Despite a high temperature of 39 degrees Wednesday, the Chicago area Thursday started out frigid, with temperatures in the low teens in most parts -- and wind chills made the air feel even colder.
But how exactly is "wind chill" defined, and how cold does it feel?
As of 5:30 a.m. Thursday, readings in Chicago clocked in around 10 degrees, with wind chill readings between -5 and -15, NBC 5 Meteorologist Alicia Roman said.
By 9:45 a.m. temperatures at Midway International Airport were at 16 degrees, with a wind speed of 28 miles-per-hour, and a wind chill of zero, the National Weather Service said.
Such cold temperatures and low wind chills come after a wind advisory brought snow squall warnings and wind gusts as high as 60 mph for some. According to Roman, gusts as high as 35 mph were expected to persist Thursday.
Defining Chicago's "wind chill"
A wind chill is the "measure of how cold the air feels on your skin," NBC 5 Meteorologist Kevin Jeanes said.
Local
"How cold it is, plus how strong the winds are," Jeanes said.
According to the NWS, this is the formula used to calculate wind chill:
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35.74 + 0.6215T – 35.75 (V^0.16) + 0.4275T(V^0.16) = Wind chill
In this equation, the “T” variable represents the air temperature, while the “V” variable represents the wind speed.
The formula also accounts for heat transfer theory and skin tissue resistance, according to the NWS.
For example, an air temperature of 10 degrees, and a wind speed of 25 miles per hour will create a wind chill of approximately minus-11 degrees Fahrenheit.
"Our bodies naturally give off heat," Jeanes explained. "The air just around our body warms up, but when when the wind picks up, that removes the heat. So our body heat that it naturally gives off isn't warming us anymore. It isn't keeping us insulated."
According to officials, the formula takes into account a measurement of the wind speed as measured at the average height of a human face, which is five feet. It also takes into account that a typical anemometer, a device used to measure wind speed, is placed 33 feet in the air.
In order for wind chill to be calculated, the air temperature must be at or below 40 degrees Fahrenheit, the NWS said.
Finally, the formula assumes that measurements are taken at night, with no sunlight influencing the “feels-like” temperature.
Chicago's weekend forecast
Thursday will remain cold, the NBC 5 Storm Team said, with highs in the mid-20s and "feels-like" temperatures below zero, thanks to gusty winds.
Friday, milder temperatures were expected to move in, Roman said, with highs in the low 30s. Even warmer temperatures were on tap for the weekend, with low 40s Saturday, and a high of 50 degrees on Sunday.
"A nice increase in temperatures as we head into the next couple of days," Roman said.
