Category: Cloud of the Month

Every month, we choose one of our favourite photographs from the Cloud Gallery to become our Cloud of the Month.

Anti-Crepuscular Rays (November 06)

Satan’s Shadows

Whilst they may not know the name, most cloudspotters will have noticed ‘crepuscular rays’. These are the radiating beams of sunlight that appear to burst from behind a Cumulus cloud, or shine down through a hole in a Stratocumulus (e.g. see Angela Cragg’s image in the photo gallery). These appear when the path of sunlight is made visible by atmospheric water droplets too scarce to appear as cloud, but plentiful enough to noticeably scatter the light. The shadows of the clouds give edges to the sunbeams. The perspective, as the rays approach, makes them appear to radiate outwards. Crepuscular rays are a common and beautiful sight.

Much more rare are ‘anti-crepuscular rays’, like the fine example shown above. These appear to emanate not from the sun, but from the point on the horizon directly opposite it. Cloudspotters will, therefore, only see anti-crepuscular rays when they stand with the sun directly behind them. The shadows of individual clouds behind the cloudspotter are cast onto the cloud layer in front. In the photo above, this is a high layer of ice crystals, called a Cirrostratus. As the shadows and rays recede to the horizon, perspective makes them appear to converge at the point opposite the sun even though they are in fact parallel.

Some call crepuscular rays ‘God’s fingers’. Does this mean anti-crepuscular rays are the ‘digits of the Devil’? We sincerely hope not.


Anti-crepuscular rays spotted over Bisbee, Arizona, US by John Annesley (Member 14,212).

Stratocumulus (October 06)

(Click image to enlarge) (Image © Antonio Bonasera)



Stratocumulus clouds look rather like someone couldn’t find the ‘Off’ switch on the candyfloss machine. They generally have low, puffy cumulus-like mounds, which are joined together into a more-or-less continuous layer. When the layer is extensive and thick, it can completely block out the sky above.

The shifting terrain of the Stratocumulus results in a great variety of light and dark shades. Sometimes, openings appear through it, which can result in ‘crepuscular rays’ of sunlight that look like torch beams shining down to the ground. At others times, as in the fine example shown above, such an opening can appear as a window up to the sky above.

What a glorious sight it is to peer up through a cavernous hole in the clouds to the firmament above. Suddenly, there is a sense of scale to the ocean of air above us. Suddenly, our atmosphere has depth and drama. Looking up through it, we become more aware of our place down here on the surface. A opening through a Stratocumulus cloud is a window on the sky and a window on the soul.

Altocumulus (September ’06)

(Click image to enlarge) (Image © Carolyne Locher)



The mid-level cloud known as ‘Altocumulus’ often takes the form of smooth rounded masses that look like a tray of bread rolls. But occasionally, as with the fine specimen shown above, this cloud can have a more ragged appearance with sharp turrets sprouting from its cloudlets. It is then known as Altocumulus ‘castellanus’, since its tops resemble the crenellations of castle battlements. When these airborne defenses proliferate across the morning sky, they suggest meteorological skirmishes to come.

Altocumulus castellanus can appear when the air is ‘unstable’ in the middle of the troposphere (this is the lower part of our atmosphere – in which weather happens). This means that the air temperatures up at that middle level will tend to encourage the vertical growth of clouds. As the sun rises through the day, it can cause low Cumulus to develop on thermals of air floating from the warmed ground. If these Cumulus grow tall enough to reach the unstable air in the mid troposphere, they can continue to build into enormous, fierce Cumulonimbus storm clouds, releasing barrages of hail, thunder and lightning.

With the distant storms still rumbling at day’s end, how obvious it seems in retropect that all those Altocumulus castles could never have have remained at peace.

Cloud of the Month for August 2006

August 2006

‘Surf’s Up’ at 24,000ft over Georgia, US.

The classic 1964 surfing documentary, The Endless Summer, followed the adventures of three surfers traveling from Malibu to Ghana, via Nigeria, Australia, New Zealand, Tahiti, and Hawaii in search of the perfect wave. Cloudspotters can experience their own perfect wave too, without ever having to leave home – all they need is a lot of patience. The only down side to the ultimate aerial breaker, is that it has the particularly un-hip name of the ‘Kelvin-Helmholtz cloud’.

The breaking waveforms of ‘Kelv-Helmz’ (as they aren’t known) are the result of shearing winds up at cloud level. A particular type of turbulence can develop in a layer of Cirrus cloud, which happens to form below an inversion* between air currents of differing speeds and/or directions. Sea waves break as their bases are slowed down upon reaching shallow water and their crests surge ahead. Cloud waves break in the same way: when their crests are pushed ahead of their troughs by the difference in air currents.

Though spotter-dudes won’t have to cross the world to see this formation, those in search of the perfect Kelvin-Helmholtz will have to wait a while. This most beautiful and transient of formations may appear over most regions of the world but it only ever does so on the rarest of occasions.

*An inversion is a region where air temperatures change with altitude in such a way as to act as an invisible ‘ceiling’ that stops clouds from rising through it.

Photograph © Brian Wong

July 2006

“Boo, Man”

When cloudspotting mountaineers and hill walkers are out on the Brocken mountain, a peak in the Herz range of Northern Germany, they often find themselves climbing through a bank of stratus cloud to emerge into the sunlight above. At this point, they can sometimes come face to face with a ‘Brocken Spectre’. This fantastical apparition is of a ghost-like shadow surrounded by rainbow colours and wearing very flared trousers.

Despite its flower-power appearance, a Brocken Spectre like the one spotted here by Dave Newton in Cumbria, England is not a ghost from the sixties. It is an optical effect that appears as sunlight, coming from directly behind the cloudspotter, falls onto a layer of cloud droplets. This situation tends to occur only on elevated ground, since the cloudspotter needs to be looking down onto the cloud layer, for the Sun to be shining from directly behind him, along his line of vision. The cloudspotter’s shadow, hugely distorted by the effects of perspective, appears in the middle of a halo of colours, which are officially known as a ‘glory’. The exact optics of this effect is still only partially understood.

Of course, cloudspotters don’t have to climb the Brocken mountain to see a Brocken Spectre – any mountain can serve the purpose. Nor is the sight available only to mountain-climbing cloudspotters. The coloured rings of a glory can also be seen around an aircraft’s shadow cast onto a cloud layer below. Is this light effect known as a ‘Boeing Spectre’? No, of course it’s not.

Glory spotted from Grisedale Pike, Cumbria, England by Dave Newton.

Moby Dick, Supercell-style

Cloudspotters who find themselves in the path of a ferocious ‘multicell’ or ‘supercell’ storm system, may have the unsettling experience of looking up into a ‘whale’s mouth’. This is the name storm chasers give to the huge, dark void that can appear in the region below and ahead of the storm.

The whale’s mouth forms as a result of very strong ‘downdraughts’ of cool air, which are associated with such severe storm systems. On reaching the ground, this cool air spreads out like a tide beneath the advancing storm. As it does so, it lifts the warmer, lighter air at ground-level, which then cools enough for its moisture to condense into cloud. This cloud is the roof of the whale’s gaping mouth, while the blubbery-looking rim along the front of the spreading air is the whale’s top lip*.

The storms that give rise to whales’ mouths are particularly fierce. Cloudspotters should avoid staring for too long into the jaws of these mighty beasts, or they may just end up as dinner.

*N.B. This is officially known as a ‘shelf cloud’, not a ‘top-lip-of-a-whale cloud’

Arcus cloud associated with a supercell storm spotted over Nebraska, US by Mike Hollingshead (Member 1,666).

Noctilucent (May ’06)

(Click image to enlarge) (Image © Chris Mabbott)


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?

Cirrus (April ’06)

06apr-cirrus.jpg(Click image to enlarge) (Image © Dorian Creber)


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.

Cap Cloud (March ’06)

06-mar-cap.jpg(Click image to enlarge) (Image © Skip Probst, 2/26/03)


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.

Fallstreak Holes (February ’06)

(Click image to enlarge) (Image © Blair Heald)


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.

Nacreous (January ’06)

(Click image to enlarge) (Image © Gill and Peter Smith)


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.’

Cirrostratus (December ’05)

(Click image to enlarge) (Image © Paul Warren)


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.

Tuba (November ’05)

(Click image to enlarge) (Image © Clay Craig)


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.

Virga (October ’05)

(Click image to enlarge) (Image © Mike Rubin)


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.

Cirrocumulus (September ’05)

sept_cc_high.jpg(Click image to enlarge) (Image © Chris Dolley)


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.

Cumulus (July ’05)

(Click image to enlarge) (Image © Laura Billings)


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?

Mamma (June ’05)

(Click image to enlarge) (Image © Bill Lowe)


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.

Pileus (May ’05)

(Click image to enlarge) (Image © Justin Moore)



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.

Lenticularis (April ’05)

(Click image to enlarge) (Image © John Lamb)



The Altocumulus lenticularis is known affectionately by glider pilots as a ‘lennie’. It is an orographic cloud formation, which appears in mountainous regions as air, forced upwards over the range, cools and sheds its moisture. If it so happens that the wind takes on a wave-like oscillation in the lee of the mountain, this handsome formation can form at the crest of each wave, resulting in a line of huge shimmering white discs.

Lennies look like enormous flying saucers. Have aliens parked beside New Zealand’s Mount Cook for a cup of hot chocolate before the long ride home through the Milky Way? No, they’ve just come to remind us how clouds can be vehicles of the imagination, hovering silently in the rarefied air between crest and crag.

Stratus (March ’05)

(Click image to enlarge) (Image © David Fuller)



After the cold, grey months of January and February, many in Britain will feel that they have seen quite enough of the overcast skies of the stratus. But they should not be too hasty in complaining about this layer cloud.

Remember that the stratus is unique amongst the cloud family: she is the only one to come knocking at our door in the form of swirling fogs and mists. Who can turn away such a mysterious, silent seductress? The expanse of the heavens feels all the more satisfying when it is revealed, little-by-little, as she teasingly removes her veils.

Cumulonimbus (February ’05)

(Click image to enlarge) (Image © Wendy Rogers)



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.

Contrails (January ’05)

(Click image to enlarge)



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)

Altostratus (August ’05)

Every Cloud Has its Day

Cloudspotters tend to think of the altostratus as a boring cloud. Indeed, it is a featureless, mid-level layer, which tends to give the sky a washed-out, overcast appearance. When it is thick, it is little more than a grey blanket that leads to prolonged light rain or snow. Much of the time, there seems little to recommend the altostratus.

But every cloud has its day – or rather, its time of day – and for this cloud, it is at sunrise and sunset. As you can see from the beautiful example above, that is when the altostratus dons her fancy clothes to paint the sky red. Her gentle undulations become visible for all to admire, her delicate surface awash with ruby hues.

She may be plain by day but, for fleeting moments at dawn and day’s end, the altostratus has a beauty to match any one of her more flamboyant cloud cousins.

Image © Irene, East Queensland, Australia