What type of cloud does a tornado typically form on the forward edge of?

A tornado typically forms on the forward edge of a cumulonimbus cloud. Cumulonimbus clouds are associated with thunderstorms and severe weather phenomena, providing the necessary conditions for tornado development. These towering, dense clouds are capable of producing significant updrafts, which are crucial for forming the rotating columns of air that constitute a tornado.

This type of cloud exhibits strong vertical growth and can reach high altitudes, creating an environment conducive to the instability and wind shear required for tornado formation. The presence of a strong thunderstorm updraft can lead to the development of a mesocyclone, a rotating updraft that can ultimately spawn a tornado.

In contrast, the other cloud types mentioned do not have the characteristics necessary for tornado formation. Altocumulus clouds are middle-level clouds typically associated with fair weather or light precipitation, cirrus clouds are high-altitude clouds indicating fair weather but are not involved in severe storms, and stratus clouds are low-level clouds that often lead to overcast skies with light rain, lacking the violent conditions conducive to tornado formation.