a low-level, wedge-shaped cloud attached to the thunderstorm. It forms above the gust front as warm air ahead of the storm rides over the cool outflow from the thunderstorm.

Long, wedge-shaped clouds associated with the gust front. Shelf clouds indicate the downdraft or outflow of a thunderstorm.

A low-level horizontal accessory cloud that frequently appears to be wedge-shaped as it approaches. It is usually attached to the thunderstorm base and forms along the gust front. The leading edge of the shelf is often smooth and at times layered or terraced. It is most often seen along the leading edge of an approaching line of thunderstorms, accompanied by gusty straight winds as it passes overhead and followed by precipitation. The underside is concave upward, turbulent, boiling, or wind-torn. Tornadoes rarely occur with the shelf cloud.

a low, horizontal wedge-shaped arcus cloud, associated with a thunderstorm gust front (or occasionally with a cold front, even in the absence of thunderstorms). Unlike the roll cloud, the shelf cloud is attached to the base of the parent cloud above it (usually a thunderstorm). Rising cloud motion often can be seen in the leading (outer) part of the shelf cloud, while the underside often appears turbulent, boiling, and wind-torn.

A low-level, horizontal, wedge-shaped arcus cloud associated with a convective storm gust front (or occasionally a cold front). The shelf cloud is attached to the convective storm's cloud base. Rising motion can be seen in the leading (outer) part of the shelf cloud, while the underside appears turbulent and tattered. Compare roll cloud.