Cloud Type: Wall Cloud
Features: A small, localized rotating cloud beneath the base of a supercell thunderstorm's updraft
They form when: Warm, moist air rises rapidly into a thunderstorm, causing air pressure and temperatures to drop. This results in a new cloud below the base of the main thunderstorm, rotating as part of the supercell thunderstorm.
Portends: A possible tornado
Chance of spotting in a year: AverageShare this on Facebook?