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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