![]() ![]() 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. An arcus cloud is a low, horizontal cloud formation, usually appearing as an accessory cloud to a cumulonimbus. ![]()
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