Chicago, IL

Chicago Watch Out For the Powerful Pneumonia Front That Could Hit Us This Spring

Natalie Frank, Ph.D.

An odd weather phenomenon could have frigid air racing down the Lake Michigan shoreline, dropping temperatures a whopping 30 degrees or more in less than an hour in Chicago this spring and early summer.

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Rare pneumonia front over Lake MichiganKam/flickr

Those who live along Lake Michigan are used to bizarre weather patterns that are grouped under the general heading of “lake effects.” In fact, any time the meteorologists can’t explain something in Chicago, they just chalk it up to “lake effects.” Snow showers with thunder and lightning? Lake effect thundersnow. Colder temperatures with snow showers along the lake with discernable warmer temperatures a few blocks away and no snow? Lake effect snowstorm. Sudden downpours that quickly cause flash flooding and make residents consider building an ark? Lake effect rain. Thunderstorms accompanied by waterspouts that become tornados that move inland to devastate the blocks closest to the lake? Lake effect rainband windstorm.

Locals may be heard to blame lake effects for almost anything from psoriasis to the reason their pet is acting crazy. Some have even blamed lake effects for the Lake Michigan Bermuda Triangle where entire ships have gone missing, people have simply disappeared from their cabins, and Northwest Airlines flight 2501 which crashed into Lake Michigan never to be found.

What is a Pneumonia Front?

The pneumonia front is yet another lake effect. It has nothing to do with pneumonia and it’s not even a technical meteorological term. However, those who live along parts of Lake Michigan are often familiar with the term.

It refers to a strong northeast-to-southwest cold front racing down the shore of Lake Michigan, usually in the spring or early summer when the lake water is still very cold. It is usually accompanied by the sudden onset of strong northeast winds and a sharp temperature drop at the lake shore, with readings sometimes plummeting more than 30 degrees from the 70s into the 40s in less than an hour. Temperature changes can occur inland as well but they are far less abrupt. To be considered a pneumonia front, the temperature has to drop at least 16° in one hour.

Exactly how pneumonia fronts are produced remains a bit of a mystery. Lake Michigan’s long, deep shape and north-south orientation may be the key to their formation.

According to Paul Roebber, distinguished professor of atmospheric sciences at the University of Wisconsin-Milwaukee, the fronts aren’t really cold fronts at all. He says that the main culprit is cold air that builds over the lake in the winter months which has the tendency to remain there well into summer, even as the air over land warms significantly.

For a pneumonia front to occur, warm air must be stalled over land, while a more typical cold front approaches the lake from the north. The cold front reaches the cold air that has collected over the lake during the winter.

“You’ve got this dome of cold air piled up over the lake. It’s not doing anything. It’s just sitting there,” says Roebber. “The approaching cold front taps into the cold air supply over the lake (and) the whole thing just roars down over the coastline of Lake Michigan and comes inland.”

It all sounds logical enough, however, scientists can’t seem to predict them. “There’s some sort of trigger,” says Roebber, who worked with a graduate student trying to create a model of the phenomenon. “Unfortunately”, he says, “We weren’t able to figure out why it kicks off when it does.”

A Pneumonia Front Hits Chicago

On April 27th just over a month ago, Chicago experienced a classic pneumonia front that sent people running for shelter or at least for sweatshirts. At 6 pm, the temperature registered at the Chicago Water Intake at about 82°. Less than an hour later, the temperature had dropped to around 49°. That’s a drop of 33° in less than an hour. At 6 pm, the wind was SSW at 21.5 mph. One hour later, it was N at 17 mph, gusting to 24 mph. At 6 pm, the relative humidity was 45.1%. One hour later it was 100%. The dew point fell from 58.8° to 48.0°.

Even more extreme instances of these fronts have occurred in Chicago. For example, on March 24, 2017, Chicago’s temperature along the lakefront plummeted 20 degrees in five minutes and 29 degrees in one hour.

So, if you plan on visiting the beach this summer, no matter how hot it is, take a jacket just in case. Pneumonia fronts may not hit all that often, but when the do they can turn a sweltering early summer day into one that feel a lot more like late October.

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