CHICAGO – A rapid drop in temperature is expected Tuesday in Chicago and Milwaukee as a “pneumonia front” races down Lake Michigan, changing a summerlike day to one more typical of the beginning of April in a matter of minutes.
Despite the name, you will not catch pneumonia from this type of cold front. The name simply refers to the dramatic temperature plunge for cities along the western shores of Lake Michigan as the front charges south through the lake’s chilly waters, pushing that lake-chilled air into eastern portions of Illinois and Wisconsin.
The current water temperatures in Lake Michigan are generally in the upper 40s and lower 50s. Temperatures Tuesday in Chicago and Milwaukee are expected to climb to 75 to 80 degrees – but not for long…
“Biggest story Tuesday will be backdoor cold front progged to accelerate down the lake, possibly bringing a dramatic end to warm temps with near instantaneous temp drop of 10-20 (degrees) along the lakefront late Tuesday afternoon and evening,” meteorologists with the National Weather Service office in Chicago wrote in a forecast discussion. “Will likely see some gusty north-northeast winds immediately behind (the) front, further adding to the chill of the crashing temperatures.”
WHAT IS A BACKDOOR COLD FRONT?
The strong difference in density and pressure between the warmer air mass ahead of the cold front and the cooler air mass behind it causes these so-called pneumonia fronts to progress rapidly southward down the lake and shoreline areas, according to the NWS Chicago. Warm air is less dense than cold air, which causes it to rise and allows the colder, denser air to replace it at the surface as the front moves in.
Pneumonia fronts are common in the Great Lakes during the spring when water temperatures are still cold after reaching seasonal lows in the winter.
CAN BIG TEMPERATURE SWINGS MAKE YOU SICK?
An extreme case occurred on March 24, 2017, when the temperature in Chicago dropped 29 degrees in one hour, falling from 77 at 5:15 p.m. to 48 at 6:15 p.m. That came up just 1 degree shy of Chicago’s most extreme temperature drop of 30 degrees in one hour set on March 26, 1908, when temperatures fell from 71 to 41 between 2 p.m. and 3 p.m.
According to a 2005 study by Cory Behnke from the University of Wisconsin-Milwaukee, a pneumonia front – a term first used in the 1960s by the NWS office in Milwaukee – specifically refers to at least a 16-degree temperature drop in one hour, particularly along the western shores of Lake Michigan.