For the first time since 2014, the polar vortex descended on North America.
More than 84 million Americans in the Midwest and parts of New England experienced subzero temperatures, CNN reported. Wind-chill advisories in the Midwest and Northeast impacted 140 million Americans.
So far, 21 deaths have been linked to the recent weather, according to Reuters. Fatalities have been reported in Illinois, Indiana, Iowa, Wisconsin, Ohio, New York, Minnesota, and Michigan.
Windchill temperatures were as cold as minus 50 degrees Fahrenheit in parts of Minnesota and Wisconsin on Thursday. Some areas in Minnesota and the Dakotas faced temperatures 50 degrees below average for this time of year. Des Moines, Iowa saw a record-breaking low of minus 20 degrees Fahrenheit yesterday, according to the BBC, while Rockford, Illinois also set a record at minus 31 degrees Fahrenheit.
The term polar vortex describes the mass of low-pressure cold air that circulates in the stratosphere above the Arctic and Antarctic regions. Sometimes the circulation of the polar vortex weakens during the winter, causing surges of frigid air to splinter off and drift south.
The freezing air is carried by the jet stream, a current of wind that extends around the hemisphere and divides the air masses in the polar region from those farther south.
This polar air mass has always been present, but scientists first dubbed it the "polar vortex" in 2014, when a similar (though less severe) cold snap hit a majority of the continental US.
Frostbite 'in a matter of minutes'
North America, Europe, and Asia can all experience polar-vortex events, which bring temperatures that are simply too cold for people to safely be outside. This NASA satellite imagery of the recent polar-vortex event shows the cold air moving from Central Canada into the US Midwest from January 20-29.
"You're talking about frostbite and hypothermia issues very quickly, like in a matter of minutes, maybe seconds," Brian Hurley, a meteorologist with the Weather Prediction Center, told the Huffington Post.
Hypothermia occurs when your body loses heat faster than you can produce it. Frostbite arises when skin and the tissues below freeze, or, in extreme cases, die.
Chicago's National Weather Service office said, "dangerously cold wind chills could cause frostbite on exposed skin in as little as 5 minutes."
Hennepin Healthcare in Minneapolis, Minnesota has treated 22 patients for frostbite since last Friday, including 13 people who were admitted to the hospital, according to the Associated Press. At least 144 people in Illinois visited hospital emergency rooms for cold-related injuries on Wednesday and Thursday, according to the Guardian.
Many of the weather-related deaths involved people dying at home, according to Reuters. Authorities found a man frozen in his Milwaukee garage, a woman died of hypthermia in an abandoned Ohio house, and three men in Minnesota and Michigan were found dead outside their homes. One Illinois man was fatally struck by a snow plow, two people died in a car accident in Indiana, while four deaths in Iowa — including a University of Iowa student — were linked to the storm.
Relief is in sight
Fortunately, forecasts show the cold snap dissipating over the weekend. Temperatures in New England and the Midwest should reach 50 degrees Fahrenheit through the weekend and Monday, according to Reuters.
The National Weather Service reports a high of 46 degrees Fahrenheit in Chicago by Sunday — nearly a 60 degree increase from the city's Thursday morning temperatures.
"I don't think there's ever been a case where we've seen (such a big) shift in temperatures" in the winter, Jeff Masters, meteorology director at Weather Underground, told the Associated Press. "Past record-cold waves have not dissipated this quickly. Here we are going right into spring-like temperatures."
At least 3,000 flights were canceled this week, according to the airline tracking website FlightAware. The US Postal Service suspended mail delivery in six Midwest states on Wednesday and Thursday, but deliveries have now resumed.
We might start seeing polar-vortex events more often
Despite President Donald Trump's assertions to the contrary on Twitter, the emergence of a polar vortex does not invalidate the scientific consensus on global warming.
The polar vortex creates weather events that take place regionally on dayslong or weeklong timescales. The latter is a planetwide phenomenon caused by increased concentrations of certain gases in Earth's atmosphere. Global average temperatures are still among the highest ever recorded, and oceans are the warmest they've been since we started keeping records.
In fact, recent research shows that the frequency of winter polar-vortex events has increased over the past four decades, perhaps because of climate change.
Although many questions remain, scientists have started to connect extreme cold waves to the warming Arctic, as Inside Climate News reported. Because temperatures are rising in the Arctic twice as fast as the rest of the globe, the difference between temperatures at the North Pole and continents at lower latitudes is decreasing, according to The Conversation. Less disparity in temperatures means less difference between air pressure levels, which weakens the jet stream. That can lead the jet stream to take longer, less direct paths.
If the jet stream wanders enough, that can disrupt the natural flow of the polar vortex.
This story has been updated to reflect the latest numbers.
Kevin Loria contributed to a past version of this story.
