It’s a late spring afternoon along the southwest coast of Lake Michigan, and people from Chicago to Milwaukee are soaking up the sun after another long winter in the Upper Midwest. The temperature climbs to 80 degrees Fahrenheit and continues to rise, unusually. The promise of an early summer draws gardeners, cyclists and beachgoers. Then it happens.
A wall of cold lake air hits the coast and moves rapidly inland. People are scrambling to find shelter, or at least a sweatshirt. The temperature drops by 20, 30, or even 40 degrees in a few minutes. Winter is coming back in force.
This is not the opening scene of a horror movie. It is a pneumonia front, a rare meteorological phenomenon that is unique to this corner of the Great Lakes and still poorly understood by scientists. The event has nothing to do with actual pneumonia: an entire region suddenly experiences chills.
Lake Michigan, like the other Great Lakes, is large enough to act as an inland sea, generating its own weather patterns, including wind. Anyone who has lived by an ocean has experienced a strong sea breeze, but pneumonia fronts are something else. “It’s much more drastic,” says Tim Halbach, an alert coordination meteorologist for the Milwaukee / Sullivan office of the National Weather Service. “This is an extremely brutal change.
The term “pneumonia front” is not officially recognized by the National Weather Service. Halbach says it was invented by “someone smart” at the Milwaukee office in the 1960s to explain the unusual local phenomenon. The accepted definition is a drop of at least 16 degrees Fahrenheit in an hour, although many fronts of pneumonia are more extreme. On May 3, 2020, Milwaukee recorded a 24-degree drop in less than 15 minutes, including a 14-degree drop in just five minutes.
Fronts of pneumonia can feel like another world. Although sometimes heralded by fog or a low plateau cloud, they can also occur on a clear day and may or may not be accompanied by wind. “It’s not like a hurricane or a wall of water,” says Halbach. “It’s hard [to visualize] because it’s like cold air. Remember on Game of thrones, when an entire landscape froze in front of the series’ undead villain? “It would be like a front of pneumonia, when the Night King arrives and everything changes drastically,” says Halbach.
While the National Weather Service officially classifies pneumonia fronts as “lake-reinforced cold fronts,” meteorologist Tom Wachs, known for his enthusiastic coverage of extreme weather events on FOX6 station in Milwaukee, says they are also called “backdoor cold fronts”.
“This means that the front is coming from a northeast direction,” says Wachs. “Usually cold fronts come from the west or the northwest. If the backdoor cold front is strong enough, dropping 16 degrees or more in an hour or less, then it is considered a “pneumonia front.”
Wachs adds that while the models have improved, pneumonia fronts can be difficult to predict. “Fronts of pneumonia are generally more difficult to predict because [they’re] on a smaller scale than, say, a big explosion of Canadian air that hits several states, ”he says. “For pneumonia fronts, the impact is only a few counties.”
Paul Roebber, a distinguished professor of atmospheric sciences at the University of Wisconsin-Milwaukee, said pneumonia fronts should not be viewed as cold fronts at all. “It’s physically called a density current, and it’s very different from what we would call a synoptic current. [or large-] cold front on the scale, ”he said.
“When you have a strong thunderstorm and you feel what is called a flow limit, it’s a very cold breeze coming in front of it, it’s a density current,” he says. “It’s basically cold air that comes out of the storm’s rain pit, cooled as it falls, hits the ground and spreads sideways, long before the storm. The pneumonia front is physically similar, but it is not generated at all the same as a thunderstorm exit limit. “
Exactly how pneumonia fronts are generated remains a mystery. Due to their localized and relatively rare nature (they usually only occur once or twice a year), they are difficult to study. The long, deep shape of Lake Michigan and its north-south orientation seems to be the key to their creation. The cold air that collects over the lake in winter tends to stay there until summer, even when the air over land warms.
Setting up a pneumonia front requires warm air stationed above land, with a more typical cold front approaching the lake from the north and meeting the cold air that has accumulated above. Lake.
“You have this dome of cold air piled up over the lake. It does not matter. It’s just sitting there, ”Roebber says. The approaching cold front “draws on the supply of cold air over the lake (and) it all roars over the Lake Michigan coast and goes inland.”
Due to the larger-scale currents and the momentum of the pneumonia front – imagine a snowball rolling down the slope – the wall of cold air swings towards the southwest coast, although the southern end of the lake, including a small part of Indiana, sometimes affected. “If that cold air supply hadn’t been there, if you were filling Lake Michigan or covering it with a big pool blanket or something, then this cold front would have kind of meandered like it was. habit, ”Roebber adds. “This is because the cold front is drawing on the dome of cold air above the lake which makes it much faster and much more dramatic.”
But every encounter between a cold front and the cold dome of Lake Michigan air doesn’t turn into a pneumonia front. “There’s sort of a trigger,” says Roebber, who worked with a student several years ago on a model of the elusive phenomenon. Unfortunately, he says, “we couldn’t figure out why it starts when it does.”
Atmospheric scientists are also unsure of what impact, if any, climate change may have on the frequency and intensity of pneumonia fronts. “They tend to happen more often in late spring than in summer. The reason is that the lake temperatures are colder, so you have a better supply of cold air, ”says Roebber. “So with climate change, if we expect the Great Lakes to be generally warmer because the air is warmer and the ice cover in winter is less, they would likely be less frequent. “
Roebber warns, however, that while climate change can push the temperature to higher temperatures, it can also create greater variability and more extreme oscillations. It could actually lead to more unique weather phenomenon. In the last days of May, the Milwaukee area experienced two fronts of pneumonia in four days. Keep this sweatshirt handy.