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 (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 March 26, 2026 Cook County, Illinois

Pneumonia front

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