![]() Cool, sinking air from a storm cloud's downdraft spreads out across the land surface, with the leading edge called a gust front. Rising cloud motion often can be seen in the leading (outer) part of the shelf cloud, while the underside often appears turbulent and wind-torn. A shelf cloud is attached to the base of the parent cloud, which is usually a thunderstorm, but could form on any type of convective clouds. Ī shelf cloud is a low, horizontal, wedge-shaped arcus cloud. An extreme example of this phenomenon looks almost like a tornado and is known as a gustnado. A very low shelf cloud accompanied by these signs is the best indicator that a potentially violent wind squall is approaching. In a severe case there will be vortices along the edge with twisting masses of scud that may reach to the ground or be accompanied by rising dust. As the cool air lifts the warm moist air, water condenses creating a cloud which often rolls with the different winds above and below (wind shear).Ī sharp, strong gust front will cause the lowest part of the leading edge of an arcus to be ragged and lined with rising fractus clouds. This outflow undercuts warm air being drawn into the storm's updraft. Cool, sinking air from a storm cloud's downdraft spreads out across the surface with the leading edge called a gust front. Roll clouds and shelf clouds are the two types of arcus clouds, slight variations in their generation and appearance being the difference. There are no reports of injuries at this time.Arcus, Shelf, Roll, Morning Glory Clouds - CrystalinksĪn arcus cloud is a low, horizontal cloud formation associated with the leading edge of thunderstorm outflow, or occasionally with a cold front even in the absence of thunderstorms. Large spires were blown off the top of a Church and came crashing down on the sidewalk below, and tall trees toppled over. So, bottom line: there's a lot of moving air going on in a shelf cloud and you do not want to be in that tumultuous windy mix when it passes over.Ĭhicago reported severe damage from strong winds on Wednesday as the thunderstorm tore through the city. ![]() "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. An imposing, ominous shelf cloud was spotted in the skies above Chicago.
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