Natural Disasters

One of the most violent tornadoes develop from super-cell thunderstorms. A super-cell thunderstorm is a long-lived thunderstorm possessing within its structure a continuously rotating updraft of air. These storms have the greatest tendency to produce tornadoes, some of the huge wedge shape. The super-cell thunderstorm has a low-hanging, rotating layer of cloud known as a “wall cloud.” It looks somewhat like a layer of a layer cake that hangs below the broader cloud base. One side of the wall cloud is often rain-free, while the other is neighbored by dense shafts of rain. The rotating updraft of the super-cell is seen on radar as a “mesocyclone.”
The tornadoes that accompany super-cell thunderstorms are more likely to remain in contact with the ground for long periods of time—an hour or more—than other tornadoes, and are more likely to be violent, with winds exceeding 200 mph.
source