Weather Explainers

'Pneumonia Front' Hits Milwaukee, Chicago, Plunging Temperatures in Minutes

A sharp drop in temperature was experienced from Milwaukee to the Chicago area Monday, courtesy of a "pneumonia front" that pushed through the region.

This type cold front does not cause pneumonia; rather, it describes a dramatic temperature drop in areas near western Lake Michigan.

Monday afternoon, a cold front moved southward through southeastern Wisconsin and into northern Illinois by the evening. The colder temperatures were enhanced by the chilly Lake Michigan water temperatures, where temperatures are generally lower than 42 degrees.

(MAPS: Check the Current Temperatures)

The strong density and pressure differences between the air mass behind the cold front and the air ahead of it allow these cold fronts to progress rapidly southward down Lake Michigan and the shoreline areas, according to the National Weather Service office in Chicago.

When a cold front moves quickly southward and is enhanced by chilly water temperatures of Lake Michigan, temperatures can rapidly drop near the shoreline.

This enhancement from the lake water is common in the spring, but not to the degree experienced on Monday.

A pneumonia front specifically refers to a drop in air temperature of at least 16 degrees in one hour – specifically near western Lake Michigan – and was first used by the Milwaukee NWS office in the 1960s. According to a 2005 study by Cory Behnke, the magnitude of the temperature drops is strongly dependent on daytime heating and the surface water temperatures of Lake Michigan.

(MAPS: 10-Day Forecast)

In this case, temperatures had reached the middle to upper 70s Sunday and again Monday ahead of the front. As the cold front surged southward, temperatures dropped more than 20 degrees in just a few minutes and gusty winds quickly shifted from the southwest to the northeast.

On Monday, several cities saw temperatures drop by more than 20 degrees in less than an hour, including Milwaukee, Kenosha and Chicago.

Milwaukee saw temperatures rapidly plunge 30 degrees from 12:52 p.m. (CDT) to 1:52 p.m., when the temperature went from 75 degrees to 45 degrees.

A drop of 22 degrees in one hour was recorded both in Kenosha, Wisconsin, and Waukegan, Illinois, north of Chicago. At 1:53 p.m. in Kenosha, the temperature was 71 degrees; an hour later, the temperature was only 49 degrees. At 2:55 p.m. in Waukegan, the temperature was 67 degrees, but by 3:55 p.m., it had plummeted to 45 degrees.

(MORE: When The Last Freeze of the Season Typically Occurs)

This system also brought thunderstorms to the region, with hail up to an inch in diameter reported in northern Illinois.

In addition to this latest occurrence, Chicago recently experienced a big temperature plunge on March 24, when the air temperature dropped 20 degrees in five minutes near the lake front and 29 degrees in one hour.

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