Velum is Latin for ship’s sail. This name is a tad misleading for it is a thin horizontal patch of cloud, rather than one that hangs downwards from a tall mast and catches the wind. You’ll spot a velum cloud either just above, or around the sides of, a group of large convection clouds, such as Cumulus congestus or Cumulonimbus, like the one shown here. Observing the clouds from a distance, the velum usually looks like a white or grey strip that can be separate from, or mixed in with, the convection clouds.

Velum is often spread over a very large area. If the cloud layer was not already around before the convection clouds grew up through it, the velum can sometimes have resulted from the tops of Cumulus clouds spreading out upon reaching a layer of stable air above. When, later, more powerful Cumulus clouds finally burst through the stable air layer, the velum remains loitering at their flanks for some time.

Velum is an accessory cloud that turns up in the same sort of places as the pileus cloud. Though they both form in a similar way, they have quite different natures. Unfortunately for velum, it is usually the less attractive and more ponderous of the pair.