Family: Main Cloud Types


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.


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.


These are high patches or layers of cloudlets that appear tiny, on account of their distance from the ground.

The best way to distinguish Cirrocumulus from lower Altocumulus is the size of the cloudlets, as well as the area of the sky covered by the layer as a whole. Being such a distance from the ground (often in the region of six miles (10km)), the cloudlets of Cirrocumulus appear so small that you often have to look carefully to notice the cloud’s grainy texture. For the cloud to be Cirrocumulus, these cloudlets must appear no larger than the width of a finger, held at arm’s length, when they are more than 30˚ above the horizon.

Composed generally of very supercooled water droplets that are on the point of freezing into ice crystals, Cirrocumulus is the rarest of the ten main cloud types. When you see a layer of small cloudlets it is more typically the mid-level equivalent, Altocumulus. When Cirrocumulus is present, it doesn’t last long. The cloud soon freezes into the other high clouds, Cirrus or Cirrostratus. As a result, you’ll often find Cirrocumulus accompanied by these other high-cloud cousins.


Cirrostratus is a subtle, understated cloud that can easily go unnoticed – except, that is, by CloudSpotters, keen to complete all ten of the main cloud types in their collection.

A delicate layer of ice crystals, often spread over vast areas of the sky, Cirrostratus can appear as no more than a light, milky whitening of the blue. It can sometimes look stripy or fibrous (the species known as fibratus) but more commonly lacks any variation in tone.

It also distinguishes itself as the best of the high clouds at producing the coloured arcs, rings and points of light known as halo phenomena. These can appear as the sunlight shining through a layer of Cirrostratus is refracted and reflected by the ice crystals, each of which can behave like a tiny prism. Halo phenomena certainly don’t always appear but, when they do, they can exhibit beautiful rainbow colours, and are a sure way to distinguish Cirrostratus from Altostratus, which, being lower and consisting (at least partially) of droplets, doesn’t produce them.


The most ethereal looking of all the main types, Cirrus clouds are also the highest – composed entirely of ice crystals. These typically fall through the high winds of the upper troposphere to appear as delicate, celestial brush strokes, known as ‘fallstreaks’.

Cirrus often look like white locks of hair (from which the Latin name is derived). Cirrus clouds thickening and spreading across the blue can be the first signs of moisture developing at high altitudes, indicating the start of a common cloud progression that leads to Nimbostratus and rain or snow in a day or so.

Apart from when it is very thick, and known as Cirrus spissatus, Cirrus can sometimes refract and reflect the sunlight to produce coloured arcs and rings known as halo phenomena.


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.


The lowest-forming of all the cloud types, Stratus can give you a strangely claustrophobic feeling, even though you’re outside. It is a featureless, grey overcast layer, which lurks around with its base generally no higher than 1,500ft from the ground. This is much lower than its equally charisma-free cousin, the Altostratus cloud. Stratus can sometimes obscure the tops of tall buildings. When a cloud like this forms so low that it is at ground level, it is known as fog or mist. Since fog can sometimes form in a different way from airborne Stratus, it has a page of its own.

One way that Stratus forms is when moist air cools as it blows over a relatively cold surface, such as a cold sea or land covered in thawing snow (‘advection fog’ is formed in the same way when winds are gentler). Another is when air cools as it rises. This might be as it blows up the lower slopes of a mountainside or as warmer air slowly rides up over a region of colder (denser) air. Finally, Stratus can appear when fog, which has formed overnight, lifts from the ground as it is stirred by a freshening wind.


The most widespread of all cloud types, Stratocumulus is a low layer or patch of cloud that has a well-defined, clumpy base. The patches are either joined up, or have gaps in between. When the sky is overcast, and the cloud base appears to be low, with tones from white to dark grey, cloudspotters can confidently add Stratocumulus to their cloud collections.

High Stratocumulus that have cloudlets with gaps in between – a variety known as perlucidus – can be confused with the mid-level cloud Altocumulus. But Stratocumulus is usually less orderly in appearance and its cloudlets are bigger (appearing larger than the width of three fingers, held at arm’s length, when they are more than 30˚ above the horizon). Due to its sun-blocking tendencies, Stratocumulus may not be the most popular cloud, but it is one of the most varied.


If you’ve never spotted a Cumulus cloud, then you need to get out more. This has to be one of the easiest types to add to your cloud collection (which explains why it only earns one star). Cumulus clouds are the cotton-wool puffs, with flat bases, that drift lazily across the sky on a sunny day. Generally forming a few hours after daybreak, they tend to dissipate before sundown, for they form on thermals – invisible columns of air rising from the ground as it is warmed by the sun.

Most forms of Cumulus produce no rain or snow, and so are known as fair-weather clouds. But in unstable air, their bright, crisp cauliflower mounds can build upwards so that they develop from the small humilis species, through mediocris to the largest form, Cumulus congestus. With its ominous, shadowy base, this cloud is no longer fair-weather. Congestus can produce brief but sizeable showers, and can keep growing into fierce Cumulonimbus storm clouds.

The little ones, by contrast, are scary only when they take the form of David Hasselhoff.