Explainer

Bomb cyclones and polar vortexes—winter's scary weather explained

What creates these dramatic-sounding weather conditions?

An intense winter storm with hurricane-force winds is expected to hit the Midwest on Wednesday. The bomb cyclone follows an active winter season where polar vortex winds created dangerous conditions throughout parts of the U.S.

It's not the first time these conditions have hit the country.

Why Niagara Falls Doesn't Freeze Solid

Last year, it was so cold Niagara Falls partially froze. Bomb cyclones, polar vortexes, and arctic air masses blanketed the region. New England saw hurricane-force winds, Maine and Boston had blizzard warnings, and it snowed in Florida.

Are these atmospheric conditions as scary as their names make them sound? And are they normal?

We answer your questions here.

What is a bomb cyclone?

It's called a cyclone, a bomb cyclone, or a cyclone bomb. Or a weather bomb. Or bombogenesis.

What is technically a "midlatitude cyclone" refers to when a storm gains strength from an extreme drop in atmospheric pressure. The effect is prompted by what is technically called "explosive cyclogenesis," and occurs when a storm drops by at least 24 millibars (a unit that measures pressure) in 24 hours.

According to the National Oceanic and Atmospheric Administration's website, a bomb cyclone occurs when a "cold air mass collides with a warm air mass." Cold arctic air colliding with warm ocean water is a common source of this collision.

After pressure plummets, air rushes in to fill the space between these two air masses, creating intense winds and strengthening the storm.

Despite the intense name, bomb cyclones are fairly common, particularly in northern Atlantic regions.

The National Weather Service reported in 2018 that states in the path of this current bomb cyclone experienced hurricane-force winds, heavy snow, and flooding.

What Is the Polar Vortex?

What is a polar vortex?

Winds spinning around the Arctic circle can push intensely cold air south.

This is thanks to winds escaping from the polar vortex.

The vortex is a mass of cold air that sits over the Arctic region. It's full of swirling eddies that, during winter months, can grow and extend farther south. It represents the boundary of cold, polar air and warmer subtropical temperatures.

As National Snow and Ice Data Center Director Mark Serreze described for a video made by non-profit Earth Vision Trust in 2014, polar vortexes are bordered by the "polar front jet stream" that's constantly shifting.

A polar vortex outbreak in farther southern latitudes can have damaging impacts on regions' transportation and agriculture practices, and scientists aren't quite sure how these outbreaks will be influenced by climate change, says Serreze.

"The weather machine is going to respond to that," he said in the video. "Just how, we don't quite know yet."

What about Nor'Easters?

A Nor'easter is geographically constrained cyclone driven by Atlantic winds. They occur anywhere from New England to Georgia but aren't seen in other parts of the country.

The term Nor'easter simply refers to a midlatitude winter storm. Many Nor'easters form when that same polar jet stream collides with warm currents from the Gulf jet stream.

This collision facilitates winter storms like bomb cyclones, notes meteorologist for Weather Underground Bob Henson.

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People walk through a frigid Manhattan on December 28, 2017 in New York City.

"Any areas that lose power due to heavy snow and strong winds will be vulnerable to intense cold," says Henson, making conditions more dangerous for people at risk.

What role does climate change play in all this?

It's well known that climate change can influence weather. The effect is known to exacerbate natural disasters like hurricanes and wildfires, and warming Arctic regions may even be making U.S. winters colder.

But linking one specific extreme weather event to climate change is tricky.

Henson notes that these types of multi-latitude storms vary greatly year-to-year, saying, "There are some indications these storms have become stronger and more frequent across the Northern Hemisphere in the last 60 years, but there isn't strong evidence of any major change in impacts along the U.S. East Coast."

"What we simply have here is a strong trough in what we call the atmospheric longwave pattern, or, as some might say, there has been a strong southward excursion of the polar vortex," said Serezze in a recent email to National Geographic.

Simply put: "It is winter. This happens."

When scientists talk about weather, they're referring to day-to-day changes, and when they discuss changes to the Earth's climate, they're referring to changes over time.

While the U.S. has experienced longer-than-normal cold weather this winter, meteorologist Eric Holthaus told National Geographic in a previous article that it's too soon to link this year's winter weather conditions to overall climate change.

"It's really difficult to say for certain if this exact weather pattern today would have happened the same way without climate change," he said. Though he added, "It's really irresponsible to say that climate change is not affecting weather everywhere on Earth."

This article was originally published on January 3, 2018. It was updated to include bomb cyclone events from winter 2019.