A low, horizontal cloud along the leading edge of a thunderstorm’s outflow is called a shelf cloud.
Thunderstorm downdrafts spread cool, dense air outward at the surface. The advancing edge, called a gust front or outflow boundary, lifts warmer, moist environmental air. That rising air condenses into a broad, usually attached cloud that can appear as a dramatic shelf or wedge ahead of the storm.
Shelf clouds are outflow features, not usually the storm’s main updraft. They may bring sudden wind shifts, falling temperatures, blowing dust, and strong gusts even when heavy rain has not yet arrived. On radar, the associated outflow boundary can appear as a fine line.
Shelf clouds are commonly confused with wall clouds and roll clouds. A wall cloud is a localized lowering beneath a storm’s cloud base and is associated with inflow and strong updrafts. A roll cloud is generally detached from the storm base, whereas a shelf cloud typically remains connected to the parent storm. Both shelf and roll clouds, however, can occur near an outflow boundary.