Cumulonimbus incus (from the Latin incus, "anvil") is the most developed and mature stage of the cumulonimbus, recognisable by its spectacular upper anvil: a horizontal spread of dense cirrus at the cloud top that forms when the updraft strikes the tropopause (11-15 km altitude) and spreads laterally.
Structure and danger
The anvil can extend 50-100 km downwind at upper levels. Its shape and size provide valuable information: an asymmetric, extended anvil indicates strong upper-level winds; an overshooting top (a dome that penetrates through the tropopause) signals an extremely powerful updraft and risk of giant hail, tornadoes, or microbursts. The presence of striations or mammatus on the anvil underside confirms intense turbulence.
A mature cumulonimbus incus produces lightning (both cloud-to-ground and intra-cloud), intense precipitation, hail, violent downdraft winds, and in the most severe cases, tornadoes. The storm can persist for hours if wind shear is favourable, especially when it evolves into a supercell.
In Spain, the most developed cumulonimbus incus are observed in spring and autumn storms over the Mediterranean, fuelled by the contrast between cold upper-level air and the warm sea. They are easily identifiable from a distance as white "mushrooms" or "anvils" dominating the horizon. See also: weather radar.