As darkness falls on anything but the brightest of nights, those clouds still awake will appear dark in the night sky as they are cast in the Earth’s shadow. But one rather mysterious cloud refuses to go to bed with the others. ‘Noctilucent’ clouds form so high up in the atmosphere that sun catches them even in the dead of night.

Most clouds form in the lower region of the atmosphere, from the ground to around 8-10 miles up, known as the troposphere. Noctilucent clouds, however, form at altitudes of between 30 and 50 miles, where temperatures can be as low as -125°C. This is a region called the mesosphere – the one above the stratosphere – and is extremely dry, making the appearance of the Noctilucent cloud’s ice crystals somewhat of a mystery. The clouds have a bluish white colour, with a rippled or undulating appearance, as can be seen in the handsome specimen above. They are most readily observed in the higher-latitude regions of the world – those above 50° – within a month or so of mid-summer.

There have been an increasing number and extent of Noctilucent cloud observations over the last 100 years. Some scientists think that this is an indication of man’s contribution to global warning. Increases in the amounts of greenhouse gases in the stratosphere will not only warm the atmosphere below, but will also cool those regions above. Colder temperatures in the mesosphere would be expected to encourage the formation of Noctilucent clouds. Could the increasing observation of this insomniac cloud be the writing across the night sky of our role in changing the climate?

Some clouds are the type to make a fuss – they demand the attention of everyone below with torrents of rain and crashing thunder and lightning. But not the Cirrus. This is a high, delicate, ice-crystal cloud, whose name means a lock of hair in Latin, and it is never the sort to cause a commotion.

Aren’t the quieter souls always the ones worth listening to? This is certainly the case with the Cirrus. The cloud’s beautiful locks were described by hippie songwriter, Joni Mitchell, as ‘rows and floes of angel hair’. These can sometimes look neatly combed, like the Cirrus ‘fibratus’ shown above, and sometimes messy and dishevelled (known as Cirrus ‘intortus’). Whatever the formation, when the streaks or fibres are spreading and stretching and joining together to form a layer that covers the sky, the Cirrus contains a message about the weather approaching.

In the mid-latitude regions of the world, such spreading and thickening high clouds can be among the first indications of a ‘warm front’ arriving. This is an advancing mass of warmer, often moister, air that can mean steady rain in a day or so. Cloudspotters in these regions should keep an eye out for the behaviour of Cirrus. They’re not so much floes of angel hair, as tufted whiskers of a wise man’s beard. He’s a genial old fellow, who’ll tell of the weather in store. But he speaks in a whisper, and only those who care to pay attention will ever be able to hear.

When dressing up for the evening, there is always the danger of overdoing things. Tonight, Washington’s Mount Rainer – clearly eager to make an impression – has decided that the classic mountain look of a single ‘cap cloud’ perched upon her summit is not dramatic enough. She’s gone and made the fashion faux-pas of opting for a pair of them.

Cap clouds, like their lenticularis cousins (see Cloud of the Month for April ’05), are known as ‘orographic clouds’. This means that they form when air cools as it rises to pass over an obstruction such a mountain. If the airstream is moist enough, such cooling can cause droplets to form and appear as a cloud – it is much like breath becoming visible as it mixes with the air on a cold morning. Two cap clouds can appear, one above the other, when the airstream consists of layers of moist air, separated by a drier one.

Mount Rainer’s sense of restraint will surely have returned by tomorrow. And in time, like the rest of us, she’ll doubtless muse upon the sartorial choices of days gone by, and gently blush in the honeyed rays of the setting sun.

Few of us like to be the first to take the plunge. We hold back at the water’s edge, waiting for others to dive in. Clouds are much the same, as is demonstrated by the phenomenon of a ‘fallstreak hole’.

Layers of high cloud, such as cirrocumulus or the high altocumulus, shown above, are often composed of water that is much colder than 0degC but hasn’t frozen into ice crystals. When water is in the form of tiny droplets suspended in the air, it can behave rather differently from that in an ice tray in the freezer. It can stubbornly refuse to freeze, remaining as ‘supercooled’ liquid at temperatures of –10, –15, –20degC… None of the droplets want to be the first to freeze, and they tentatively wait as liquid, until some brave souls decide to make their move.

For reasons that are none too clear, a particular region of supercooled cloud can throw caution to the wind and decide to freeze into ice crystals that grow and fall below. A hole is left behind, and this spreads outwards as neighbouring droplets are swept up in the excitement and start freezing too.

No sooner have some droplets made the change, than they are all joining in. How appropriate, that the trail of falling crystals can look like a bird taking flight.

Most clouds are happy to coexist within the lower region of the atmosphere, known as the ‘troposphere’. Nacreous clouds, however, like to be a little different. They form in the rarefied heights of the stratosphere, and look down on the troposphere – what an obvious place it is for a cloud to exist.

Also known as mother-of-pearl clouds, Nacreous usually appear between 10 and 20 miles up, in the higher-latitude parts of the world. They are the most colourful-looking clouds, exhibiting refined and delicate pastel hues*, just as one would expect from such lofty individuals.

It seems these clouds are caused by waves of air penetrating the stratosphere, due to the effect of mountain ranges all the way down on the surface. They only tend to be spotted just before dawn and after dusk, when the lower sky is in shadow and the stratosphere is still lit up.

‘Let those common clouds jostle for people’s attention during the day,’ they no doubt whisper to each other.‘We would never lower ourselves to compete on such a overcrowded stage.’

The cirrostratus is surely the most understated of all the cloud types. It is a thin, milky veil, spread out high across the heavens, and it is often almost transparent.With such subtle shades, it is a cloud that is ignored by most members of the public.

Of course, cloudspotters are not most members of the public. They know to keep a keen eye to the skies when this delicate layer of ice crystals first appears. Though it arrives without fanfare, the cirrostratus (along with some of the other high clouds) often comes with the most beautiful of optical surprises up its icy sleeve. These result from the diffraction of sunlight through the cloud’s array of tiny ice-crystal prisms.

Our favourite of these ‘halo phenomena’ is being demonstrated by the delicate layer of cirrustratos above. It is officially known as a ‘circumzenithal arc,’ for it forms high above the Sun, with its axis at the zenith. The name is, of course, most inappropriate for such a beautiful effect. It should really be called a ‘cloud smile’.

This is just one of twenty five or more arcs and halos that ice-crystal clouds can cause at different angles and orientations to the Sun or Moon. They have names like ‘parhelic circle’, ‘anthelion’, ‘120¾ parhelion’, ‘Tricker arc’, ‘Parry arc’ and ‘Hastings arc.’ Some only form in the dry, icy air around the Poles.

Most people may never notice even the most common of the halo phenomena, but does the cirrostratus care? No, it just smiles down silently from the heavens, content in the knowledge that the colours of its own arc are both brighter and purer than those of the oh-so-familiar rainbow.

A ‘tuba’ is when a cloud extends a finger towards Earth. Young children often yearn to reach up and touch the soft mounds of a fair-weather Cumulus. So who can blame a cloud for wanting to know what the ground feels like?

It does have to get worked up into a vigorous spin, however, before it can summon the energy to do so. In and around the intense downdrafts associated with large Cumulonimbus and Cumulus congestus clouds, a vortex of swirling air can develop – like that of water draining down a plughole. The air in the center drops in pressure as a result of all the spinning, which can make it cool enough for some of its water vapour to combine into droplets.

Like the slender pair of swirling digits forming above, tuba are columns or cones of cloud extending down the middle of these vortices. They do not always end up reaching the surface, however. More often than not, the cloud loses heart before touching terra firma. Maybe it knows that, in another guise – on a different day – it will return as fog or mist, only to hug the ground until it is heartily sick of it.

Those unfamiliar with the habits of our fluffy friends will claim that it never snows on warm, clear, sunny days. Of course, a cloudspotter knows that they are talking complete and utter nonsense.

In fact, high clouds often produce snowfalls, which evaporate in the warmer, drier air below, well before reaching the ground. When this is the case, they exhibit supplementary features, known as ‘virga’. These dangling tendrils of ice crystals (or droplets) often have a wavy appearance as they fall through the varying wind currents.

Sometimes virga, like the handsome specimen shown above, can hang in the air after the clouds that created them have given up the ghost. Other times, they can dangle below their cloud bodies like a school of quivering jellyfish (see the cloud gallery image). Luckily for glider pilots, hang gliders and little birds, virga do not sting.

Wizened mariners pay careful heed to dramatic displays of the cirrocumulus cloud, which they refered to as a mackerel sky. It can be a precursor of storms at sea, warning them to batten the hatches and stow the mainsal.

More often than not, however, this high cloud appears in less extensive patches – usually in the company of its relatives the cirrus and cirrostratus.

The term mackerel sky is often erroneously used to refer to this cloud’s lower cousin, the altocumulus, whose elements, or ‘cloudlets’, are larger. Cloudspotters can distinguish the two by a simple rule of thumb. If the cloudlets directly above appear smaller than the width of a finger held at arm’s length then they can be confident they are looking at a cirrocumulus.

Care should be taken with the use of this rule, however, for the cirrocumulus is a beautiful and uncommon cloud, to which cloudspotters wouldn’t want to give the impression that they are making a rude gesture.

If the average person were to close their eyes and think of a cloud, chances are they would picture this little fellow, for the cumulus feels like the most generic of all the cloud types. It is a fair-weather specimen, born on the invisible thermals of air that rise from the ground on a sunny day.

As any six-year-old child will tell you, cumulus are the most comfortable of all the clouds to sit on. They are like gentle tufts of cotton wool on which dreamers can kick back and shoot the breeze. This fact was not missed by the Italian painters of the Baroque period who rarely depicted a saint or an angel without a comfy cumulus cloud for them to recline on.

Whilst there are many rare and fancy cloud types vying for our attention, cloudspotters should never forget to appreciate the light-hearted cumulus. He may be common, but how many other clouds can claim to be the sofas of the saints?

With her pendulous udders, this mother’s cooking up a storm. She’s known as the mammatus cloud (mamma is the Latin for ‘breast’) and you can find her hanging out in the company of any one of a number of clouds. She is at her most impressive, however, when she is wed to the mighty cumulonimbus thundercloud.

Mammatus can form on the underside of the enormous anvil that often spreads out at the top of a cumulonimbus. They appear when the top of the anvil cools by losing some of its heat into the atmosphere above. Parts of it sink into warmer, moister air below, which forms the cloud droplets that make up the mammatus.

When they are plump and full, like the buxom lass above, mammatus tell you that there is a major thunderstorm nearby – one that is big enough to send most people running for mummy.

Enlightened meteorologists will tell you that the ‘pileus’ is not so much a cloud as it is a cloud haircut. It is a supercooled-droplet bouffant, worn exclusively by the fashionable cumulus family. If you keep your eyes to the sky, every once in a while you’ll notice a proud cumulonimbus thunder cloud or his younger brother, the cumulus congestus, wearing this dashing haircut.

The pileus can form when a towering cumulus, like the suave model shown above, is pushing a moist layer of air upwards as he grows. If conditions are right, this elevation can be enough for the moist layer to cool and form into droplets, appearing as a smooth cloud over the cumulus congestus’ puffy summit.

But cloud fashion is more transient than most and, as the cumulus cloud continues to grow, his head soon pokes through the top of the pileus cloud. Within a few short minutes, it has slipped to his shoulders, leaving the poor fellow with all the indignity of a lopsided wig.

This proud cumulonimbus is the King of Clouds. Stretching around ten kilometres into the sky, he harnesses the power of ten Hiroshima-sized bombs. It is he – the tallest of all clouds – that we ultimately thank for the phrase ‘to be on cloud nine’, for he was originally classified as number nine in the list of cloud types published in the International Cloud Atlas of 1896. This proud fellow has taken on a beautiful anvil shape (known as cumulonimbus incus), as the ice crystals of his upper reaches spread out at the tropopause – the thermal lid to the region of atmosphere in which clouds mostly form. But don’t let his handsome chiselled features deceive you. He is a ferocious beast who’ll terrify young ladies with his thunder, lightning and hailstones.

A couple of contrails* blowing an air kiss to their friends in the Cloud Appreciation Society. Born at 40,000ft, these cheeky clouds aren’t in a hurry to go anywhere while they spread into gentle feathered cirrus layers called cirrostratus. If you watch their locks grow long in the wind, you can be sure these lovely ladies share a little secret – that rain will probably come in a day or two.

(* Contrails are the long straight man-made clouds that form behind high-altitude aircraft)