For a while there, Chicago’s 2019 weather was normal. With just three days left in January, temperatures had averaged a few tenths warmer than usual. It had been a bit snowy, but no big deal.

Then the Polar Vortex happened.

On Wednesday, Chicago may well be having its coldest day in recorded history. Its all-time record low temperature, -27 degrees Fahrenheit set in 1985, and its all-time lowest high temperature, -11 degrees set in 1994, are both in serious danger.

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For that matter, people throughout the Midwest are in serious danger, too. Extreme cold weather is nasty stuff, especially with winds as strong as they are. Ordinary happenings can quickly become fatal when wind chills are below -50 degrees.

Got a flat? It might have been caused by the extreme cold. Now try changing the tire while wearing a heavy coat, scarf, and thick gloves, while snow is whipping across your face and building up on your exposed skin. Take off your gloves to work with the lug nuts, and you could immediately get frostbite. That car coming up behind you might not notice you crouched next to your left rear tire, because visibility is so poor with the blowing snow, and any jerk of the steering wheel could send the car into a spin.

Better stay inside the car. It’ll get bone-chillingly cold pretty fast, especially if you forgot to bring along a blanket. If you run the engine to keep the heater going and don’t keep clearing the snow from the exhaust pipe, the carbon monoxide buildup will probably kill you. Or if your idling car finally runs out of gas, maybe the cold will kill you anyway.

Don’t want to wait it out? Sure, you could try walking. But if it’s more than a couple hundred feet to safety, hypothermia might cause disorientation, and then it could be several days before someone stumbles across your body.

It’s much safer just to stay at home. But if your house is not well insulated, you might need to break out the extra space heater. Hopefully, there won’t be a power outage. And you’d better hope the extra load on your electrical system doesn’t cause a short, because that could start a fire. Fires are pretty hard to fight when the water from the fire hoses freezes on contact with the ground and makes everything as slippery as, well, ice.

That’s assuming you have a home.

This is shaping up as one of the most intense cold-air outbreaks ever for the Midwest. Probably not the worst, though, from a climatological perspective.

The one in January 1994 that set Chicago’s record for coldest high temperature was pretty much as cold, but covered a wider area.

The one in February 1899 was even colder and set lots of records that still stand.

The cold in December 1983, around Christmastime, ranks right up there, as does January 1982. Both of those had several waves of cold weather, lasting a week or more overall.

This one, at least, will be here and gone fairly quickly. The coldest low-level air in the entire Northern Hemisphere is moving southeastward through the Midwest, but it’s going to circle across the Great Lakes and up toward the Canadian Maritimes before reaching the relative warmth of the open waters of the Atlantic Ocean.

By Saturday, Chicago will be back above freezing and warmer than normal, at least temporarily.

The cold air came, of course, from the Arctic. Events were set in motion several weeks ago, when the normal westerlies circling the Northern Hemisphere midlatitudes at an altitude of 20 to 50 miles, forming what’s called the stratospheric polar vortex, broke down into a disorganized mess. This breakdown, which seems to be more common in recent years, sets the stage for large excursions of warm air toward the pole and cold air toward lower latitudes.

Then, over a week ago, a seemingly inconsequential storm spun up in the Sea of Japan and moved eastward across the Pacific. As it continued spinning, it was able to ingest part of a plume of moisture, an atmospheric river originating in steamy tropical air near the Philippines. The moisture reinvigorated the storm as it was carried aloft and condensed into precipitation.

Partly because of this invigoration, the downstream jet stream bent northward, then toward the east and then back southward, creating an upper-level ridge. Often, you’ll get a new winter storm forming downstream of such a ridge, in a process called downstream development.

In this case, the ridge extended far enough north to impinge on a pool of cold air that had drifted close to the Canadian Arctic. The southward-moving airflow latched onto the (by now extremely) cold air over the weekend and brought it southward, much like food on your dinner plate starts sliding off if you suddenly tilt the plate. Directly in the path of this cold air was the Upper Midwest, which has been awaiting the arrival of cold air not nearly as eagerly as your dog awaits falling food.

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Climate change could be affecting all aspects of this process, except maybe the dog. The warming Arctic could be responsible for the disorganized stratospheric polar vortex becoming more common. The warmer temperatures also provide more moisture to feed winter storms, possibly making it easier to build the upper-level ridges, although other temperature changes have a weakening effect. The combined effect of climate change and natural variability has actually led to increasingly severe winters during the past few decades across much of the Northern Hemisphere land masses.

On the whole, though, extreme cold temperatures are rarer than extreme warm temperatures, just because temperatures are warmer overall. Still, a few degrees of warming has no chance of neutralizing an air mass that’s up to 45 degrees Fahrenheit colder than normal.

John Nielsen-Gammon is Regents professor of Atmospheric Sciences at Texas A&M University and serves as the Texas state climatologist. He is a fellow of the American Meteorological Society. Follow him on Twitter at @ClimaTexas.