Altitude: Mid


Like one of those tiny fish that swim in the slipstream of sharks, the accessory cloud known as velum is easily missed beside the mighty forms of its companion Cumulus congestus or Cumulonimbus clouds. It usually appears here as a thin, dark horizontal streak half-way up the side of the main cloud.

Named from the Latin for the sail of a ship, or the flap of a tent, velum are found in the vicinity of large convection clouds that have spread outwards for a time during their growth before breaking through and continuing their ascent. A strip of cloud is left behind, which lingers at the flanks of the towering mounds.

Despite their flimsy appearance, velum often hang in the sky long after the showy convection clouds that formed them have dissipated. We are sure there is a lesson hidden in there somewhere.


These are typically mid-level layers or patches of cloudlets, which form clumps or rolls. They are white or grey, and shaded on the side away from the sun. This distinguishes Altocumulus from the shade-free cloudlets of Cirrocumulus. Another’s the size of its cloudlets. These appear between the width of one and three fingers, held at arm’s length, when they’re more than 30˚ above the horizon.

The form of Altocumulus that stands out from all the others is when it is the species known as lenticularis. Rather than a layer of cloudlets, this Altocumulus is in the form of large, smooth individual clouds. Altocumulus clouds produce the most dramatic and beautiful cloudscapes, especially in the rays of a low sun.


It feels wrong to devote as much space to the rather drab and featureless Altostratus cloud as to its relative, the gloriously varied Altocumulus. Few CloudSpotters will be seen to punch the air and high-five upon adding this one to their cloud collection. Altostratus is, after all, generally considered the most boring of all the cloud types. Although, even to say that, makes it sound rather more noteworthy than it deserves.

Altostratus is a mid-level, generally featureless, grey, overcast layer – a Tupperware sky that often extends over several thousand square miles. True to its dull nature, Altostratus produces little more than a lingering drizzle or light snow. Once it is thick enough to produce more significant precipitation, it has generally developed into the Nimbostratus cloud.

The most common way for Altostratus to form is by the thickening of high Cirrostratus, when a large region of warmer air pushes against one of colder air. The warmer air, being less dense, rises gently en masse over the colder.

Generally darker than Stratus, Altostratus never produces halo phenomena, as Cirrostratus does. The sun showing through the cloud appears as if through ground glass.