A shelf cloud is a horizontal wedge-shaped cloud which hangs low in the air, and is usually associated with thunderstorms. The shelf cloud will attached itself to the underside of a parent cloud.
The rising cloud motion looks severe as it rises but it is the lower cloud that appears turbulent and wind torn.
The cool sinking air from the downdraft of the shelf cloud spreads across the surface, and the leading edge is known as the gust front.
When the warm air is drawn into the storm’s updraft and it mixes with the cooler air, the moist air condenses and this creates a rolling effect people see which usually develops into a heavy rainstorm.
A shelf cloud is also known as a “Tsunami” cloud due to what looks like a rotating motion as the cloud draws nearer to the land.