# Pneumonia front

A pneumonia front, also known as a lake-modified synoptic scale cold front, is a rare meteorological phenomenon observed in coastal areas of Lake Michigan, in the United States, most commonly between the months of April to July. The phenomenon, according to the National Weather Service, consists of a cold front that accelerates southerly down Lake Michigan, rapidly dropping temperatures in coastal areas of the lake by 16 °F (8.9 °C) or greater. These fronts are often followed by fog clouds, and, less commonly, rain.
Pneumonia fronts are most often observed when there is a large temperature difference between the cold lake waters and the warmer air over land, sometimes as much as 35–40 °F (19–22 °C). These conditions are present in spring and early summer. 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.
Pneumonia fronts occur most frequently on Lake Michigan's southwestern shore, in cities such as Chicago, Milwaukee, and Kenosha. However, they are also commonly observed elsewhere on the lakeshore, including cities such as Michigan City, Benton Harbor, Green Bay, and Traverse City.

## History

The first documented pneumonia front was on June 13, 1909, in Michigan City, Indiana. The term 'pneumonia front' was coined by the National Weather Service in Milwaukee in the 1960s.

## Causes

Pneumonia fronts occur when a cold front (generally of synoptic scale), typically approaching from the north or northeast, encounters a mass of cold, dense air that has persisted over Lake Michigan, typically a remnant of winter conditions. The air mass fuels the cold front, allowing it to grow in density and momentum as it travels south along the lake. This movement displaces the warmer, less dense air over land, leading to an abrupt and significant temperature drop. Lake Michigan's elongated north-south shape and two long north-south bays (Green Bay and Grand Traverse Bay) allow for pneumonia fronts to pick up great speed and change air temperatures relatively far inland.

## Documented occurrences

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 | Southeastern Wisconsin, Northeastern Illinois |