incus (inc)

A Latin word meaning anvil, which in the Atlas Internacional de Nubes of the WMO designates a supplementary feature associated with Cumulonimbus. The aforementioned feature is easily identifiable. It describes the typical anvil shape adopted by the upper part of a cumulonimbus, with its smooth, fibrous, or striated outlines, opening out fan-like - in this latter case - into countless frayed-looking cloud filaments. This very characteristic element is a consequence of the top of the storm cloud reaching the tropopause. The presence of that natural lid (thermal inversion) forces that upper part to spread out horizontally, which shapes the long, flattened structure characterizing the anvil. It is often asymmetric, extending further to one side than the other, which allows one to deduce from where the dominant wind aloft is blowing, at the highest atmospheric level reached by the storm, which usually coincides with its movement over land.