SOUTH MISSISSIPPI (WLOX) - of thunderstorm clouds from Wednesday... some saying it looked like a mothership in the south Mississippi sky. Well, it certainly wasn't anything extra-terrestrial.
In fact, it's called a shelf cloud. Shelf clouds are usually wedge-shaped and are formed by thunderstorm air currents. They typically signify an approaching storm or line of storms.
As rain falls from the base of a thunderstorm cloud, it pulls down an air current called a downdraft. Downdrafts are sinking air currents that pool cool air along the ground underneath a storm cloud. The leading edge of that cool air beneath the storm cloud is known as a gust front (also called an outflow boundary).
It's this gust front that causes the shelf cloud formation.
The gust front pushes out away from a thunderstorm, cutting underneath warm air already in place at the surface. As that warm moist air is forced upward by the gust front, it condenses and forms the shelf cloud.
If you witness a shelf cloud approaching, you may actually feel a brief gust of wind... that's the gust front.
Gust fronts are commonly kicked out ahead of dissipating thunderstorms. Meteorologists can detect gust fronts with weather radars, usually appearing as a thin line of weak reflectivity. And as a gust front collides with another gust front, it can actually aid in new thunderstorm development.
Shelf clouds are often confused for wall clouds. Both are low-hanging clouds. But, wall clouds are associated with the lowering of the base of a thunderstorm cloud and usually precede tornado development. While shelf clouds can sometimes signal a strong wind gust approaching, they aren't directly associated with tornadoes.