"So what ends up happening then is as the air hits the ground, it then has to rise back up, and as it rises back up it cools the air and forms this cloud in this arc shape and that is the leading edge or gust front of a thunderstorm." That cooler air sinks compared to the warmer air around it, thus feeding the downdraft and increasing wind speed.Įventually, that column of cold air hits Earth's surface where it "spreads out into a circular shape." Sort of like a ripple, when you drop a pebble in a pond, Wysocki said. "As that rain falls, it evaporates, cooling the surrounding air," Wysocki said. It starts with a rain shaft that occurs when a downdraft of cool air funnels a large amount of rain into a small region. But something very specific has to take place to form that giant, ominous arc across the sky. Put simply, they form when cool and warm air mix. Shelf clouds are just one of the many types of clouds involved with a thunderstorm, Wysocki said. Shelf clouds form an ominous looking arc across the sky and are a warning sign to seek shelter. "You really should take shelter because once the cloud gets over you the winds pick up from like 0 mph to 45-70 mph in no time whatsoever," Wysocki told Insider.Ī video shared on Twitter shows a timelapse of the cloud passing over Chicago. If you see one of these apocalyptic-looking shelf clouds coming toward you, it's important to seek shelter, Wysocki said. So pretty much anywhere from 50 degrees north or south of the equator," Wysocki told Insider. "If you can get yourself a strong thunderstorm or line of thunderstorms will develop. In fact, shelf clouds are quite common and can develop just about anywhere there's a thunderstorm brewing, said Mark Wysocki, a senior lecturer at Cornell's Earth and Atmospheric Sciences Department and New York State climatologist. Video shows the cloud moving through the city, bringing strong winds up to 60 mph with it.Īt first glance, the shelf cloud that passed through the skies of Chicago on Wednesday morning looked like something out of "Independence Day." But don't worry - this ominous-looking formation isn't signaling the end of the world. The cloud is a sign of severe thunderstorms and you should take shelter if one is coming your way. Wall clouds can be as large as 2 miles in diameter and are located near the strongest updraft within the storm.An imposing, ominous shelf cloud was spotted in the skies above Chicago. In some case when a supercell has heavy precipitation, known as a high precipitation supercell (HP supercell), wall clouds can form on the east-southeast side of a storm. Wall clouds are precursor to funnel clouds and tornadoes. Wall clouds are most often associated with supercell thunderstorms. They can be found on the backside of a storm system, typically to the south-southwest of precipitation free area. Wall clouds most likely have vertical rotation associated with them. Wall clouds are isolated lowering clouds attached to the rain free base. Notice the dirt being blown across the lake? Shows the cold air pushing out ahead of the storm lifting the warm moist air to help produce the shelf cloud.Īll too often shelf clouds get reported as wall clouds. Here is another shelf cloud this time at night passing over a lake near Jamestown, ND. Notice the dust that blows over the lake ahead of the shelf cloud. Shelf cloud passing over a lake near Jamestown, ND June 5th, 2018.
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