What is a Pneumonia Front?

Figure: Graphical explanation of land and lake breezes.  (Courtesy of Vaughn Weather)

​The Great Lakes in the Midwestern United States go through many phases based on the various season that are experienced.  In this part of the country, the calendar tells what season it is, but precipitable phenomenon (i.e. snow) can definitely occur outside of winter months.  We will steer our focus on the effects the lakes have on surrounding areas in the early spring.  After the lakes have spent the duration of the winter experiencing cold air temperatures and partial freezing, and we progress into spring where air temperatures warm up, the lakes are bound to have an effect on the surrounding land.

The topic of discussion is somewhat of a slang term that came out of the Milwaukee National Weather Service office in the 1960s.  The term Pneumonia Front refers to a sharp drop of air temperature on land of 16 degrees Fahrenheit in the period of an hour in the form of a cold front.  Often times, the drop of temperature is generalized to 20-30 degree changes over the course of a few hours.  This cold front moves inland in the form of a lake breeze and modifies the air around it, thus creating much cooler temperatures inland.

A recent example occurred on March 24, 2017 where a day of strong warm air advection with high winds from the southwest led to temperatures rising to a very warm high temperature for March of 81 degrees.  The average high temperature for the O’Hare International Airport station on this date is 50 degrees, according to wunderground.com.  Right around noon, local time in Chicago, the winds turned to northeast and temperatures began to drop.  Due to the significantly cold lake paired with the northeast orientation of the winds flowing over the lake, the effects of significant cooling were felt inland.  Below is the 7-hour period of data that shows a quick temperature drop which drastically changed conditions in a short period.
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Figure: Historical Data from wunderground.com of observations on March 24, 2017 at O’Hare International Airport in Chicago, IL showing changes over a 7-hour period.

Lake breezes are not uncommon, but definitely strong during the springtime period.  Lake breezes will form due to significant temperature differentials between the land and lake, where air can be easily modified and brought inland.  Lake breezes can form and noticeably cool an environment inland in the early spring, but progressing closer towards summer, both the air temperature and the lake will warm up, and the effect of a lake breeze will not be as significant.  As summer begins and warmer temperatures are more frequent, a lake breeze can bring decent relief in temperatures during afternoon and evening hours, but is likely not to change an environmental temperature by 30 degrees.

The effect of air modification due to a breeze from a body of water are also possible in other circumstances.  For instance, Florida feels sea breeze that can modify weather conditions that usually lead to thunderstorms because of the great convergence over land due to two surrounding bodies of water being the Atlantic Ocean and Gulf of Mexico.  Any sizable body of water near land can modify air around it.  There have even been reports of breezes from rivers creating temperature fluctuations!

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© 2019 Meteorologist Jason Maska