Cumulonimbus incus (Cb inc; Latin for heap-raincloud anvil) is a typical cumulonimbus cloud which has expanded to the tropopause due to powerful updrafts and convection and which has fully developed an anvil shape. The tops of cumulonimbus incus clouds usually reach 10500m in altitude but may grow to 18000m, particularly near the equator; this is dependent upon the altitude of the tropopause at the location of cloud formation. As updrafts do not extend past the tropopause, the cloud is thrust upward until this point, where it is forced to spread radially into a characteristic "anvil" composed of ice crystals. Mammatus clouds may develop on the underside of this anvil. On rare occasions, part of the anvil may protrude into the stratosphere due to the updraft associated with anvil formation