Shelf clouds are low, horizontal, wedge-shaped clouds that typically form at the leading edge of thunderstorms or along gust fronts where cold, dense air from a storm's downdraft meets warm, moist air ahead of it. This interaction causes the warm air to lift, condense, and create the distinctive shelf-like cloud formation, often appearing as a broad arc across the sky that may sometimes seem to rotate horizontally

. They are a type of arcus cloud and are closely associated with severe weather phenomena such as strong wind gusts, heavy rain, and sometimes hail. Shelf clouds indicate the presence of a gust front, which is the boundary where the cool air spreads out along the surface, pushing under the warmer air. This can cause turbulent conditions beneath the cloud and often precedes a sudden temperature drop and strong winds

. Shelf clouds are often mistaken for wall clouds, but they differ in location and formation: shelf clouds form at the leading edge of storms and are linked to outflow boundaries, while wall clouds are found at the rear of storms and are associated with rotating updrafts that can produce tornadoes

. In summary, shelf clouds are visually striking indicators of an approaching thunderstorm's outflow, signaling potentially strong winds and heavy precipitation, but they do not typically produce tornadoes themselves