Why the definition of 'extreme cold' is different in Duluth and Dallas

With a mass of frigid Arctic air descending on much of the United States this upcoming week, meteorologists and public health officials are well aware that a North Dakotan and a South Texan are unlikely to agree on what amounts to "extreme cold."

As such, the National Weather Service and the Centers for Disease Control and Prevention stress that there is no set definition for extreme cold, and the point at which forecasters will warn residents that the plunging temperatures are a danger to them is calibrated to the region's prevailing climate.

"Everyone's extreme is a different level," said Ketzel Levens, a meteorologist in the weather service's office in Duluth, Minnesota, where the average daily temperature in January is a crisp 9.4 degrees. "Folks up north might have better protection. They might have a lot more clothes and layers. Our houses, our water infrastructure, they're built to a different standard."

And so if you warned Minnesotans every time it is merely freezing cold, you would struggle to get their attention when it was dangerously cold. Levens' office issues a wind chill advisory only when the temperature reaches minus 25 and a wind chill warning when it reaches 40 below.

While it won't be historically cold in Duluth this week — temperatures are expected to reach as low as minus 7 degrees — Levens said that residents might still feel the bite of the cold as the city is coming off its second warmest December on record.

Roughly 1,000 miles south, the Dallas-Fort Worth region won't get as cold as Duluth this week, with a low temperature of 14 degrees. But the freezing temperatures there could pack a more significant punch, said Tom Bradshaw, a meteorologist in the service's office in Fort Worth, Texas, where the average high temperature for January is 56 degrees.

"We could be looking at as much as 80, 85 hours of below-freezing temperatures here," he said, "which for us is definitely on the colder side of what we normally experience."

On the plus side, he said, this week's cold snap isn't expected to come with precipitation and icing, which battered Texas during a storm in February 2021. More than 200 people died, and millions were left without electricity and heat in the cold. Many also had no clean water, as pipes had burst or water treatment plants had failed.

Not only is the infrastructure in the South not as comfortable in such extremes, but also its population — which is less acclimated to severe winter weather — can also struggle more, according to Dr. Gordon Giesbrecht, an expert on cold weather physiology and hypothermia at the University of Manitoba.

The body responds to feeling cold, he said, by tightening blood vessels to stanch blood flow to the extremities. That protects a person's core and vital organs from getting cold. The body can also shiver to increase heat production, he said. But both those protective tendencies mean that the skin gets colder and is at a higher risk of frostbite.

"People who are more adapted to cold, like if you live in Minnesota, your skin will not get as cold," Giesbrecht said. "Your body figures out through continued exposure that, 'Well, we can afford to lose a little heat at the expense of keeping our skin warm because it takes a lot to actually cool the core.'"

People who are not used to such cold will probably experience more constriction, he said, leaving them more at risk of frostbite. Giesbrecht advised keeping the head, fingers and toes well-covered.

"People often get fooled. They don't feel anything and say, 'OK, I'm OK now,'" Giesbrecht said of the numbness induced by icy temperatures. "You have to do something: Either put more insulation on, or do some exercise to increase your heat production, or go inside."

And bear in mind that even Minnesota is relatively warm by some standards. Natalie Hasell, a warning preparedness meteorologist for the Canadian government, said that on the shores of the Hudson Bay in northern Manitoba, the threshold for a wind chill advisory is minus 50 degrees Celsius, or about minus 58 degrees Fahrenheit.

It's not that people there are "super human," she explained, because anyone's skin could freeze in five to 10 minutes of exposure to temperatures below minus 40 or even more quickly in strong winds.

"But it just happens so often there that they'd never hear the message of when it would be really bad," Hasell said. She added, "If you only have two minutes before your skin can freeze, you want to hear that if you're used to having 10 minutes."