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 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. 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, 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 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":
DateLocation
June 13, 1909Michigan City, Indiana
May 21, 1938Will County, Illinois
June 6, 1967Cook County, Illinois
June 30, 1975Lake County, Indiana
July 1, 1983Kenosha County, Wisconsin
July 17, 2003Lake Michigan
May 20, 2008Eastern Wisconsin
May 21, 2008Eastern Wisconsin
May 26, 2008Cook and Lake Counties, Illinois; Lake and Porter counties, Indiana
April 25, 2009Cook and Lake Counties, Illinois
June 1, 2009Cook and Lake Counties, Illinois
April 21, 2010Cook, Lake and Will Counties, Illinois; Lake and Porter Counties, Indiana
May 13, 2011Cook, Lake and Will Counties, Illinois; Lake and Porter Counties, Indiana
March 15, 2012Cook and Lake Counties, Illinois; Lake County, Indiana; Milwaukee, Racine and Kenosha Counties, Wisconsin
April 13, 2014Cook, Lake and Will Counties, Illinois; Lake and Porter Counties, Indiana
May 27, 2014Cook, Lake and Dupage Counties, Illinois; Milwaukee, Racine and Kenosha Counties, Wisconsin
September 29, 2014Cook, Lake, Dupage and Will Counties, Illinois; Milwaukee, Racine and Kenosha Counties, Wisconsin
March 24, 2017Cook, Lake, Dupage and Will Counties, Illinois; Milwaukee, Racine and Kenosha Counties, Wisconsin
April 10, 2017Waukesha, Milwaukee, Racine and Kenosha Counties, Wisconsin
June 1, 2018Cook County, Illinois; Porter County, Indiana
May 3, 2020Northeast Illinois; Milwaukee, Racine, and Kenosha Counties, Wisconsin
April 27, 2021Cook County, Illinois; Lake, Porter, and LaPorte Counties, Indiana
May 10, 2022Emmet, Charlevoix, Grand Traverse, and Leelanau Counties, Michigan
June 17, 2022Milwaukee County, Wisconsin
April 14, 2023Grand Traverse County, Michigan
May 16, 2023Southeastern Wisconsin, Northeastern Illinois