What is the icnus?

Tornadoes often form from a type of cumulonimbus cloud called a?

Cumulonimbus incus, which has an obvious anvil top. Only very strong ones, called supercells, spawn tornadoes.Cumulonimbus icnus is the most likely type of storm cloud to produce a tornado. These cumulonimbus clouds are often supercells.Tornadoes can form from any cumulonimbus cloud, but they usually form from Cumulonimbus incus (heaped rain cloud with anvil) with a rotating updraft called a mesocyclone. The type of storm this rotating cumulonimbus cloud brings is a strong thunderstorm called a supercell, the thunderstorm most likely to form a tornadocumulonimbus clouds