What is a ‘bomb cyclone,’ the weather pattern battering B.C.’s coast and Haida Gwaii?

After atmospheric rivers and heat domes, there appears to be a new extreme-weather term British Columbians should get familiar with: “bomb cyclones.”

These storms are caused by a rapid drop in air pressure and can bring heavy rain and powerful winds.

On Monday morning Environment Canada warned of wind gusts of up to 90 km/h on Vancouver Island and 110 km/hour on Haida Gwaii due to a bomb cyclone off the B.C. coast, which could be particularly destructive as trees, weakened from the summer drought, are susceptible to breaking — which itself could lead to power outages if they fall on wires and other infrastructure.

Ferry sailings are also being cancelled due to the risks of high winds.

Here’s some information about the weather phenomenon.

How is a bomb cyclone different from a cyclone?

Bomb cyclones and cyclones are the same thing — the “bomb” portion is just a reference to how quickly it is created.

All storms are the result of cold and warm weather systems colliding with one another, and a cyclone is the term used when these air masses create spiralling winds.

In order to qualify as a “bomb cyclone,” there needs to be a drop of 24 millibars (the unit used to measure air pressure) in 24 hours, said Yimei Li, a meteorologist with Environment and Climate Change Canada.

“It’s a way to describe a rapidly dropping pressure system,” Li said, noting that “bomb cyclone” is not an official term used by the weather agency, but is becoming more popular among meteorologists as a way of describing the rapid development of a storm resulting in spiralling winds.

Bomb cyclones most often occur in the fall and winter, as temperatures drop.

Where did the term come from?

The term’s origins can be traced back to John R. Gyakum, a professor of atmospheric science at McGill University, who coined it alongside Frederick Sanders when the pair were at the Massachusetts Institute of Technology in the 1980s as a way to describe intense storms that formed during fall and winter.

Gyakum told the Washington Post that they wanted a better way to communicate the intensity of the pressure systems so the public would be aware of their dangers, even after the hurricane season ended.

“Our goal was to help raise awareness that damaging ocean storms don’t just happen during the summer,” he told the newspaper last year.

What about ‘bombogenesis’?

The term “bombogenesis” is sometimes used to refer to the formation of a bomb cyclone.

“Genesis” means the formation of something and the term “cyclogenesis” is used to refer to the creation of a cyclone.

How common are bomb cyclones?

As a sub-type of cyclones, bomb cyclones aren’t considered common but they aren’t considered rare, either.

In his interview, Gyakum noted that in the leadup to his 1980 research paper, there had been several big fall storms, some resulting in causalities.

Generally speaking, they are only of note when they come close enough to land in order to prompt warnings, as they have in B.C. this week.

The real question for meteorologists is how powerful the associated winds are once they reach the land.

“It doesn’t have to be a bomb to give us wind warnings,” Li said. “The impact depends on where the system is.”

How dangerous are they?

Bomb cyclones can be devastating, as seen in December 2022 when the eastern United States saw massive damage and at least 50 deaths due to a winter storm that hit just before Christmas.

Again, Li said, it’s about where the weather system lands.

“It’s like a hurricane: If it doesn’t make landfall, then the impact is not as strong.”

For now, the cyclone forecast for B.C. is expected to skirt Vancouver Island and B.C.’s South Coast before heading north through the water up to Haida Gwaii, bringing wind gusts of up to 110 km/h.

“If the system were to move more on shore, it could bring even stronger winds,” Li said.