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.^[1][2] 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.

The first documented pneumonia front was on June 13, 1909, in Michigan City, Indiana.^[3][4] The term 'pneumonia front' was coined by the National Weather Service in Milwaukee in the 1960s.^[5]

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.^[6] 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.

The following are documented occurrences of a lake-modified synoptic scale cold front or a "pneumonia front":

• Bomb cyclone
• Cold front
• Hurricane Huron
• Lake-effect snow
• November gale
• Pneumonia

• "Synoptic and Local Controls of the Lake Michigan Pneumonia Front", Cory Behnke, M.S. Thesis, University of Wisconsin - Milwaukee (2005)

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