What is a 'bomb cyclone'?

Eastern US braces for winter "bomb cyclone".VIDEO: REUTERS
For weeks, the eastern half of the US has been locked in a weather pattern where it has been continually blasted by cold arctic air coming from the North Pole.
For weeks, the eastern half of the US has been locked in a weather pattern where it has been continually blasted by cold arctic air coming from the North Pole.PHOTO: AFP

NEW YORK - A "bomb cyclone" is heading towards the East Coast of the United States, threatening to bring temperatures colder than Mars for millions who are already battling one of the coldest winters on record.

Why is it called a 'bomb cyclone'?

When discussing the impending storm, some weather forecasters have referred to a "bomb cyclone" or "weather bomb", unofficial terms for what is known as explosive cyclogenesis.

When a storm's central pressure drops by at least 24 millibars in 24 hours, it is referred to as a bomb or "bombing out", meteorologist Ryan Maue told BuzzFeed News.

These storms can also be called extratropical cyclones, mid-latitude cyclones, or just plain old Nor'easters.

In the 1940s, some meteorologists began informally calling some big coastal storms "bombs" because they develop "with a ferocity we rarely, if ever, see over land", retired Massachusetts Institute of Technology professor Fred Sanders told USA Today. He brought the term into common usage by describing such storms in a 1980 article in the journal Monthly Weather Review.

How is it formed?

Like any other storm, a bomb cyclone is formed when a warm front collides with a cold front, causing deep drops in barometric pressure.

The air starts to move, and the rotation of the Earth creates a cyclonic effect. The direction is counterclockwise in the Northern Hemisphere (when viewed from above), leading to winds that come out of the north-east - a Nor'easter.

For weeks, the eastern half of the US has been locked in a weather pattern where it has been continually blasted by cold arctic air coming from the North Pole.

That cold air over land and the warm ocean temperatures in the Atlantic, along with the interaction of another weather system higher in the atmosphere, were all key ingredients in forming this fast-growing storm, Jonathan Martin, a meteorology professor at the University of Wisconsin-Madison, told BuzzFeed.

Why will there be powerful gales?

Where there are different pressures in the atmosphere, the wind will flow from high pressure to low pressure to try and balance out the difference.

With this storm, which has been named Grayson, the biggest concern is very strong winds gusting as much as 100kmh along much of the East Coast. Those strong winds are probably going to cause a lot of power outages, according to Scientific American.

Power outages bring the threat of potentially deadly heating disruptions.

Forecasters say the Northeast states will see threatening hurricane-force winter wind gusts and blinding snow, reported CNN.

Are 'bomb cyclones' rare?

Calling it a "bomb" sounds dire, but those kinds of storms are not exceedingly rare.

Prof Martin told BuzzFeed that about 10 of these cyclones occur across the Northern Hemisphere every year. About one occurs in the US every year. 

A similar phenomenon was seen at the end of October, when warm air from the remnants of a tropical cyclone over the Atlantic collided with a cold front coming from the Midwest. Among other impacts then, more than 80,000 electricity customers in Maine lost power as high winds toppled trees.

How much damage can it cause?

Experts agree that this storm will be particularly powerful because "the bigger the contrast between that cold Arctic air on the one side and the warm moist air from the ocean on the other side, the stronger your storm is going to be". 

Ryan Maue of the US weather service that the coming storm would be "in the upper echelon of 'bomb cyclones'" - adding that the term refers to "simply a more extreme variety of 'cold season' storm that usually harmless mix fish, generate huge waves, and do their job of moving Earth's heat.


Its effects include record snowfalls, strong winds that can quickly create blizzard conditions and coastal flooding. 

The bone-chilling cold this winter has been blamed for at least 12 deaths across the US, thousands of flight cancellations and countless traffic accidents.

Every state from Maine to Georgia along the east coast has either a winter weather advisory, a winter storm warning or a blizzard warning. It snowed for the first time in 28 years in the Florida city of Tallahassee on Wednesday morning.

Was it influenced by climate change?

Climate change is known to exacerbate natural disasters like hurricanes and wildfires, and warming Arctic regions may even be making US winters colder.

But linking one specific extreme weather event to climate change is tricky, according to the National Geographic.

The Arctic is not as cold as it used to be – the region is warming faster than any other – and studies suggest that this warming is weakening the jet stream, which ordinarily acts like a giant lasso, corralling cold air around the pole.

“There’s a lot of agreement that the Arctic plays a role, it’s just not known exactly how much,” said Marlene Kretschmer, a researcher at the Potsdam Institute for Climate Impact Research in Germany. 

“It’s a very complex system,” she told New York Times.