Change in wind chill factor means current marks can't be directly compared to old ones.
63 below no doubt bone-chilling, but not the same as before 2001
When parents in Minot, N.D., tell their youngsters that Thursday's wind chill of 63 degrees below zero wasn't as low as the wind chill when they were kids, they're not just blowing hot air.
In fact, it would have been a 78-below wind chill prior to 2001, when the National Weather Service revised its wind chill index, said Rich Leblang, weather service meteorologist in Bismarck.
Wind chills got a lot of attention this week as even the lightest breeze became bone-chilling and record low temperatures were set in Bismarck and Grand Forks.
But, while temperature comparisons are consistent over the years, one can't compare today's wind chill figures with those used before 2001.
The National Weather Service made the switch that year to a revised formula developed by a U.S.-Canadian coalition and designed as a more realistic guide to how the wind feels to exposed skin.
The formula is "ridiculously complicated," and questions linger about the best way to calculate wind chill, Leblang said.
"When you get into thermodynamics, it gets ugly, (with) so many molecules moving at different speeds," he said.
Leblang said the coldest wind chill he recalls during his 35 years with the weather service in Bismarck was 86 below on the night of Dec. 23-24, 1983. That same night, Williston flirted with a wind chill of 100 below, which would be 71 below by today's index, he said.
The current index dips into the negative wind chills faster than the old index, but the wind chill drops faster and further in extreme cold temperatures under the old index, Leblang said.
For example, a temperature of 5 degrees with a 5-mph wind yields a wind chill of 5 below under the current chart and zero degrees under the old chart. At a temperature of 40 below with a 20-mph wind, the wind chill is 74 below under the current chart and 95 below under the old chart.
The current index was based on a human face model at an average height of 5 feet, but different people will feel different levels of cold from the wind, Leblang said.
"Because it involves biology and perception from person to person, there is no perfect formula," he said.
North Dakota is expected to take a huge temperature swing out of the deep freeze today, with forecasted highs of 10 degrees in Fargo and 27 to 28 degrees in Bismarck, which set a record low of 44 below Thursday morning. Fargo should warm up into the upper 20s on Tuesday, said Jim Kaiser, meteorologist at the weather service office in Grand Forks.
Fargo's low temperature Thursday of 30 below was about 25 degrees below normal, and forecasted highs of 25 to 30 early next week would be 10 to 15 degrees above normal, Kaiser said.
"When you combine those two in that short amount of time, that's fairly rare," he said.