years.

What exactly is a bomb cyclone?

By definition, it is a low pressure system that experiences a fall in pressure of 24 millibars in 24 hours. Millibars are a unit used to measure atmospheric pressure.


Also known as bombogenesis, it's essentially a storm that rapidly intensifies over a period of 24 hours. This is the result when atmospheric pressure drops significantly, which is usually the result of warm and cold air masses overlapping, according to the National Ocean Service.

Bombogenesis can happen when a cold air mass collides with a warm air mass, such as air over warm ocean waters.

It's important to keep in mind however that the above benchmark is also dependent on the latitude of the storm, not just the millibar requirement. 

The term bomb cyclone goes back to a meteorological research paper published in a 1980 edition of Monthly Weather Review.

The authors of that paper, MIT meteorologists Fred Sanders and John Gyakum, added to the foundational work of Swedish meteorology researcher Tor Bergeron, by adding varying ground rules based on latitude. That's when the "bomb" part of bomb cyclone originated, due to the explosive power that these storms derive from when rapid pressure drops.