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Altitude: Multiple

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Cumulonimbus

No cloud collection is complete without the big one, the boss, the King or Queen of Clouds: Cumulonimbus. This enormous storm cloud, which often spreads out at the top to resemble a blacksmith’s anvil, can form individually or co-ordinate with neighbours to form multicell and supercell storms.

CloudSpotters should note that the cloud’s anvil shape is visible only when spotting it from many miles away. It develops from a very large Cumulus, known as Cumulus congestus, and has officially grown into a Cumulonimbus only once its summit has ‘glaciated’, meaning that its particles have changed from droplets to ice crystals. This is revealed by the cloud top developing softer edges. Below a Cumulonimbus, you will see just its dark, ragged underside, which (being so low) appears to cover the whole sky. In this case, you can distinguish it from similar-looking Nimbostratus by the fact that its precipitation is in the form of sudden, heavy showers and the fact that it produces thunder, lightning and often hail.

Cumulonimbus also gives rise to a whole range of distinctive cloud features, such as incus, mamma, pileus, velum, shelf clouds, roll clouds and tuba.

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Nimbostratus

When people claim clouds are depressing, they’re often thinking of Nimbostratus. This thick, grey, featureless rain cloud gives all the other ones a bad name. Not only does it block much of the sun’s rays, casting everything in a dim, miserable light, it also produces rain – and lots of it.

Nimbostratus is one of only two cloud types that are defined as always producing rain or other precipitation. The other is the Cumulonimbus storm cloud. From below, both appear as dark and ominous skies, but they can be distinguished by the nature of their precipitation. Compared with the brief heavy showers from individual Cumulonimbus clouds, the precipitation from Nimbostratus is much more steady, and can last for many hours.

Surreptitiously and without fanfare is how the Nimbostratus arrives. It generally results from the thickening and lowering of Altostratus. Since one cloud leads to the other, the point of distinction between Alto- and Nimbostratus is rather academic. But when the cloud is dark, and the rain moderate to heavy, and its diffused base shows darker ragged patches of Stratus fractus, which is also known as pannus, you can confidently add Nimbostratus to your cloud collection. Your photograph of this less-than-handsome cloud type is unlikely to ever win you any awards. This is the cloud that gives all the others a bad name.

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