What defines a heat wave?

CBS News

14-05-2025

Minnesota is in the midst of a sizzling stretch of summer-like weather. Something unique for this time of year.
A packed Nokomis Beach, soaked in sunshine and steaming close to 90 degrees, would make you think it's a mid-summer day. But it's just mid-May.
Several beachgoers WCCO talked with described the day as hot but bearable thanks to a lake breeze.
Still, temperatures that feel like late July for a four-day stretch had them calling it a heat wave.
What defines a heat wave?
"I think it's in the eye of the beholder," said Ken Blumenfeld, a senior climatologist with the Minnesota Department of Natural Resources.
According to the National Weather Service (NWS), a heat wave is a "period of abnormally hot weather generally lasting more than two days. Heat waves can occur with or without high humidity."
There was no specific temperature listed, but the NWS added that a heat wave often involves dangerously hot weather that could harm people.
Blumenfeld looks at heat waves from a statistical perspective. "In climatology it's just extreme warmth relative to the time of year," he said.
May 11 to May 14, the daily high temperature hit or was forecasted to land around 90 degrees. The average high for this time of year is around 67 to 68 degrees according to the National Oceanic and Atmospheric Administration (NOAA). That means the Twin Cities is running least 20 degrees warmer than usual.
"If it's supposed to be like 60 right now but right now, we're having all this heat, it's gotta be a heat wave," said Ambuskah Little Voice while at the beach with friends.
Blumenfeld also understands that public health workers have their own definition.
"[They] might consider a heat wave to be something that could cause human bodies to have problems keeping itself cool naturally," said Blumenfeld.
That's usually in peak summer when high humidity combines with high temperatures, creating dangerous conditions leading to heat stroke and exhaustion.
"Heat during late May and even late September and October tends to be really dry compared to the kind of heat you get in July and August," said Blumenfeld.
If nights are comfortably cool, is it still a heat wave?
"That's where it gets a little iffy," Blumenfeld said.
A heat wave in mid-summer could have nights in the 70s while the air remains thick. In May, the daily low temperatures have settled in the 60s and even 50s, creating a difference of 25 to 30 degrees, even larger during this current heat wave. The air also pleasantly dry.
Could you have a heat wave in the winter?
Beachgoers said both yes and no. Heat isn't a word typically associated with the coldest time of year, but there's no denying that it still arrives in surprising fashion.
"We can have heat waves in winter if you're looking at it statistically," Blumenfeld said.
Consider late December 2023, the Twin Cities hit 55 degrees on Christmas Eve, then 54 degrees on Christmas day along with relatively high humidity. That's extreme warmth for the holiday season, heating up the debate on how to label these steamy stretches.
"Defining a heat wave really kind of depends on who's asking," Blumenfeld said.
The most recent extreme heat for the month of May came in 2018. That's when much of Minnesota hit 100 degrees on Memorial Day.