What Caused This Beautiful But Terrifying Rotating Wall Cloud?

May 04, 2012; 1:58 AM

A wall cloud is the parent to a tornado. It's the rotating part of a thunderstorm that squeezes down a fast-spinning tornado, like an ice-skater pulling himself into a faster and faster spin.

The National Weather Service said that "rotating wall clouds usually develop before strong or violent tornadoes, by anywhere from a few minutes up to nearly an hour."