Shelf clouds can certainly be alarming to see and most definitely an awesome weather phenomenon.
This type of arcus cloud, or low-level, horizontal cloud formation, is notorious for its wedge-shape and associated with approaching thunderstorms. This cloud gets its name since it often appears like a shelf in the sky.
Shelf clouds are located at the leading edge of a thunderstorm and indicate a storm downdraft or an outflow.
When it rains, cooler air is pulled along with it. This cooled air rushes down and spreads outwards as it touches the ground.
Since cold air is denser than warmer air, the warm and moist air is forced upwards. As this air travels upwards and cools, it results in the formation of a shelf cloud. They can also indicate the possibility of strong winds and intense thunderstorms.