Right on cue, the much hyped “pneumonia front” hit Chicago shortly after 5:30 p.m. Tuesday, sending the official temperature at O’Hare International Airport plummeting from 81 degrees to 66 degrees in short order.

The effect was even more dramatic closer to the lakefront.

The same story played up further north, were Milwaukee dropped nearly 25 degrees in less than 30 minutes.

These so-called “pneumonia fronts” aren’t rare, but they have a way of catching people off guard — like the30-degree nosedivethat occurred over a period of 20 minutes on April 28, 2021.

“It [a pneumonia front] indicates what happens very often in the spring time with cold air coming off of the lake so the winds shift around to the north east and that cold air off of Lake Michigan just spreads inland,” said Paul Sirvatka, professor of meteorology at the College of DuPage. “You get some really radical temperature drops.”

The record for the biggest, fastest drop is the27-degree freefall that took place in 10 minutes on April 20, 1936.

“We get this quite often in the springtime, anywhere between April and early June when the lake temperatures are still rather cold,” Sirvatka said. “It takes much longer to heat up that cold water. When we have days like today with dry conditions, and it was sunny and warm, that sense of a really big temperature difference across from the water to the land, that allows the pressure to build up over Lake Michigan. Eventually that cold air, as we get an upper-level disturbance coming southward starts to push inland … and it continues to push to the southeast, maybe as far as Joliet.”

The National Weather Service played the role of Every Mom warning Chicagoans well in advance, "Take a jacket, you might get chilly."

A lake charged cold front (sometimes called a pneumonia front b/c of dramatic temp fall with it) will blow in off the lake late Tuesday. Near the lake, temps could fall from near 80 into 50s in minutes, with a more subdued temp fall expected the farther inland the front gets. pic.twitter.com/9ahhNtxuIC

— NWS Chicago (@NWSChicago) May 15, 2023