If you keep up with the weather in the winter, you’ve probably heard or read about a “bomb cyclone”. This week, it’s been quite the buzz as our friends to the north brace for a strong nor’easter this weekend.

The coastal low pressure system that will bring all that rain, snow (over a foot for some spots) and strong wind to the Northeast is organizing itself off the Southeast Coast and will rapidly strengthen off the North Carolina Coast tonight.

Whether it is a new term to you or seems like it belongs in a military movie, in the weather world, bomb cyclone is a normal term.

But what exactly does it mean?

First, a quick vocabulary lesson. A millibar is a unit of pressure.

You are likely quite familiar with a high pressure (denoted with the letter H on a weather map) and a low pressure (denoted with the letter L on a weather map).

High pressure systems are anticyclones and spin clockwise in the Northern Hemisphere. Low pressure systems are known as cyclones and spin counterclockwise in the Northern Hemisphere. (Fun fact: the reverse is true for the Southern Hemisphere!)

A lower pressure reading means a stronger storm. A low pressure system “bombs out” and is called a bomb cyclone when the pressure drops 24 millibars in 24 hours. This is known as bombogenesis.

The rapidly dropping pressure indicates that the storm is rapidly strengthening. Such rapid intensification is due to the combination of the cold air from the north and the warmer air from the south. A stronger storm can mean more destruction to the areas impacted.

For those wondering how long this term has been used, the term “bomb” originated in a meteorological research paper from 1980. The authors (MIT meteorologists Fred Sanders and John Gyakum) were expanding on earlier work from Swedish meteorologist Tor Bergeron.

Now of course we know storms aren’t confined to certain latitudes, so does the 24 millibar/24 hour rule apply everywhere?

The answer is no. You have to look at where the storm developed, so the millibar requirement may be different depending on the storm’s location.

For storms north of 60 degrees latitude, the 24 millibar/24 hour rule applies.