State Climatologist Jim Angel of the Illinois State Water Survey says the recent subzero temperatures were the result of several weeks of below-normal temperatures --- made even colder by a section of the Polar Vortex of frigid air that usually sits on the North Pole. But Angel says that sort of combination is rare, and the Polar Vortex itself only shows up in the Midwest every few years.
“We sometimes go from year to year without having this kind of situation come up. In fact, what’s more typical is that one year we get it, and the next year Europe gets it. and the next year Asia gets it, or something like that. So we kind of trade places with other parts of the northern hemisphere.”
Angel said the last time Illinois saw an attack of subzero temperatures like this was in January of 2009 --- when the Polar Vortex was also a factor. Angel says another common factor between now and 2009 was lots of snow on the ground, which contributes to cooler temperatures.