A tornado often extends from Earth’s surface to the base of a cumulonimbus cloud. Cumulonimbus is the towering thundercloud genus associated with thunderstorms, lightning, heavy precipitation, hail, and sometimes tornadoes. Its abbreviation is Cb.
The cloud base supplies the visible connection from which a tornado or condensation funnel may hang, while the rotating wind column continues downward to the ground. Many tornadoes form within severe thunderstorms, especially supercells, whose organized rotating updrafts are called mesocyclones.
Cumulonimbus clouds are not automatically tornadic. They need suitable moisture, instability, lifting, and wind shear, and only a small fraction produce tornadoes. The other options are different cloud genera or features: altocumulus is a mid-level cloud, stratus is a generally layered cloud, and cirrus is a high, icy cloud.