Shelf clouds usually appear on the leading edge of a storm, while wall clouds are usually at the rear of the storm.Ī sharp, strong gust front will cause the lowest part of the leading edge of a shelf cloud to be ragged and lined with rising fractus clouds. This is likely to be a mistake, since an approaching shelf cloud appears to form a wall made of cloud. People seeing a shelf cloud may believe they have seen a wall cloud. As the lower and cooler air lifts the warm moist air, its water condenses, creating a cloud which often rolls with the different winds above and below ( wind shear). This outflow cuts under warm air being drawn into the storm's updraft. Cool, sinking air from a storm cloud's downdraft spreads out across the land surface, with the leading edge called a gust front. Rising air motion can often be seen in the leading (outer) part of the shelf cloud, while the underside can often appear as turbulent and wind-torn. Roll clouds may also arise in the absence of thunderstorms, forming along the shallow cold air currents of some sea breeze boundaries and cold fronts.Ī shelf cloud is a low, horizontal, wedge-shaped arcus cloud attached to the base of the parent cloud, which is usually a thunderstorm cumulonimbus, but could form on any type of convective clouds. They most frequently form along the leading edge or gust fronts of thunderstorms some of the most dramatic arcus formations mark the gust fronts of derecho-producing convective systems. Roll clouds and shelf clouds are the two main types of arcus clouds. This upward action is apparent in this time-lapse video of a different shelf cloud taken in Cleveland in 2016.An arcus cloud is a low, horizontal cloud formation, usually appearing as an accessory cloud to a cumulonimbus. Surprisingly, the direction these jagged clouds move is not down, but upward with the warm air draft. They gave this shelf cloud a waterfall-like appearance. These jagged features, which some people think look like fingers reaching down, are called fractus clouds. Not every shelf cloud has the ragged appearance of the cloud featured. ![]() Heading west on IL 146 from Vienna, Illinois, to Anna, Illinois. ![]() #tristatewx #ilwx Check out these photos by Donna Smith. The National Weather Service report of " Storm Data and Unusual Weather Phenomena" from August 2018 does not list a storm on August 7, and the National Oceanic and Atmospheric Administration (NOAA) does not have any storm events recorded in Union County, Illinois, in the month of August 2018.Īdditional photos of this August 7, 2018, cloud taken by Donna Smith were posted on Twitter by meteorologist Beau Dodson. (Image source: 10weather screenshot taken on Thu May 05 21:13:00 2022 UTC)Īlthough dramatic to see, this cloud did not produce a strong thunderstorm. The illustration shows the sharp contrast between the cold air of the gust front (blue), which rushes out ahead of the storm cloud, and the warm moist air (red), which is displaced and lifted by the cold gust front. This weather feature about the images includes an infographic about how a shelf cloud forms. (Image source: Facebook Screenshot taken on Thu May 05 19:23:16 2022 UTC)īenefield's images of the cloud went viral and appeared in news station weather features in places as far away as Tampa, Florida. This is how the post appeared at the time of writing:
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