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.

Cloud of the Month for September 2010

September 2010

Cloud of the Month for September 2010

The Northern Lights

This month’s image shows the beautiful atmospheric display known as the ‘aurora borealis’. It was given this name by Pierre Gassendi in 1621, after the Roman goddess of dawn, Aurora, and the Greek name for the north wind, Boreas. Also known as the northern lights, the display of shifting colours results from the collision of energetic charged particles from solar storms with atoms in the Earth’s high atmosphere. Because of the Earth’s magnetic field, the displays only occur in northern latitudes, like this beautiful example over Denmark.

Photograph © Jesper Grønne.

Cloud of the Month for August 2010

August 2010

A Cloud Smile

The most beautiful of halos, officially known as a circumzenithal arc, is created by tiny ice crystals. It is sometimes known as a ‘cloud smile’, and rightly so, it can’t fail to bring joy to any cloudspotter. The delightful example shown here was spotted over Stratford-Upon-Avon, UK.

Photograph © Emma Jukes.

Cloud of the Month for July 2010

July 2010

Cloud of the Month for July 2010

Sunset Lenticularis

The image chosen this month shows a group of U.F.O. lookalikes, namely altocumulus lenticularis clouds, highlighted in a colourful sunset over Fiordland, New Zealand.

© Photograph Shirley Moodie.

Cloud of the Month for June 2010

June 2010

Cloud of the Month for June 2010

On the Wing

This month’s image is a high flying Altocumulus cloud over Southern Spain.

© Photograph Christina Chelmick.

Cloud of the Month for May 2010

May 2010

Cloud of the Month for May 2010

Squall Line

This month’s image taken in Derby, Australia, is a line of severe thunderstorms known as a squall line, they form along or ahead of a cold front.

© Photograph Hans Leenaarts.

Cloud of the Month for April 2010

April 2010

Cloud of the Month for April 2010

Touching the Clouds

This month’s image of Altocumulus stratiformis clouds was taken at Disneyland, Paris. ‘Altocumulus’ are mid-level clumps of cloud that often appear in a layer which, when it is also described as ‘stratiformis’ spreads over the majority of the visible sky.
Photograph © Dawn James.

Cloud of the Month for March 2010

March 2010

Cloud of the Month for March 2010

A Bow from the Air

This month’s image comes from the above the clouds category of our gallery. It is different view of a rainbow from the normal one, and was spotted on a flight above Anglesey, North Wales.

Photograph © John Rowlands.

Cloud of the Month for February 2010

February 2010

Cloud of the Month for February 2010

Diamond Dust over Silvercreek

This month’s image is a true gem, a cloud of Diamond Dust. Early morning temperatures hovering around minus 27 degrees and falling ice crystals, created this varied collection of arcs and halos spotted by Jay Brazel at approx 8,700 feet above a small United States town called Silvercreek in Colorado.

© Photograph Jay Brazel.

Cloud of the Month for January 2010

January 2010

Cloud of the Month for January 2010

Twin Peaks

This month’s image portrays classic Altocumulus Lenticularis forming as cap clouds over Mount Lidgbird and Mount Gower on the south east coast of Australia. Photograph taken from Lord Howe Island.

© Photograph Matthew Brennan.

Cloud of the Month for December 2009

December 2009

Cloud of the Month for December 2009

Looking Down at The Clouds

As any angler will tell you, sitting in front of still water can give you a different perspective on the sky – one that allows you to look up at the clouds without straining your neck. And when the expanse of water is large, it can act like an enormous mirror that reflects huge areas of the sky, like this glorious Altocumulus sunset over southern Australia.

Perhaps the most dramatic example of water mirroring the sky is found on the Salar de Uyuni in Bolivia. This is the world’s largest region of salt flats, and is situated in the Andes, at an elevation of over 3,500m. The salt deposits are incredibly level and become flooded at certain times of the year to be covered in just an inch or two of water. Too shallow to develop any waves, the reflections appear to double the sky.

But you don’t have to cross the world to enjoy the pleasures of cloud reflections. Watch the clouds in any pond or slow river when the day is still, and you will see them sway dreamily with the water’s undulations, as if dancing to a music only they can hear.

© Photograph Heather Hartkamp.

Cloud of the Month for November 2009

November 2009

Cloud of the Month for November 2009

Numbers in the Clouds

Every now and then, someone sends us a cloud in the shape of a number. This month, for instance, it is a Cirrus cloud in the shape of the number three.

In fact, number three is the most common. We’ve also had threes in from Palm Desert, California, Uppsala, Sweden, Margate, UK. There have been a couple of number twos over Cambridge and Stafford. But only one number one over Mercogliano, Italy, (also photographed by Modestino Carbone, who took the image above). We’ve even had a bunch of zeros, mostly photographs of the same cloud, over Dorset, UK.

But we only ever get low numbers. What’s going on? What about the other digits? Now that the collection is started, we think it’s time we graduated to higher numbers. In fact, we have developed an obsession about completing the set. What about fours? Fives? Someone must have seen a six, a seven… an eight? Most of all, we’re desperate for one of our members to capture the elusive cloud nine.

So please do send us your cloud digits – all will be gratefully received.
Except for threes. We are not interested in is any more of those.

© Photograph Modestino Carbone.

Cloud of the Month for October 2009

October 2009

Cloud of the Month for October 2009

The Poor Man’s Optical Effect

Sun pillars are vertical streaks of light that appear above and below a low Sun as it shines through clouds that contain ice crystals, such as the Altocumulus clouds shown here. At night, they are known as moon pillars.

The optical effect is one of the ones that are collectively described as ‘halo phenomena’, and appears on about 25 days of the year over Europe. Sun pillars are caused by the sunlight reflecting off the surface of the ice crystals. So they are rather like an aerial version of the ‘glitter paths’ that appear below the bright sun on the surface of the sea, but it is the faces of tumbling ice crystals that cause the reflections here, rather than then rippled peaks on the ocean. A pillar extending above the Sun like this fine example, spotted over Lively Ontario, looks brightest when the Sun is just below the horizon.

Most of the other the arcs, rings and spots of light that are called halo phenomena only appear when the clouds’ crystals are optically pure, regularly shaped and neatly aligned. But this is not the case for sun pillars. They are like a poor man’s halo phenomenon, for they appear when cloud crystals aren’t so exquisitely refined. Since the sunlight needs only to glance off a surface of the crystals, they can be rough, irregular and jumbled.

© Photograph Paul Laplante.

Convection Clouds (September 09)

Thick and Puffy

‘Convection clouds’ are ones that form and grow by the process known as convection. Cumulus and Cumulonimbus are examples of this type of cloud.

Convection in the atmosphere is the way air floats upwards on account of being warmer than the surrounding air. You can see this sort of movement close up when a shaft of sunlight picks out the tiny motes of dust suspended in the air of your room. Since the dust reveals the invisible swirling currents of the air, it lets you see the air near to a radiator rising upwards. The radiator warms the air beside it, making it expand and become less dense than the air further away. This causes the air near the radiator to gently float up towards the ceiling. The same convection currents, on a much larger scale, drive the formation and development of convection clouds like the large Cumulus shown above.

Out in the atmosphere, patches of sun-warmed ground can act as enormous radiators that set the air rising. These convection currents are known as thermals, and produce fair-weather Cumulus clouds. But convection can also produce clouds when the Sun is not shining. This happens when a huge region of colder air from the poles collides with a region of warmer air from the tropics. When this happens, some of the colder air can ride up over the warmer. The result is in an unstable region of the atmosphere that produces large convection clouds. The less dense, warmer air below rises rapidly through the denser, cooler air above. And whenever air rises like this it cools. This means that some of the invisible water vapour it contains can condense into droplets that appear as cloud.

Since they tend to form rapidly in the rising columns of air, convection clouds are ‘optically dense’. They contain a lot of very small droplets, the surfaces of which scatter the sunlight more than in clouds made of fewer, larger droplets or ice crystals. For this reason, convection clouds often look bright white on the sides facing the Sun and dark grey on the sides away from it.

 Cumulus congestus spotted over Cividale del Friuli, Italy by Tommaso Zamò (Member 14,276)..

August 2009

Your Turn To Do the Washing Up

The cloud species known as lenticularis is one of our favourite formations — not least because it often has the appearance of a UFO. The cloud’s name comes from the Latin for a ‘lentil’, presumably because no one could work out what the Roman’s would have called a flying saucer.

We like the way that this cloud hovers in place, even though a brisk wind is blowing at the cloud level. The cloud forms when the lower atmosphere is ‘stable’, which means that an airstream passing over raised ground, such as a hill or mountain, tends to rise and dip in a wavelike path. It is rather like the standing waves that hover on the surface of a fast-flowing stream in the lee of a rock. Lenticularis clouds can form downwind from the peak, at the crests of these invisible sanding waves of flowing air*.

But we particularly like it when lenticularis clouds take on the stacked formation, known as a ‘pile d’assiettes’, or ‘stack of plates’ in French, like the handsome Altocumulus lenticularis shown above. This type of stacked lenticularis only appears when the airstream encountering the raised ground consists of alternating moister and drier layers. The cloud droplets form as the moister layers cool upon rising at the crest of the standing wave, but are far less plentiful in the drier layers of air in between. Pile d’assiettes is the only internationally accepted cloud term that is in French rather than Latin.

When the atmosphere in the region is particularly stable, even just a gentle raising of ground level can be enough set up the waves of air that produce lenticularis clouds.

Photograph © John Maltas.

Cumulus congestus (July 09)

Pump it up

There are few clouds as pleasing to watch develop as the Cumulus congestus. From below, the darkening base of this shower cloud looks ominous and brooding but, from a distance, it has the appearance of a gigantic inflatable creature: its crisp, voluminous shape swelling into the middle atmosphere, as if it is being pumped up, from behind the scenes, by some enormous, invisible dad.

The largest of the four possible species of Cumulus, congestus can develop from the smaller humilis and mediocris species when the atmospheric conditions are ‘unstable’. This means that the way the air temperature changes with altitude tends to encourage the rising column of warm, moist air at the centre of the cloud to keep lifting higher and higher. Such unchecked convection makes the cloud swell to formidable proportions. While it has the same crisp, cauliflower summit as the smaller fair-weather Cumulus, the congestus species is so tall that it readily produces sizeable showers.

This stage of Cumulus growth is when it is at the point of maturing into a different cloud type. Once the top of the cloud begins to ‘glaciate’, its droplets freezing into ice crystals, the crisp, sharp edges of its summit soften and become more blurred. This is the point at which the cloud has officially turned into a Cumulonimbus storm cloud.

Such a fully-fledged storm cloud is the stage in the cloud’s development that tends to get all the attention – what with its heavy downpours, its spreading canopy of ice crystals and dramatic thunder and lightning. But we rather prefer the congestus stage just before it becomes such an ostentatious individual. We like to gaze up and chart the cloud’s progress, blow by blow, as this brother to the fair-weather Cumulus swells into adulthood.

 Cumulus congestus spotted over Naples, Italy by Henning Thing (Member 16,294).

Cloud of the Month - Asperatus

June 2009

Cloud of the Month - Asperatus

A New Cloud Name?

A few years back, we chose a Cloud of the Month which we didn’t feel very confident about identifying. Since it looks rather like the surface of a choppy sea viewed from below, we gave it the nickname of the ‘Jacques Cousteau cloud’, after the legendary 1970s French diver and ecologist.

But as we started receiving more and more dramatic examples of this cloud formation from members and visitors across the world, we decided that it warranted a more official-sounding name. We looked for a Latin one that would sound at home amongst the official cloud classifications and settled on ‘asperatus’, which is the Latin for ‘roughened up’. The term was used by Classical poets to describe the seas being agitated by strong winds.

We propose that asperatus should be adopted as a new ‘variety’ of cloud, meaning that it is a particular characteristic that appears in one or other of the main cloud types. This would mean that the rough and choppy looking Altocumulus cloud shown above would become known as ‘undulatus asperatus’.

Our proposed new cloud variety shares some similarities with existing formations such as the more regular waves of undulatus clouds and the hanging pouches of mamma clouds, but we feel that this cloud is different enough from them to be classified as a variety of its own – or a ‘supplementary feature’, if you want to be precise about it.

Who knows whether asperatus will eventually be accepted as an official variety? If it ever is, we will celebrate with a snorkelling holiday.

Asperatus over Schiehallion, Perthshire, Scotland © Ken Prior

Clouds at Night (May 09)

By the light of the silvery moon

There aren’t many images of clouds at night on the society gallery. Perhaps this is because the low light conditions present challenges for digital photography. Perhaps it is because cloudspotters like to go to bed early. Whatever the reason, we think that it is a shame, as the right sort of clouds lit by a full Moon is a sight of great beauty.

The high, streaked ice-crystal clouds, such as Cirrus, look glorious in the moonlight. Thin enough to let the light through with ease, they seem even more delicate and lace-like with a dark backdrop than they do during the day. But the best night clouds have to be Altocumulus – particularly those with gaps between their clumps, a variety known as ‘perlucidus’. If the Altocumulus layer is not too thick, the clumps of cloud develop a striking silvery glow, which is interspersed with inky rivers of the starry sky beyond.

And sometimes, as in these night-time clouds over Bangalore, India, the droplets of the Altocumulus cloud can diffract the moonlight to produce a fragmented ring of reddish colours, around a blue-white disc of light, which is known as a ‘corona’. As different clouds drift in the wind across the face of the Moon, these colours jump nearer or further from it, shifting position depending on the size of the cloud droplets.

Nothing beats a clear starry night when the Moon is less than full. But when it is shining at its brightest, the drifting schools of iridescent, silvery clouds are enough to make even astronomers, usually frustrated by the obstacle of clouds, put down their telescopes and admire the spectacle.

Altocumulus at night spotted over Bangalore, India by Monish Mathiaas.
 

Sundogs (April 09)

Every Sundog Has its Day

Also known as ‘mock suns’ and ‘parhelia’, sundogs are large spots of light that can appear on one or both sides of the Sun. They appear on about 70 days of the year in mid-latitude regions of the world, and are formed as sunlight is refracted through the ice crystals of thin layers of high clouds, such as Cirrostratus, Cirrocumulus and Cirrus, as is the case in April’s Cloud of the Month. Like all the ‘halo phenomena’ produced by sunlight shining through ice clouds, sundogs can also appear in ground-level ice-crystal cloud known as diamond dust.

These spots of light appear level with the Sun, and at a distance away from it that’s equivalent to the outstretched span of a hand held up at arm’s length. Sundogs are brightest when the Sun is low, and only generally visible when it’s lower than 40˚ above the horizon. They generally appear orange and red on the side nearest to the Sun, because of the way the wavelengths of sunlight are separated by tiny ice-crystal prisms of the cloud.

Cloudspotters should learn to recognise the sort of sky that produces sundogs and other halo phenomena. This is when the blue first begins to pale with subtle wisps and milky patches of the ice-crystal clouds. The clouds must be thin for the light effects to appear. Once they’ve grown thick enough and white enough to be noticed by the general riffraff, the sundogs will have silently slipped away.
 

Sundog spotted in Cirrus clouds over Mount Jade, Taiwan by Jochun Ho (Member 10,497)..

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)

Mamma: 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.

Photographed over St Albans, U.K © Vanessa Sonnabend. See this photo in the Cloud Gallery here.

Kármán Vortex (August 08)

Take Your Partners…

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.

Photographed over the Cook Inlet, southwest of Anchorage, Alaska, US © Barbara Bundy. See this photo in the Cloud Gallery here.

The Summertime Halo (July 08)

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’?

Photographed over Fort Worden State Park, Port Townsend, Washington, USA © Russ Hendricks. See this photo in the Cloud Gallery here and here.

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)

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.

Photographed over Long Island, in The Bahamas © Dene Georgelin. See this photo in the Cloud Gallery here.

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.

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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.

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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)

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.

Photographed over Saalbach, Austria © Thomas Dossler. See the image in our Photo Gallery.

Lightning (January 08)

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

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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)

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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)

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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)

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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)

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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)

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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)

Distrails are like Contrails in Negative Form

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.

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