A pneumonia front is a rare meteorological phenomenon observed on the western shoreline of Lake Michigan during spring and early summer. These fronts are defined as lake-modified synoptic scale cold fronts that result in one-hour temperature drops of 16 °F (8.9 °C) or greater. The term was created by the National Weather Service in Milwaukee in the 1960s.
Pneumonia fronts do not necessarily have to be synoptic, or large scale, cold fronts. They are most common between the months of April to July when the temperature difference between the cold lake waters and the warmer air over land can be as much as 35–40 °F (19–22 °C). Under weak prevailing winds, a density current can often develop in the form of a lake breeze that moves from that water to the adjacent shoreline and several miles inland. This "lake-breeze cold front" can drop temperature in places like Chicago, Milwaukee, Benton Harbor, and Green Bay significantly as they cross the area.
The following are documented occurrences of a lake-modified synoptic scale cold front or a "pneumonia front".
| Date | Location |
|---|---|
| June 13, 1909 | Michigan City, Indiana |
| May 21, 1938 | Will County, Illinois |
| June 6, 1967 | Cook County, Illinois |
| June 30, 1975 | Lake County, Indiana |
| July 1, 1983 | Kenosha County, Wisconsin |
| July 17, 2003 | Lake Michigan |
| May 20, 2008 | Eastern Wisconsin (Lake Michigan) |
| May 21, 2008 | Eastern Wisconsin (Lake Michigan) |
| May 26, 2008 | Cook and Lake Counties, Illinois; Lake and Porter counties, Indiana |
| April 25, 2009 | Cook and Lake Counties, Illinois |
| June 1, 2009 | Cook and Lake Counties, Illinois |
| April 21, 2010 | Cook, Lake and Will Counties, Illinois; Lake and Porter Counties, Indiana |
| May 13, 2011 | Cook, Lake and Will Counties, Illinois; Lake and Porter Counties, Indiana |
| March 15, 2012 | Cook and Lake Counties, Illinois; Lake County, Indiana; Milwaukee, Racine and Kenosha Counties, Wisconsin |
| April 13, 2014 | Cook, Lake and Will Counties, Illinois; Lake and Porter Counties, Indiana |
| May 27, 2014 | Cook, Lake and Dupage Counties, Illinois; Milwaukee, Racine and Kenosha Counties, Wisconsin |
| September 29, 2014 | Cook, Lake, Dupage and Will Counties, Illinois; Milwaukee, Racine and Kenosha Counties, Wisconsin |
| March 24, 2017 | Cook, Lake, Dupage and Will Counties, Illinois; Milwaukee, Racine and Kenosha Counties, Wisconsin |
| April 10, 2017 | Waukesha, Milwaukee, Racine and Kenosha Counties, Wisconsin |
| June 1, 2018 | Cook County, Illinois; Porter County, Indiana |
| May 3, 2020 | Northeast Illinois; Milwaukee, Racine, and Kenosha Counties, Wisconsin |
| April 27, 2021 | Cook County, Illinois; Lake, Porter, and LaPorte Counties, Indiana |
| May 10, 2022 | Emmet, Charlevoix, Grand Traverse, and Leelanau Counties, Michigan |
| June 17, 2022 | Milwaukee County, Wisconsin |
| April 14, 2023 | Grand Traverse County, Michigan |
| May 16, 2023 | southern and southwest Wisconsin |