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.