Tornadoes most commonly form from cumulonimbus clouds, also known as thunderstorm clouds.

Here's why:

* Strong Updrafts: Cumulonimbus clouds have powerful updrafts that create the spinning column of air that is a key ingredient in tornado formation.

* Instability: These clouds are associated with unstable atmospheric conditions, where warm, moist air rises rapidly, creating the potential for powerful thunderstorms.

* Shear: Cumulonimbus clouds often develop in areas with wind shear, where winds change direction and/or speed with height. This shear helps to rotate the updraft and initiate the formation of a tornado.

While tornadoes can form from other types of clouds under certain conditions, cumulonimbus clouds are the most common source of these powerful storms.