A “pneumonia front” dropped south down Lake Michigan Tuesday PM, bringing a rapid drop in temperature, a wind shift and a sharp rise in the relative humidity. Despite the strong temperature contrast with the front, there was no rain and virtually no clouds along the front.  (top picture is a calm Lake Michigan the hour before the “pneumonia front” came through).

The term “pneumonia front” has been attributed to Milwaukee NWS meteorologist Rheinhart Harms (no relation to Blake Harms), who also coined the term “Alberta Clipper”. 

A pneumonia front is a sharp cold front that moves north to south down Lake Michigan.  It is most likely to occur from mid-April to early June, when the land is warm and the lake water is still cold.  Fronts like this have been known to drop the temperature 30 degrees in half an hour in Chicago.  It also produces a sharp wind shift to the NNE and the wind can gust to 30-40 mph. The strong wind gusts that sometimes happen with these fronts are a hazard to boaters. The lake can get choppy quickly with the passage of a pneumonia front.   Here’s what happened yesterday (Tue.):

Graph of temperature Tuesday afternoon in Milwaukee WI

At 3:15pm the temperature was 80°at Milwaukee with a west wind off the land. Twenty minutes later at 3:35pm it was 60° with a NNE wind off cold Lake Michigan. That is a 20-degree drop in 20 minutes!

Graph of temperature at the Oak St. Beach in Chicago

Here’s a graph of the temperature at the Oak St. Beach in Chicago. Tuesday PM, the temperature rose to a high of 77°. Then the temperature dropped 16° in one hour.

Waukegan, IL dropped from 81° at 4 pm to just 58° at 5 pm – a drop of 23° in one hour. They had a gust to 37 mph with the front.

At the Chicago Water Intake (Harrison-Dever Crib – about 2.75 miles east of downtown Chicago), the temperature at 5 pm was 75.9° with a south wind at just 4 mph.  When the front hit, the wind turned to the NNE and jumped to 30 mph with a peak gust of 38 mph. The relative humidity jumped from 26.0% at 5 pm to 71.4% at 6 pm.

O’Hare Airport was 81° at 5 pm.  At 6 pm they were 66° with a NNE wind at 26 mph gusting to 36 mph. At the same time, Midway Airport was 79° with a west wind at 13 mph.

At Michigan City, Indiana – the temp. at 6 pm was 71.4° with a SW wind at 5 mph and a relative humidity of 25%.  One hour later at 7 pm, the temp. was down to 57.4°, the humidity rose to 73% and the wind was north at 13 mph gusting to 23 mph.

On the Michigan shore, the front was not nearly as dramatic…The prevailing west wind was coming off the cold lake to start with…the South Haven Beach never got to 56° all day.  When the front passed, the temperature fell from 55.8° to 52.3°.

At the Muskegon Beach, the temperature “bounced”. At 6 pm, the temperature was 55.8° with a relative humidity of 72.8%. At 7 pm, the wind swung to the northeast (off the land) and the temperature went up instead of down…rising to 66.4°. The relative humidity dipped to 47.4%. Then the wind shifted slightly to the NNW, coming back off the colder water. The temperature fell to 54.5° by 8 pm and the humidity was back to 75.3%.

The water temperature at the mid-Lake Michigan buoy is still just 44°. At the Port Sheldon buoy, the water temperature is 47°. When the wind is off that cold water, it’s not going to be very warm until the lake temperature is warmer.

Finally, I pulled this pic. from archives – from 11 am on April 8, 2019.

This pic. is at the Muskegon Beach.  I think what we have here is a subtle fogbow (not a rainbow).  It’s mostly a faint white, but you may be able to make out a smidgen of color.