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Fractus

When Stratus clouds or Cumulus clouds appear as broken fragments, they are known by the species fractus. And while all example of fractus clouds are united by their frayed, shred-like appearance, they can form in a number of quite different ways.

Cumulus fractus is the more straightforward. This is when a fair-weather Cumulus cloud is just beginning to form or is in the process of dissipating away. In both cases, the cloud lacks the crisp outlines and solid appearance of a fully-fledged Cumulus, looking instead rather more wispy and translucent. When viewing photographs, cloudspotter have been known to confuse Cumulus fractus with the high, wispy trails of ice crystals known as Cirrus.The distinction is clearer viewing the cloud in reality, since the fractus formation is much lower and soon changes either into Cumulus or just clear air.

The fractus form of Stratus tends to appear in two contexts. One is in the form of dark shreds beneath a raincloud. This form of Stratus fractus has the more specific name of pannus. The other appears as wisps of cloud snagging on hillsides, where they catch in the trees, forming as damp air gently ascends up the slopes.

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Velum

Like one of those tiny fish that swim in the slipstream of sharks, the accessory cloud known as velum is easily missed beside the mighty forms of its companion Cumulus congestus or Cumulonimbus clouds. It usually appears here as a thin, dark horizontal streak half-way up the side of the main cloud.

Named from the Latin for the sail of a ship, or the flap of a tent, velum are found in the vicinity of large convection clouds that have spread outwards for a time during their growth before breaking through and continuing their ascent. A strip of cloud is left behind, which lingers at the flanks of the towering mounds.

Despite their flimsy appearance, velum often hang in the sky long after the showy convection clouds that formed them have dissipated. We are sure there is a lesson hidden in there somewhere.

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Distrail

An aircraft dissipation trail, or distrail, is a specific example of the cloud hole known as a cavum, or fallstreak hole. In the case of a distrail, the cloud hole is formed by the passage of an aircraft through the cloud layer, and so it shaped more like a line than a circular hole.

As with a regular cavum, a distrail forms when very cold droplets in a cloud layer start to freeze in one region and fall below as ice crystals. If these dissipate away in warmer, drier air below the cloud, all that is left is a hole – or in this case, a line. An aircraft can trigger this freezing process by the cooling that happens within its wing vortices or by the tiny particles in its exhaust acting as ‘freezing nuclei’. Airborne particles, whether natural ones like dust, ash or plant material or ones introduced artificially like this, serve as the tiny seeds onto which cloud droplets can begin to freeze. Without them droplets can stay in ‘supercooled’ liquid state at temperatures as low as –40°C (–40°F).

Depending on whether or not the ice crystals evaporate in the air below, a distrail can appear as a completely clear line cut out of a cloud layer or one with a trail of ice falling crystals visible beneath it.

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Floccus

When higher clouds are in small clumps or patches with very soft, fluffy edges, they are described as floccus. The name comes from the Latin for a tuft of wool, a piece of fluff, the little thing you find in your belly button. (Not the last one, actually). Floccus formations can be found up at the high-cloud level, as forms of Cirrus or Cirrocumulus, and at the mid-cloud level, as a soft looking, ice-crystal form of Altocumulus cloud.

Floccus clouds often have trails of ice crystals falling from them. For the lower examples, you’d describe these trails as virga. For the high examples of floccus, you wouldn’t – the trails are just another feature of the general cascade of ice crystals that we describe collectively as cirroform clouds.

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Uncinus

Uncinus is a species of Cirrus where the streaks of the high cloud have a hooked appearance at one end. To be a true uncinus, these hooks should look like the handles of walking sticks, and should not take the form of rounded tufts or clumps. Those would be more likely to be the floccus form of Cirrus and trails of virga falling from Altocumulus.

Described in maritime folklore as ‘mare’s tails’, uncinus have traditionally been associated with the arrival of unsettled weather. There is some truth to the phrase ‘mare’s tails and mackerel scales make tall ships carry low sails.’ The hooked ends of these clouds indicate that strong winds up at the high-cloud level vary greatly with altitude – being much faster above the cloud than they are below. Such ‘shearing’ wind conditions do indeed herald the arrival of a weather front.

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Fluctus

The fluctus cloud, also known as the Kelvin-Helmholtz wave cloud, looks just like a series of enormous waves breaking on the shore. It is rare, fleeting and the favourite of surfing CloudSpotters. A well-defined fluctus is the crown jewel in many a cloud collection, for it requires the CloudSpotter to be blessed with eagle-eyed sky awareness and sheer blind luck. In one spotting alone, this cloud can help observers overtake their fiercest cloud-collecting rivals.

It appears at all three cloud levels, and can be thought of as a very specific example of the undulatus cloud variety, tending to be found in Stratocumulus, Altocumulus or Cirrus clouds. The distinctive curling waves can also sometimes be spotted along the top edge of a lenticularis cloud, a small Cumulus or even a layer of fog. In all cases, the formation lasts no more than a minute or two.

The breaking-wave appearance is caused by wind shear. When cloud develops at an abrupt boundary between layers of colder air below and warmer air above, and the upper layer is moving more rapidly than the lower one, undulations can develop along its upper surface. If the amount of shearing is just right, these undulations can roll up into a succession of vortices. The mechanism is rather different from that of ocean breakers, but fluctus cloud do look like a cloudspotting surfer’s idea of heaven.

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Horseshoe vortex

What a subtle little wisp of cloud the horseshoe vortex is! It is easily missed by anyone other than the most keen-eyed CloudSpotter, intent on adding it to their collection. The rare and fleeting horseshoe vortex cloud appears for just a minute or so before evaporating. Anyone lucky enough to spot one must take a photo if they want to be believed by their cloud-collecting friends.

This cloud forms in a region of rotating air, or vortex. While the familiar orientation for a vortex is vertical (see the tuba cloud), they can occasionally develop on a horizontal axis. This is when the gently rotating crescent of the horseshoe vortex cloud can form. The movement of air seems to result from thermal that is sent into a spin as it reaches stiff horizontal winds above. Only rarely are conditions right for cloud to appear as the low pressure within this horizontal vortex causes the air to cool slightly. When they are, the twisting ribbon of cloud that forms is soon lifted upwards at its centre by the rising thermal, and distorts into a crescent shape.

This rare and beautiful little cloud won’t lead to any precipitation, but it will rain down luck upon anyone fortunate enough to spot it – as well as five CloudSpotter stars.

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Nacreous

Forming 10–20 miles up, in the stratosphere, at –85˚C (-121˚F), nacreous clouds show beautiful iridescent pastel hues as they scatter the light from the Sun when it is just below the horizon.

Sometimes called ‘mother-of-pearl clouds’, their tiny, uniform ice crystals are very good at diffracting sunlight. This separates the light into bands of colour, to create a much more dramatic version of the iridescence sometimes seen in lower clouds.

Also known as ‘polar stratospheric clouds’ since they tend to appear over higher-latitude regions of the world, nacreous clouds are like a stratospheric version of the lenticularis species of wave cloud. They form when the atmosphere is so stable that waves produced as air flows over mountains down at ground level are transferred up through the atmosphere, and push moisture into the lower stratosphere. The best time of year to spot them is in winter, when temperatures are lowest.

Nacreous are known as Type II polar stratospheric clouds, and are made of ice crystals. There is another type of cloud up in the stratosphere, known as a Type I polar stratospheric cloud, which consists of droplets of nitric and/or sulphuric acid. These clouds are actually destructive for our environment. Their droplets act as catalysts that encourage the destruction of the protective ozone layer. They have only subtle, flat colouration, and so they’re not even described as nacreous clouds. Best stick, instead, with appreciating the harmless, colourful ones.

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Noctilucent

The mysterious noctilucent clouds are higher than any other cloud in the atmosphere. Also known as ‘polar mesospheric clouds’, they have an eerie, bluish-white appearance, often showing delicate ripples or billows.

Noctilucent clouds form in the mesosphere, at altitudes of 30–50 miles (50–80 km) – almost at the limit of the atmosphere. Being so high means that, in the higher latitudes, where they are most frequently seen, noctilucent clouds shine out against the night sky well after the Sun has dropped over the horizon. They still catch the sunlight when the rest of the sky is dark. Their name comes from Latin for ‘night shining’.

Quite how noctilucent clouds form is by no means clear. The mesosphere is a region where air temperatures can be as low as –125˚C (–190˚F) but there is very little moisture at all. No one knows why the ice crystals that make up this cloud arise in such a dry and remote part of the atmosphere.

Historically, noctilucent clouds have tended to be spotted at latitudes higher than 50˚ during the summer months. It now seems that they are appearing over much larger regions of the world and more frequently. Some scientists have speculated that this change might be related to global warming.

The best times for CloudSpotters to try to add noctilucent clouds to their collections is a few hours after sunset or before sunrise from May to August in the Northern Hemisphere, and November to March in the Southern Hemisphere.

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Lacunosus

A rare, fleeting formation, the lacunosus variety is identified in terms of the gaps between cloud elements, rather than the clouds themselves. It is when a cloud layer is composed of more or less regular holes, around which fringes of cloud form, like a net or rough honeycomb. Even though lacunosus forms at all three cloud levels, it is an elusive prize for any cloud collector, since it is so short-lived.

The holes of this variety are formed by sinking pockets of air, and the cloud fringes around them by air rising up between the pockets to replace them. Such sinking can occur when a layer of cooler air finds itself over a warmer one. Being more dense, the cooler air sinks down through the warmer air. The appearance is similar to the rough honeycomb pattern you occasionally see on the surface of a hot cup of tea. As the tea on the surface cools and contracts, it sinks in pockets through the hotter tea below, which bubbles up in between to replace it. That said, no one is completely sure why sometimes the cool air sinks to form lacunosus, while other times the warm air rises in pockets to form the opposite arrangement of cloudlets with gaps between.

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Contrail

Before the start of the First World War and the advent of high-altitude flight, our skies appeared very different from the way they do today – there were no condensation trails, or contrails, which form in the exhaust of aircraft.

There’s no confusing these man-made clouds with the natural ones. Following the aircraft’s path, contrails tend to appear as long, straight slashes of white across the blue. In the vicinity of airports, however, they can sometimes form large loops, due to the stacking formation of aircraft waiting to land.

The length of time contrails remain in the sky – or indeed whether they form at all – varies greatly depending on the air conditions up at cruising altitude. When it’s cold enough and moist enough, the water vapour contained in the plane’s hot exhaust gases mixes with the very cold air to condense and form ice crystals. In some conditions, these soon evaporate. In others, they can persist for hours, the ice crystals absorbing water vapour from the surrounding air to grow in size and spread out in the high winds. In this way, contrails often encourage the formation of Cirrus, Cirrocumulus and Cirrostratus ice-crystal clouds.

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Cavum

They look bizarre, but cavum, also known as fallstreak holes, are not actually that rare. They are crisp gaps in mid- or high-level cloud layers, below which dangle trails of ice crystals.

To form a cavum, the cloud layer must consist of supercooled droplets – when its water is in liquid form despite temperatures at cloud level being well below 0˚C (32˚F). This is actually quite common, for pure water suspended as droplets in the air behaves very differently from tap water in the freezer. If there aren’t enough of the right sort of tiny particles in the atmosphere to act as icing nuclei, on to which they can start to freeze, droplets remain liquid until temperatures drop to around –40˚C (–40˚F). They ‘want’ to freeze, but can only do so when there are seeds on which the crystals can begin to grow.

The fallstreak hole forms when one region of the cloud finally starts to freeze and begins a chain reaction. All the moisture from the supercooled droplets in the area rushes to join the ice crystals, which quickly grow big enough to fall below. A form of virga, the trail of ice crystals doesn’t tend to reach the ground, but evaporates before getting that far.

What starts the freezing? Sometimes it’s ice crystals falling into the cloud’s droplets from a higher Cirrus cloud. Most often, it is caused by an aircraft climbing or descending through the cloud to form a ‘distrail’. Low pressure in the vortices around the plane’s wings can cool the air enough to set off the freezing.

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Radiatus

When a layer of cloud rolls or clumps extends in long lines that stretch off to the horizon, the effect of perspective makes these lines converge, like railway tracks, towards a point. Such a formation is a variety known as radiatus, and it can be found at all three cloud levels.

The parallel cloud lines form along the direction of the wind at cloud level. (When they form perpendicular to the wind, they are of the undulatus variety, rather than radiatus.)

Radiatus in low Cumulus clouds are known as ‘cloud streets’. These formations cause glider pilots to wet themselves with excitement, for they indicate avenues of lifting air along which the pilots can reliably gain altitude.

When it comes to high, ice-crystal clouds, the most dramatic examples of radiatus result from jet streams – the ribbons of 180mph winds that encircle the globe in the mid-latitudes at the top of the troposphere. Known as ‘jet-stream Cirrus’, these radiatus varieties of Cirrus can be spread over great distances by the high winds. Occasionally they appear to extend all the way from one horizon right overhead to the opposite one. The perspective causes the cloud rows to bulge dramatically above, while converging at ‘radiation points’ on each horizon. Such an impressive radiatus formation will be a source of great pride for any cloud collector but it is practically impossible to photograph in its entirety, since it stretches over such a large part of the sky.

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Virga

When you look up to find jellyfish floating above, you are either diving or beneath the cloud known as virga.

In essence, this is just a cloud raining or snowing, but with one important difference: the precipitation never reaches the ground. If the droplets or ice crystals (or anything between the two) fall through air that is warm enough and/or dry enough, they can evaporate before ever landing.

The appearance of virga from the ground is of trails that hang down like tentacles from a clump or layer of cloud, waving not in the currents of the ocean, but in those of the atmosphere. When virga occur below low-level clouds, they are composed of water droplets, and appear grey. When they consist of ice crystals, having fallen from mid- or high-level clouds, they have a much paler appearance. But beware: this distinction is a tenuous one, because our eyes judge colour and tone relative to the brightness of the background. The same trail of virga can appear whiter or greyer depending on the sky behind. Fallstreak holes are specific cases of virga falling from a layer of supercooled droplets to leave a hole behind.

When a cloud’s precipitation can be seen to reach all the way to the ground, it is no longer called virga, but ‘praecipitatio’.

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Fog

The distinction between fog and mist relates to visibility. Officially, you can see 1–2km in mist but no more than 1km in fog – one’s just a thicker version of the other. Though fog is sometimes described as ground-level Stratus cloud, since that’s the lowest of the main clouds, it often forms quite differently.

Fog appears if air is cooled enough by its proximity to the ground or water surface for its moisture to condense into droplets. There are two main ways this cooling can happen.

‘Radiation fog’ forms after long, cold and clear nights. With no blanket of cloud cover to keep the warmth in, the ground quickly radiates the day’s warmth into the night sky, and can cool the air enough to form droplets. On higher ground, the cold, foggy air can sink downhill and gather as ‘valley fog’.

‘Advection fog’ occurs when air cools as it blows over a warmer surface to a colder one. If these are ocean surfaces, it’s called ‘sea fog’. Then there’s ‘steam fog’ – when cold air blows over warmer water, such as a lake, and the water vapour that evaporates off the surface instantly cools to form droplets.

That’s not the end of it. There’s also ‘upslope fog’, ‘hill fog’, ‘ice fog’, ‘haar’ and ‘frontal fog’. No matter which one it is, cloudspotters will never get closer to a cloud than when they’re enveloped in fog or mist.

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Volutus

This is a long, low tube of cloud, which can appear to extend horizontally from horizon to horizon. Known more generally as a roll cloud, it often has a very smooth, silky surface. At other times, it can appear quite rough and bumpy. Volutus can move at speeds of up to 35mph (55km/h), with the roll appearing to rotate as it travels along. The direction of rotation is not as it would be for a solid tube rolling along the ground. In fact, the roll cloud rotates against its direction of travel – the cloud surface lifting at the front and dropping down at the back.

One famous volutus, the Morning Glory cloud, appears in Northern Queensland, Australia. This forms in a solitary wave of air and is caused by colliding sea breezes over the Cape York Peninsula. Volutus are often associated with sea breezes, and so they tend to be found in coastal waters. At other times, volutus are caused by storm systems. In this case they are rather like arcus, or shelf clouds, that have become detached from the storm system that produced them. As the storm dissipates, gusting winds of cold air can continue to spread out ahead of it, and form a roll of cloud that separates away from the rest of the storm.

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Fibratus

The high, ice-crystal clouds of Cirrus and Cirrostratus are called fibratus when they have been drawn out by the wind into long, fine filaments. These close strands of cloud appear rather like hair run through with a comb. Such an orderly atmospheric hairstyle depends on high, continuous winds. These are more common up at Cirrus and Cirrostratus level, since the higher you climb through the troposphere, the faster the average wind speed becomes, and the less the wind is messed about by the influence of the ground.

The way to distinguish fibratus from the other Cirrus species that can also have somewhat parallel filaments, floccus and uncinus, is to look at the ends of the strands. In fibratus, the filaments do not descend from the fluffy tufts of cloud found in floccus, nor do they curve down from thicker heads to give the hooked, comma-like appearance of uncinus. Fibratus are simply thin, delicate strands of high cloud.

As expressions on the face of the sky, clouds can be indicators of the atmosphere’s moods, but not so in the case of fibratus clouds. Other than indicating high, continuous winds up at cloud level, they tell nothing of the weather in store. Perhaps they are just there to look nice.

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Cirrocumulus

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

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

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

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Cirrostratus

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

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

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

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Cirrus

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

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

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

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Undulatus

When the surface of a cloud layer, or the arrangement of its cloudlets, develops an undulating appearance that suggests waves, it’s defined as the undulatus variety.

Waves and clouds have always had a close relationship. The interaction of currents in the atmosphere, and the effects of the terrain on the passage of winds, can result in a whole range of undulating currents of air. Generally, these are invisible, unless the rising parts of the undulations cool the air enough to produce clouds of droplets or ice crystals, which are thinner or absent in the sinking parts of the undulations. In such circumstances, the waves show up on the surface of the cloud or as cloud billows with gaps in between.

Undulatus usually forms when the air above and below the cloud layer is moving at differing speeds and/or in different directions. It is the shearing effect of the two airstreams that gives rise to the cloud billows, which form perpendicular to the wind direction and can resemble ripples on a sandy beach caused by the movement of water.

Wave formations in clouds are so common that the undulatus variety is within six of the ten main cloud types. Their presence is a reminder, to any who might forget, that the atmosphere around us is just as much of an ocean as the sea below.

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Lenticularis

Lenticularis clouds are contenders for the Weirdest-Looking-Clouds-in-the-Sky awards. Their name is Latin for a lentil, on account of their very distinctive lens shapes. They often look remarkably like flying saucers. Presumably, when they were named, no one could think of the Latin word for ‘shaped like a UFO’.

Lenticularis can be found at low, medium and high cloud levels, although the most striking and dramatic ones tend to be the mid-level Altocumulus lenticularis. At whatever altitude they form, they are usually caused by a moist airstream flowing over raised ground, such as a hill or mountain peak. When the atmosphere in the area is stable, the air can develop a wave-like motion downstream, invisibly rising and dipping in the lee of the peak. If the air rises and cools enough, lenticularis clouds can appear at the crests of these waves. Unlike most clouds that drift along with the breeze, these hover even in the strongest winds (so long as air speed remains constant). Their positions in the airstream remain fixed, like the stationary waves of water behind a boulder in the current of a fast-moving stream.

When the airstream contains layers of moist air separated by drier air, a stacked formation can appear, known as ‘pile d’assiettes’ (which is French for ‘your turn for the washing up’).

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