What is a bomb cyclone? The extreme winter storm hitting the US

Though it might not sound like it, a “bomb cyclone” is a legitimate scientific term, so named because the rapid formation of the storm is like a bomb going off. 

Millions of Americans have been warned to brace for a bomb cyclone with blizzard conditions, wind chills and temperatures plunging as low as -45.6C (-50.1F) – cold enough to get frostbite in less than five minutes.

A bomb cyclone or “weather bomb” is a term used to describe a rapidly deepening area of low pressure.

“More correctly, it should be called explosive cyclogenesis, which is when the central pressure of a low pressure system falls dramatically – by 24 millibars in 24 hours,” Sky News weather presenter Kirsty McCabe said.

“These intense storms bring heavy precipitation and very strong winds. In the US right now, very cold Arctic air is being pulled in, with the freezing weather causing further complications.”

How does a bomb cyclone form?

The sudden change in pressure is due to interactions with a powerful jet stream, McCabe said.

“This is the fast-moving ribbon of air high in the atmosphere that steers our surface low pressure systems around.

“The contrast between cold, Arctic air in the north and warm, tropical air in the south has strengthened the jet stream, which in turn deepened the area of low pressure.”

Why is it called a bomb cyclone?

Meteorologists have likened the sudden drop in pressure to a bomb going off, using words such as “explosive cyclogenesis” and “bombogenesis” to describe the storm’s formation process.

The term specifically refers to the speed at which a storm forms, Daniel Swain, a climate scientist at the University of California, said.

“It doesn’t necessarily refer to the absolute strength of the storm system,” he said.

Does the UK get bomb cyclones?

The UK has experienced weather bombs, most famously the Great Storm of 1987.

How does a bomb cyclone differ from a hurricane?

“All bomb cyclones are not hurricanes,” Mr Swain said. “But sometimes, they can take on characteristics that make them look an awful lot like hurricanes, with very strong winds, heavy precipitation and well-defined eye-like features in the middle.”

Hurricanes tend to form in tropical areas and are powered by warm seas. For this reason, they’re most common in the US in summer or early autumn, when seawater is warmest.

In contrast, bomb cyclones don’t need balmy ocean waters to form.

They can appear over land as well as the sea and are most common between late autumn and early spring, when warm tropical air bumps up against frigid Arctic air.

“They have to occur at a time of year when there is some possibility of both warm and cool air at the same time,” Mr Swain said. It’s the difference in temperature that fuels the drop in pressure.

Are bomb cyclones dangerous?

It depends. Sometimes, bomb cyclones behave like conventional winter storms.

But sometimes they produce heavy flooding, blizzard conditions and wind speeds comparable to a Category 1 hurricane.

“Fundamentally, the impacts of a bomb cyclone are not necessarily different from other strong storm systems, except that the fast strengthening is usually a signature of a very powerful storm system,” Mr Swain said.

Much of the danger lies in the fact that bomb cyclones can take people by surprise, he added.