The cold front bringing its icy grip to parts of the US and Canada is a result of a weather condition called a polar vortex.
It might seem counter-intuitive, but the phenomenon is produced by a sudden blast of warm air in the Arctic, and North Americans need to get used to it – the polar vortex has been wandering more often in recent years.
It all started with misplaced Moroccan heat. Last month, the normally super chilly air temperatures 30km above the North Pole rapidly rose by about 70 degrees, thanks to air flowing in from the south. It is called “sudden stratospheric warming”.
That warmth split the polar vortex, leaving the pieces to wander, said Judah Cohen, a winter storm expert for Atmospheric Environmental Research, a commercial firm outside Boston.
“Where the polar vortex goes, so goes the cold air,” Mr Cohen said.
By Wednesday, one of those pieces will be over the lower 48 states of the US for the first time in years.
Officials throughout the Mid West are taking extraordinary measures to protect the homeless and other vulnerable people from the bitter cold, including turning some city buses into mobile warming shelters in Chicago.
Temperatures plunged as low as minus 32 degrees in North Dakota with wind chills as low as minus 52 degrees in Minnesota. It was nearly as cold in Wisconsin and Illinois.
Governors in Wisconsin, Illinois and Michigan declared emergencies as the worst of the cold threatened.
The National Weather Service forecast temperatures in Chicago as low as minus 33 degrees, with wind chills to minus 46 degrees. Detroit’s outlook was for overnight lows around minus 26 degrees, with wind chills dropping to minus 40 degrees.
“These are actually a public health risk and you need to treat it appropriately,” Chicago mayor Rahm Emanuel said. “They are life-threatening conditions and temperatures.”
Freeze skin
A wind chill of minus 32 degrees can freeze skin within 15 minutes, according to the National Weather Service.
At least four deaths were linked to the weather system, including a man struck by a snow plough in the Chicago area, a young couple whose SUV struck another on a snowy road in northern Indiana, and a Milwaukee man found frozen to death in a garage.
The unusual cold could stick around for another eight weeks, Mr Cohen said.
“The impacts from this split, we have a way to go. It’s not the end of the movie yet,” Mr Cohen said. “I think at a minimum, we’re looking at mid-February, possibly through mid-March.”
Americans were introduced to the polar vortex five years ago. It was in early January 2014 when temperatures dropped to minus 27 degrees in Chicago, and meteorologists, who used the term for decades, started talking about it on social media.
This outbreak may break some daily records for cold and is likely to be even more brutal than five years ago, especially with added wind chill, said Jeff Masters, meteorology director of the private weather firm Weather Underground.
When warm air invades the polar region, it can split the vortex or displace it, usually towards Siberia, Mr Cohen said.
Recently, there have been more splits, which increase the odds of other places getting ultra-cold, he said. Pieces of the polar vortex have chilled Europe, Siberia and North America this time.
When the forces penning the polar vortex in the Arctic are weak, it wanders, more often to Siberia than Michigan, and it is happening more frequently in the last couple of decades, said Oklahoma meteorology professor Jason Furtado.
A study a year ago in the Bulletin of the American Meteorological Society looked at decades of the Arctic system and found the polar vortex has shifted "toward more frequent weak states".
When the polar vortex pieces wander, warmth invades the Arctic, Alaska, Greenland and Canada, Mr Masters said.
While the Mid West chills, Australia has been broiling to record-breaking heat. The world as a whole on Monday was 0.4 degrees warmer than the 1979-2000 average, according to the University of Maine’s Climate Reanalyser. – AP