Pneumonia front

A pneumonia front is a rare meteorological phenomenon observed in coastal areas of Lake Michigan during spring and early summer. These fronts are defined as lake-modified synoptic scale cold fronts that result in one-hour temperature drops of 16 °F (8.9 °C) or greater.^[1] While pneumonia fronts occur primarily in the states of Illinois and Wisconsin, pneumonia fronts are also observed in Indiana and Michigan.

Pneumonia fronts do not necessarily have to be synoptic, or large scale, cold fronts. They are most common between the months of April to July when the temperature difference between the cold lake waters and the warmer air over land can be as much as 35–40 °F (19–22 °C). 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. This "lake-breeze cold front" can drop temperature in places like Chicago, Milwaukee, Benton Harbor, Green Bay, and Traverse City significantly as they cross the area.