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.

Diamond Dust (March 09)

Cloudspotter Collision Alert

Collections of water droplets that appear as cloud can form at many different altitudes. They often appear down here at ground level, when they are known as fog. But the ice crystals that make up higher clouds are far less common at low altitudes. Few have been lucky enough to see the beautiful ice-crystal equivalent to fog, shown in March’s Cloud of the Month, which goes by the appealing name of ‘diamond dust’.

Diamond dust is rarely thick enough to reduce visibility much. Its presence is then only revealed by the way the ice crystals glint in the light as they tumble through the air, producing a magical sparkling effect. It is sometimes also known as ‘ice fog’, though this term usually refers to a thicker ice-crystal fog. For classic diamond dust to appear, temperatures need to be lower than –20˚C. When the ice crystals grow slowly in such temperatures, they take the form of tiny, regularly shaped hexagonal prisms. As well as glittering, these can also produce some of the most dramatic and beautiful ‘halo phenomena’.

This is the name for the rings, arcs and bands of colour that appear when sunlight is refracted and reflected as it passes through a cloud’s ice crystals. If diamond dust is made of particularly regular crystals, as is often the case in polar regions, it produces better halo phenomena than the high clouds do. But cloudspotters needn’t travel all the way to the Poles to see these light effects. The onset of the skiing season brings diamond-dust observing opportunities much closer to home, for the same tiny ice-crystal prisms often form downwind of snow machines at ski resorts. Just remember to take cover when they do so on a black run. Fellow cloudspotters will be so distracted by the beautiful halo effects that they are bound to be completely out of control.
 

Diamond dust spotted Apollo, Pennsylvania, US by Grover Schrayer (Member 14,209).

February 09

Please Form an Orderly Line

Nature can exhibit surprising displays of order in seemingly chaotic situations: lining up grains of sand into regular, parallel ripples below the ebb and flow of the surf; coordinating bees to form geometric honeycombs as they construct their hives; arranging fair-weather Cumulus clouds into regimented lines, known as ‘cloud streets’, like those in February’s Cloud of The Month.

Orderliness in clouds can seem more surprising than when it is found elsewhere in the natural world. Are clouds not the most disorderly things around, shaped as they are by the unruly motions of the atmosphere? So irregular is their movement that when physicists came up with Chaos Theory in the 1970s, some had been inspired by gazing up at the clouds.

While the particular curls and wisps of these ghosts of the atmosphere are indeed chaotic, the clouds themselves occasionally exhibit quite regimented formations. This tension between order and disorder is surely one reason we find them so intriguing to watch. Some cloudspotters love them for their stubborn refusal to abide by rules. Others, who tend to like to have their CD collections arranged in alphabetical order for easy access, find neat formations such as cloud streets a blessed relief.

 

Cumulus cloud streets spotted over Amsterdam, Netherlands by Carlos Herrero.

Crepuscular Rays (Jan 09)

A New Dawn

While they may not know the name, most cloudspotters will be more than familiar with crepuscular rays. These optical effects, caused by the shadows of clouds, are the familiar beams of sunlight that appear to shine down from a hole in a layer of Stratocumulus or burst out in dramatic fashion from behind a Cumulus, as is the case in January’s The Cloud of the Month, above.

Crepuscular rays 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. Like fingers through a torch beam in a smoky room, the shadows caused by the clouds provide edges to the path of the light, making the rays appear as sunbeams. These rays seem to radiate outwards from behind the cloud, in spite of the fact that they are actually almost parallel. Like like railway tracks approaching from the horizon, the effect is due to perspective.

The rays that form when the Sun is high in the sky often look like they are pointing downwards, especially when they break through a hole in a cloud layer (see here, for example). But when the Sun is low on the horizon, they invariably point upwards. They look like the fingers of a hand outstretched, waving to the day that is departing, or welcoming the one that’s just arrived.

Crepuscular rays spotted over Puesta, Mexico by Fabian Gonzalez.

Valley Fog (December 08)

During the longer autumn and winter nights, the ground has more time cool than at other times of the year. This is particularly the case when it is a starry night, devoid of cloud cover, for the ground loses (or ‘radiates’) its heat out to space much more readily without any blanket of clouds to keep it in. When such a situation coincides with no more than the slightest breeze of moist air, conditions are ideal for the formation of ‘radiation fog’.

This is when the low air is cooled by the plunging ground temperatures to such an extent that some of its moisture condenses into droplets. It is a ground-level Stratus cloud that consists of chilled air, and so has a tendency to sink into and run down valleys, in much the same way that rainwater runs down them as rivers. If fog gathers at low ground like this, it is known also as ‘valley fog’.

And that is what they call the beautiful fog shown above, flooding down the course of the Potomac River early in the morning, near Washington, DC, in the US. Forming further upriver, towards the the Blue Ridge Mountains, the low-level cloud creeps silently through the night to find its level in the lower reaches of the river valley. It is little wonder, therefore, that this city district, past which the river flows, has the nebulous name of Foggy Bottom.

Stratus from a 300-ft aerostat balloon over the Potomac River, Washington DC, USA. © Curt Westergard.

Cloud Shadows (November 08)

Shadow Play

Have you ever noticed cloud-shaped shadows creeping over the ground on a sunny day? These can be cast by drifting Cumulus, as they are swept overhead in the breeze. Such shadows are a pleasing connection between sky and land. They might even catch the attention of those poor individuals who, blind to the beauty of the sky, walk around staring at their feet.

But the most dramatic and spectacular cloud shadows don’t actually reach the ground at all. They are the ones cast from one part of the sky to another.

You can’t beat cloud shadows to give a sense of perspective and scale to the atmosphere. Take these majestic ones cast at sunrise from the line of Cumulus towers on the horizon in November’s Cloud of the Month. Here the shadows aren’t cast down onto the ground, but onto a layer of Altocumulus, through which the Cumulus summits have grown.

The effect is rather like when you use your hands to make shadow shapes on the wall. But we are very pleased to say that the cloud version is considerably more beautiful and, in contrast to our own, it doesn’t inevitably end up producing the shape of bunnies, flying doves, or some mutant hybrid of the two.


 Cloud shadows cast from Stratocumulus up to Altostratus spotted over Murcia, Spain by Steen Hastrup-Stilling (MEmber 9,009).

Contrails (October 08)

Love Them or Hate Them?

Should we appreciate contrails or not? That is the question vexing cloudspotters the world over.

Contrails are those lines of cloud that you can see forming behind high-altitude aircraft. These man-made clouds only appear when the air up at cruising altitude is cold enough and moist enough to cause the water varpour in the engine’s exhaust to freeze into ice crystals and remain in the sky in the aircraft’s wake. So they are clouds – but ones that man has added to the natural cloudscapes.

But are these man-made clouds worthy of appreciation by the Cloud Appreciation Society?

The argument for appreciating contrails:
They can serve as early indicators of a change in the weather, for when contrails persist and spread across an otherwise blue sky, they can be the first sign of the arrival of a weather front, which will eventually bring rain. Also they can be very beautiful. When the conditions are right for contrails to persist in the air, they overlap, bisect and spread in the high-speed winds at cruising altitude, adding a modernist counterpoint to the chaotic, impressionistic formations of the natural clouds.

The argument against appreciating contrails:
The water vapour element of aircraft exhaust may not be the most significant from the point of view of climate change (when compared with the other greenhouse gasses it contains), but it is the most visible expression of the effect that aviation is having on our atmosphere. Contrails also encourage the formation of other high clouds, like Cirrus and Cirrostratus, which tend to trap in the Earth’s warmth, rather than reflect away the Sun’s heat like low clouds. This only affects ground temperatures while the clouds are in the sky, but the ever-increasing amount of air travel means the overall warming effects caused by contrails might well be significant.

 

Contrails spotted over Neath, South Wales, UK by Mike Davies (Member

Mamma (September 08)

September 08 Cloud of the Month
(Click image to enlarge) Photographed over St Albans, U.K © Vanessa Sonnabend. See this photo in the Cloud Gallery here.

September 08 Wiro

The Silly-Season Clouds

Gloriously arresting cloudscapes are, of course, always good news. But when this particularly dramatic formation of mamma clouds* appeared over the UK during August – the holiday season when newsrooms are desperate for something to report – the cloud made the national headlines (The Sun, The Daily Mail, The BBC).

Most meteorology books will tell you that the name of these pendulous cloud features is the Latin for ‘udders’, but few ever agree on what actually makes mamma clouds appear. Why? Because no one really has a clue. ‘Relatively little is known about the formation mechanisms of [mamma],’ stated a 2005 study of them in the Journal of the Atmospheric Sciences (see here). As much as anything, this is because it is very hard to predict when and where they will show up, making it a challenge to be in the right place at the right time to study them.

What we do know is that these pouches can form below of a number of different types of cloud, but the more dramatic examples show up on the underside of the spreading anvil of Cumulonimbus thunder clouds (e.g. see here). Each individual pouch is typically one to three km across and, while they tend to be associated with powerful and dramatic storm clouds, they don’t actually forecast severe weather. This is because mamma generally form at the back, rather than the front, of moving Cumulonimbus clouds. By the time you see them, the storm cloud is already heading off to make news elsewhere.

* Also known as mammatus.

 

Kármán Vortex (August 08)

August 08 Cloud of the Month
(Click image to enlarge) Photographed over the Cook Inlet, southwest of Anchorage, Alaska, US © Barbara Bundy. See this photo in the Cloud Gallery here.

August 08 Wiro

Take Your Partners by the Hand

We are used to the swirling masses of cloud on those satellite photographs of storm systems. As the winds flow in towards the centre of the storm, they develop into an enormous rotating pattern simply because they are passing around the surface of a rotating sphere – the Earth. The spin of our planet causes storms to rotate in different directions in the Northern and Southern Hemispheres: they go anti-clockwise in the north and clockwise in the south.

Cloudspotters might therefore assume that the swirling pattern of clouds in this fine photo, taken from a plane off the coast of Alaska, is just such a storm system. They would, however, be wrong, for these aren’t the deep, furious Cumulonimbus clouds that form the cells of a storm. They are in fact just shallow, innocuous clouds, known as Altocumulus. Nor is this pattern spinning on account of the Earth’s rotation. The cloudy swirl here is due to something much more localised: an island in the path of the wind.

Anyone who has contemplated the undulating surface of a babbling stream will know that, as the current parts to flow around a small stone, it can develop a succession of tiny eddies. Each of these swirls, known as ‘Kármán Vortices’, rotates the opposite way from its neighbours. Lined up in a beautiful procession of weaving curls, they look dancers in a Scottish reel.

Exactly the same Kármán Vortices can form on a much, much larger scale within the atmosphere, as winds part to pass around an island rising from the sea. This month’s photograph is in fact a rare record of one such atmospheric vortex, in which the winds spin with gay abandon in an island’s wake. The dance would, of course, be invisible were it not for the Altocumulus clouds. They can never resist the wind’s energetic embrace, and so follow every graceful twirl of its Highland dance in the sky.

 

The Summertime Halo (July 08)

July 08 Cloud of the Month
(Click image to enlarge) Photographed over Fort Worden State Park, Port Townsend, Washington, USA © Russ Hendricks. See this photo in the Cloud Gallery here and here.

July 08 Wiro

The Arc of Many Names

The ‘summertime halo’, the ‘fire rainbow’, the ‘circumhorizon’ or the ‘circumhorizontal arc’… Whichever of its names you decide call it, this enormous, rare, pastel-coloured optical effect will only appear up in the sky when several different factors happen to occur at the same moment.

Firstly, there needs to be the right sort of cloud around, for this is one of the many ‘halo phenomena’ that can only appear as the light shines through ice-crystal clouds, such as this fine Cirrus. The sun also has to be high in the sky (hence the ‘summertime’ name); in fact, it needs to be at least 58° above the horizon (hence the ‘horizon’ name).

In order to refract the high sunlight in the right way, the ice crystals that make up the Cirrus cloud must be shaped like microscopic hexagonal plates. They must also be aligned more or less horizontally (hence the ‘horizontal’ name), for the colours are formed as light rays enter the clear crystals through a vertical side face and exit through their flat bases. So long as the crystals don’t wobble too much in the wind, the 90˚ inclination of these two sides results in pure rainbow-like colours. When the crystal alignment is just right, the entire cirrus cloud shines like a flaming rainbow (hence the ‘rainbow’ name, even though it is not, of course, a rainbow at all).

While most halos can be observed over any parts of the planet, this one is generally found only in those countries that lie below 55° latitude. North and south of this, the sun never rises above 57.8° in the sky. (That said, you might still be able to see the summertime halo in higher-latitude countries so long as you manage to climb a high mountain just as all the other requirements happened to coincide.)

Another example of the summertime halo was spotted by 11-year-old cloudspotter, Hugo, and can be seen in the gallery pages here.

Not only does this rare phenomenon have more than its fair share of names, it is also very beautiful. So while we are at it, why don’t we also start calling it the ‘beautiful arc’, the ‘arc of lovely colours’ and the ‘really rather delightful-looking halo’?

 

June 2008

Some like it Hot

There is a strong northwesterly wind that blows across New Zealand’s South Island which is said to have a disturbing effect on the local population.

The Nor’wester, as it is known, sends temperatures soaring as it sweeps across the Canterbury Plains and has been statistically linked to increases in suicide and domestic violence. Some people feel elated when the dry, hot air hits, but most feel depressed, irritable and drained of energy.

However The Nor’wester also brings with it one of the rarities of the skies, a cloud with its own name. The Nor’west Arch is a distinctive altocumulus formation which partly takes its name from the wind that hurries it along and partly from its shape, because it appears as an arch of high white cloud in an otherwise clear blue sky. It is also often referred to as the Canterbury Arch.

This weather pattern is peculiar to the South Island’s east coast. Warm moist air from over the Tasman Sea is pushed up by the Southern Alps, causing it to cool rapidly, and much of the moisture is dumped on the west coast. As the air passes over the alps, the water vapour remaining becomes visible in a band of cloud over the mountains at the top of each wave of air. To a viewer on the eastern side, this appears as an arch of cloud.

The standing wave of The Nor’west Arch is caused by the moisture condensing and becoming visible towards the top of the wave and then evaporating again as the air descends to the trough of the wave.

Let’s hope this impressive sight helps lift the depression caused by the fiery blasts that accompany it.

The Nor’west Arch spotted by Peter Rees over South Island, New Zealand.

Microbursts (May 08)

Cloud of the Month for May 2008
(Click image to enlarge) Photographed over Long Island, in The Bahamas © Dene Georgelin.
See this photo in the Cloud Gallery here.

May 08 Wiro

Where were you brought up?

The Cumulonimbus could never be called a refined cloud. It is the beast of the lower atmosphere: roaring with deafening thunder; spitting shards of lightning and hailstones; whipping up twisting tornadoes and spawning dramatic cloud features, such as the udder-shaped mammatus clouds, cigar-shaped roll clouds and, well, wall-shaped wall clouds. Sometimes, the Cumulonimbus also produces something called a ‘microburst’, which is best thought of as a cloud sneeze. Don’t be deceived by the ‘micro’ part: the sneeze of a Cumulonimbus is anything but restrained.

A microburst is a very localized column of air – up to 2.5 miles (4km) across – which shoots earthwards from the base of the cloud, spreading outwards with violent force upon reaching the ground. With gusting speeds of up to 150 mph, these winds can damage buildings, fell trees, and have been known to cause aviation crashes.

Much like human sneezes, microbursts can be either dry or wet. They do however last rather longer than ours: between 5 and 15 minutes. Along with chilly winds, the wet varieties produce sudden downpours. These can be quite heavy: enough to give you a cold, should you be in the firing line without the right clothing. Could the Cumulonimbus not muffle its vulgar microbursts? Presumably, no one has ever had the courage to try and teach this beast of the atmosphere any manners.

 

Iridescent Clouds (April 08)

April 08 Cloud of the Month
(Click image to enlarge) Photographed over Perth, Western Australia © Dane Gerneke.
See this photo in the Cloud Gallery here.

April 08 Wiro

A Cloud of Many Colours

Closely related to the optical effect called a corona, the bands of pastel colours produced by this cloud are known as ‘iridescence’. When the Sun is at the right position, preferably hidden by thicker cloud making observation safer and easier, the cloud’s droplets or ice crystals can diffract the sunlight passing through. The array of microscopic cloud particles bends the coloured constituents of sunlight by different amounts, separating them out into bands.

Iridescence can often occur in Altocumulus and Cirrocumulus clouds, but appears most often in lenticular clouds. The effect tends to be observed at the clouds’ fringes, but it can occasionally appear over large areas, as it has in this fine Cirrocumulus over Perth, Western Australia. Iridescent colours look most dramatic in a young cloud, whose newly formed droplets all tend to be around the same size.

We are genetically programmed to see the beauty in our own children, regardless what little terrors they actually are. But what of baby clouds? Surely everyone loves the iridescent glow of their tender cheeks – especially as they don’t even demand to have their nappies changed.

 

Northern Lights – Aurora Borealis (March 08)

March 08 Cloud of the Month
(Click image to enlarge) Photographed over Karasjok, Norway © Mark Humpage.
See another of his photographs in the Cloud Gallery here.

March 08 Wiro Header

Thor's Contrails

As any astronomer will tell you, clouds are certainly not limited to the Earth’s atmosphere. There are many different types of cloud to be found in space. This month’s Cloud of The Month is a break from the norm. It shows the Northern Lights, or Aurora Borealis, which are the effects on our atmosphere of a cloud that reaches us from the Sun.

The immense explosions and flares on the Sun’s surface throw out ‘plasma clouds’ of charged particles, such as electrons, protons and ions. These travel at speeds of around a million miles per hour (400km/s). In spite of moving so fast, they still take up to three days to reach us from the Sun.

The plasma clouds themselves are invisible. But we can see their effects as they come colliding into the Earth’s upper atmosphere. When enough of the charged particles arrive, they cause a ‘magnetic storm’ and interact with the atmospheric gases to release energy in the form of light. This is what causes the beautiful and elusive aurora colours in the Northern and Southern Hemispheres.

Cloud Appreciation Society Member, Mark Humpage, recently travelled to Norway to view this amazing phenomenon. You can read about his exploits here: markhumpage.blogspot.com.

Mark tells us that the Vikings thought the Northern Lights were the contrails of Thor’s chariot being pulled by three goats. This sounds no less believable to us than magnetic storms caused by million-mile-an-hour plasma clouds.

 

Ice halos (February 08)

February 08 Cloud
(Click image to enlarge) Photographed over Saalbach, Austria © Thomas Dossler

February 08 Wiro Header

Halo, what's that?

There are many coloured arcs and rings, known as ‘halo phenomena’, which can appear as the sunlight passes through tiny ice crystals in the sky. Over many parts of the world, these optical effects are most commonly seen through high clouds such as Cirrostratus or Cirrus. But the most dramatic displays of halo phenomena are often when the ice crystals are in the form of a ground-level cloud known as ‘diamond dust’. This is what produced the beautiful light display shown above.

Diamond dust is so named because of the way it sparkles in the sunlight. It is rather like mist, but consists of ice crystals rather than water droplets. When crystals form near ground level like this, they can grow more slowly than they do in clouds high in the sky. This can make them very pure from an optical point of view – clear like glass, rather than cloudy like ice cubes. If they also happen to form in the right shapes, the crystals in diamond dust can behave like countless microscopic prisms. These are what diffract and reflect the sunlight to form halo effects.

There are a whole range of possible arcs, circles and points of light that can appear. The fantastic display shown here, over Saalbach, in Austria, exhibits no less than five halo phenomena. The circle around the sun is called a ’22˚ halo’, while the points of light on either side of it are known as ‘sun dogs’, or ‘parhelia’. The line joining these with the sun (which hidden behind the trees) is called the ‘parhelic circle’. Finally, the arc appearing at the top of the image, just kissing the 22˚ halo, is known as an ‘upper tangent arc’.

Those seeking a fuller understanding of the different halo phenomena and how they form, should make haste to www.atoptics.co.uk. It is a fantastically informative site produced by Les Cowley, who is of course an Honorary Member of the Cloud Appreciation Society.

 
From a photograph alone, it can be hard to see whether optical effects like these are caused by ice crystals near to the ground or very high in the sky. Our first thought on seeing this image was that they were caused by a thin layer of Cirrostratus. Only on closer inspection did we notice the sparkles, and realise that diamond dust was responsible.

 

Lightning (January 08)

January 08 Cloud of the Month
(Click image to enlarge) Photographed over Darwin, Australia © Jason Smith (see it in the gallery)

Jan 08 Wiro Header

When clouds show off

Lightning forms in the thunderous belly of a Cumulonimbus storm cloud. It is the sudden and enormous movement of electric charge within the cloud or to its surroundings. The huge cloud tends to develop negative charge towards its base and positive charge towards its summit. This appears to be caused by the collision between the large hail stones and smaller ice particles being blown around within the chaotic body of the cloud. When they collide, the larger hail seems to pick up some negative charge from the smaller particles. Being heavier, the hail is not blown upwards as easily by all the vertical currents of air in the cloud so the negative charge tends to build up around its base, the positive charge towards its summit.

As this distribution of charge increases, it becomes more and more unstable. Suddenly, a massive rush of electricity – the lightning bolt – redistributes the charge. Thunder is the explosive expansion of the air which is heated extremely fast as it conducts the electricity.

There are a number of different types of lightning, many of which are being demonstrated by the flamboyant thundercloud shown above. ‘Sheet lightning’ is when a bolt is hidden by the body of the cloud, so that the whole cloud appears to light up as the light passes through it. ‘Forked lightning’ tends to be classified in terms of where it travels: ‘Cloud-to-ground’ (just visible at the bottom left corner of the photograph); ‘Cloud-to-cloud’ (at the top right); ‘Cloud-to-air’ (top centre); and ‘in cloud’ (which is what is causing the sheet lightning at the centre of the image).

On the whole, clouds are the more subtle manifestations of the weather. Theirs is usually an understated beauty, which is why it is so often overlooked. No wonder, then, they want to show off every once in a while with such explosive, heart-stopping fireworks.

 

Roll Cloud (December 07)

December 07 Cloud of the Month
(Click image to enlarge) Photographed over Kristianstad, Skane, Sweden
© Rene Verduijn (see it in the gallery)

Dec 07 Wiro

Rolls of Delight

Over the years, many people have asked us whether the Morning Glory Cloud of Australia is a unique occurrence. It is such a dramatic phenomenon that it certainly seems like it should be. This long tube-shaped cloud, often stretches well over 500 miles, forming over the outback of Northern Queensland in a wave of air which glider pilots can surf just like regular surfers on an ocean wave. The Morning Glory cloud is not unique, however. ‘Roll clouds‘ like this can and do appear in many other parts of the world.

While the Morning Glory tends to form at a much greater scale than anywhere else, roll clouds like the fine example above, photographed over Kristianstad in Sweden, can sometimes be seen traveling ahead of a storm. These more run-of-the-mill roll clouds can be formed by the sinking cold air associated with storms.

Vertical columns of warmer air surge up the middle of storm clouds, while cooler air sinks downwards around them – helped along by all the falling rain and hail. In certain conditions, this sinking air can hit the ground in such a violent way that it causes a solitary wave of air to advance ahead of the storm. In the middle of this advancing wave of air, a roll cloud can occasionally appear, rotating as it travels.

What makes the Morning Glory roll cloud stand out from the others is not only its dramatic size. It is also one of the few cloud formations whose appearance is quite regular. It tends to form in a particular region of Queensland at a certain time of year – the end of September / beginning of October. How un-cloudlike to be so dependable. Your average roll cloud is much more true to form: none of us have the slightest idea when or where one will come rolling overhead.

November 2007

Flying the Flag for Clouds

‘Orographic clouds’ are ones that form when wind has to rise to pass over mountains. As it does so, it can sometimes cool enough for its moisture to condense into droplets that appear as cloud. Orographic clouds are called ‘lenticular’ when they hover some distance from the peak and look like flying saucers. They are called ‘cap clouds’ when they sit right over the mountain. But the orographic cloud shown above, trailing elegantly from the North Face of the Eiger in Switzerland, is known as a ‘banner cloud’.

Banner clouds tend to have a more even appearance, but this one consists of fluttering ribbons with gaps in between. It is an effect you see sometimes when the wind is stiff and the mountain peak is pronounced, which results in a lot of turbulence in the airstream.

The mighty mountain and the ephemeral cloud could hardly have less in common with each other. And yet they are always faithful companions – ones who proclaim their friendship by unfurling this glorious banner to the world.

Banner cloud spotted over the North Face of the Eiger in Switzerland by Bob Peterson (Member 12,047).

Stratocumulus (October 07)

October 07 Cloud of the Month
(Click image to enlarge) Photographed on a flight from Hanoi to Kuala Lumpur © Ruziana Mohd Mokhtar (see it in the gallery)

October 07 Wiro

Cities in the Sky

To view the clouds from an aircraft – to be up at their level – is to lose yourself in another world. When the low sun casts long shadows from the towering edifices of a Stratocumulus cloud, like the beauty shown above, that world seems to take the form of a fantastical cloud city.

It was just such a city that the American naturalist, Henry David Thoreau, observed in the sky off to the western horizon, and described in his journal on 10 July 1851:

“Between two stupendous mountains of the low stratum under the evening red, clothed in slightly rosaceous amber light, through a magnificent gorge, far, far away as perchance may occur in pictures of the Spanish coast viewed from the Mediterranean, I see a city, the eternal city of the West, the phantom city, in whose streets no traveler has trod, over whose pavements the horses of the sun have already hurried, some Salamanca of the imagination.”


The Unclassified Cloud (September 07)

Cloud of the Month Sept 07
(Click image to enlarge)
Photographed over Isle of Mull, Scotland © Bob Norvill (see it in the gallery)

Sept 07 wiro heading

Gesundheit!

Over the years, we’ve tended to use the Cloud of the Month section as the place to discuss particular cloud classifications. We select a nice photograph from the Cloud Gallery, and say something about the particular genus and species of cloud it represents. This month, however, we are going to break with tradition. We are not going to classify the cloud formation at all.

The art of cloud classification is just a small element of enjoying the sky. At the end of the day, who cares if that cloud over there is a fine example of Altocumulus undulatus*? Does it really matter if that one off towards the horizon is a classic Kelvin-Helholtz wave cloud**? These classifications are just names – and often complicated Latin ones that are hard to pronounce. They merely reflect man’s desire to impose order and regularity onto his world. To try and pigeonhole these most chaotic, free and ephemeral of nature’s phenomena is mere vanity. No sooner have we managed to work out a cloud formation than it mocks us by changing its guise.

Therefore, in recognition of the ultimate futility of classifications, we will not identify the cloud in this month’s photo. Let’s just accept it for what it is, and say nothing more about it.

Then again, it does look rather like an elephant sneezing.


* Of course, if you do care, you need only click here.
** Likewise, if it does matter, you can always click here.


Alexander’s Dark Band (August 07)

Cloud of the Month Aug 07
(Click image to enlarge)
Photographed over the Alpi Marittime in Liguria, Italy © Remo Mattè (see it in the gallery)

August 07 Wiro Header

Alexander's Dark Band

No, this is not the name of a goth group from the 1990s.
‘Alexander’s Dark Band’ is in fact an optical phenomenon associated with rainbows. It is the darker-looking region of sky between the arcs of a primary and a secondary rainbow. The band is named after Alexander of Aphrodisias, who first described the phenomenon in around 200AD.

Standing with the sun shining from behind onto a rain shower ahead, a cloudspotter can see a rainbow when the light passes through the raindrops and reflects back off their inside surface. As the it enters and emerges from the droplets, the sunlight is separated into different wavelengths, producing the colours of the rainbow.

Sometimes, a secondary bow is also visible outside the primary one. This is the result of the light passing into the droplets at a different range of angles and being reflected not once but twice off their inside surfaces back towards the cloudspotter. This secondary bow is fainter than the primary one, with a reversed order of colours. Due to the optics*, the rain in the space between the two bows scatters less light towards the cloudspotter than elsewhere, causing it to appear darker. This is Alexander’s Dark Band.

Besides his description of this subtle optical phenomenon, Alexander of Aphrodisias is best known for his commentaries on the philosophy of Aristotle. He did, however, write some of his own original works too. Since one of these was entitled ‘On The Soul’, we wonder whether this early cloudspotter did in fact have a Classical Greek pop group of his own called something like ‘Alexander’s Funky Band’.


* For fuller explanations of this and other optical phenomena of the air, see Les Cowley’s excellent Atmospheric Optics website.


Fumulus Snail (July 07)

July 07 Cloud of the Month
(Click image to enlarge)
Photographed over Cwmcarn, Wales © Huw Collings (see it in the gallery)

July 07 wiro header

Flying Snails

Cloudspotting is not an activity to be rushed. Most clouds appear to move at a more sedate pace than we do down on the ground and so one has to slow down to watch them develop. It is this requirement to wind down that makes cloud gazing so relaxing.*

What better expression of the change in pace that comes with a bit of meteorological meditation than this cloud in the shape of a snail, emerging from the fog over Newport in Wales, UK?

It is a formation that is sometimes known as a ‘fumulus’. The name comes from the combination of fumes and Cumulus, for this cloud has developed from the moisture contained in the gases rising from a steelworks. How ironic that such a beautiful allusion to the calming drift of the clouds should have been caused by the relentless march of heavy industry.

* In fact, clouds often move very fast but their movement appears gentle when viewed from some way away.

 
 

Distrail (June 07)

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Photographed over Hampshire, UK © John Norris (see it in the gallery)

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Negative Contrails

We’ve all observed the worrying proliferation of condensation trails, or ‘contrails’, criss-crossing our skies in the wake of high-altitude aircraft. They look like man-made scars across the blue. But how many have noticed the much less common but related cloud effect, known as a ‘distrail’?

Short for ‘dissipation trail’, this is not so much a cloud, as a gap in cloud cover. It can appear when an aircraft happens to pass through a fairly thin cloud layer composed of ‘supercooled’ droplets. These are in unfrozen, liquid form, even though temperatures are well below 0degC. Water droplets can stubbornly refuse to freeze when there is a lack of air-borne particles to act as nuclei onto which the ice crystals can start forming.

As an aircraft climbs or descends through one of these supercooled clouds, the turbulence of its wake and the many minute particles contained in its exhaust encourage the cloud’s droplets to freeze. This happens when some of the particles act as the nuclei onto which the droplets can start freezing. As the crystals form, they grow in size and fall below. Left behind, is a just gap in the cloud – the distrail.

Altocumulus undulatus (May 07)

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Photographed over Limekilns, Fife, Scotland © Dennis Cowan (see it in the gallery)

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Calling weather sleuths

All clouds with ‘undulatus’ in their name have wave-like features to them. The name is derived from unda, the Latin word for a wave. Sometimes, they are in the form of a continuous layer with an undulating surface. Other times, they are like lines of cloud with gaps in between, as in this fine Altocumulus undulatus, spotted over Fife in Scotland.

It is the way the winds vary with altitude that tends to cause these formations. When blowing at different speeds above and below the cloud layer, the wind can cause the cloud to bunch up into ridges, which curve, join and divide, like the lines on your finger.

Of course, the shearing winds that cause undulatus clouds are never visible. You have to be a cloudspotter to know that they are there. Who else would recognise the enormous fingerprints they’ve left across the sky?

Cumulonimbus capillatus (April 07)

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Photographed over Laura, South Australia © Tina Moore (see it in the gallery)

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The Hairy Mammoth

The dramatic, and often violent, storm clouds known as Cumulonimbus are divided into two species: ‘calvus’, meaning bald, and ‘capillatus’, meaning hairy. The difference, unsurprisingly, is to do with the appearance of the cloud’s summit, which can reach 11 miles (17km) into the sky.

If the head of a thundercloud has only slighly fluffy edges, the Cumulonimbus is defined as the younger, balder calvus species. Once it has developed into a lumbering, mature beast, it will often have grown the ice-crystal locks of the capillatus. These might be in an afro style, like that sported by the handsome Australian specimen, above. Or they might be in one of many other styles, such as the quiff, the thinning-on-top or the just-stepped-out-of-a-salon.

We’ve heard enough nonsense about the hairy mammoth dying out with the last ice age. In fact, it is terrorising our skies on a regular basis. And the only thing frozen about it is its buffon.




Lacunosus (March 07)

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(Click image to enlarge) Image © Marco Cingolani (see it in the gallery)

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Sweetness and Light

Sometimes a cloud will express itself more by its absence, than by its presence. It may do this by forming a hole in the shape of something (such as Graeme Ferris’s Blue Zombie). Or it may do it by turning into a cloud variety known as ‘lacunosus’, which is the latin for ‘full of holes’.

A lacunosus cloud looks rather like a loose honeycomb: a layer of rough gaps surrounded by cloud fringes. This formation can appear at all cloud levels, and tends to be more common at the high level (when it is known as Cirrocumulus lacunosus, see here, and here) and middle level (when it is called Altocumulus lacunosus, see here). But the fine example shown above, photographed over the Marche region of Italy, looks to be in the lower cloud level and so should be called Stratocumulus lacunosus. Its cloudy honeycomb holes appear a lot larger than those in the higher examples simply because they are closer to us.

Stop and take in the sweet sight of the lacunosus cloud whenever you come across it, for this is a transient species that will melt away before your eyes.


Horseshoe Vortex Cloud (February 07)

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At the Cloud Appreciation Society, we don’t like to claim one cloud is better than another. But some clouds are undeniably rarer, making them seem a little more special. One of the rarest of all is the ‘horseshoe vortex cloud’. In fact, of the 2,200 or so images on our cloud photo gallery to date, only two feature this elusive formation. The fine example above appeared over Jasper, Alberta, Canada (the other one is here).

This cloud forms in a region of rotating air, or vortex. Such vortices usually form vertically, sometimes leading to waterspouts or even tornados, but occasionally they can develop with a horizontal axis and give rise to a gently rotating crescent of cloud. Such a movement of air seems to happen when an updraught is sent into a spin upon reaching shearing horizontal winds. Rarely are conditions right for a cloud to appear within the spin.

Some say that a horseshoe brings luck only when it is pointing upwards and holding the luck in. Others claim that, when pointing down, it allows luck to fall onto those below. Horseshoe vortex clouds tend to point downwards and so any cloudspotters already fortunate enough to see one will want to stand below it to top up on their luck.

Jet-Stream Cirrus (Janurary 07)

Serpents of the Sky

Cirrus clouds form towards the top of the troposphere*, where the winds are much faster than below. As a result, Cirrus tend to be the fastest moving of all cloud formations.

High-level winds can reach speeds of up to 180mph (300km/h) in the strong westerly air currents known as ‘jet streams’. These are winding flows of air that encircle the globe at altitudes of 30,000 to 35,000 feet, developing from the temperature and pressure differences between polar and tropical air masses, and whipped-up by the Earth’s spin. Cirrus clouds forming along the path of jet streams – such as in the dramatic example shown above over Katikati, New Zealand – are generally known as ‘jet-stream cirrus’.

The paths of jet streams are in constant flux, as they wind and bend their way around the globe like enormous, invisible serpents. These snakes of the sky play a crucial role in the weather of temperate regions, for they strengthen and steer storm systems in an eastward direction around the globe. Most people never know that a jet stream serpent is passing over them – unless, that is, the silky streaks of jet-stream Cirrus have decided to render it visible to the keen eyed cloudspotter.

(* This is the lower region of the atmosphere where almost all clouds form.)

 

Jet-Stream Cirrus spotted over Uretara Paddocks, Katikati, New Zealand by Brian Chudleigh.

Altostratus/Altocumulus/Altowhateveritis (December 06)

View from the Deep

It is easy to forget that we are all creatures of the ocean. Not the ocean of water, but that of the air. We call it our atmosphere and we live – of course – down in its depths. To the floating clouds above, we must look like tiny crabs, scuttling across the ocean floor.

The clouds render visible the various eddies and currents that flow through the atmosphere. They remind us of our status as bottom-dwellers. And none do this more vividly than the beautiful formation shown here, which looks like choppy waves of water seen from below the surface.

There are a few examples of this cloud type on the gallery pages (particularly the ones that appeared in Iowa, US, in the summer of 2006 – see here and here). But it is by no means clear to us what their classification should be. In fact, it seems to us that we lack a suitable name for this formation. So, a while back, we asked C.A.S. members to come up with their own suggestions. Look here to see members’ name ideas.

At the end of the day, it doesn’t matter what this cloud formation is called. More important, is that it reminds us that we’re animals of the air, as well as creatures of the deep.

 

Altocumulus undulatus clouds spotted over Segno, Liguria, northern Italy by Riccardo Dall’Acqua.

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)

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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)

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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)

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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)

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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)

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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)

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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)

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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)

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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.