Family: Optical Phenomena

Reflection bow

A reflection bow is a form of rainbow that appears between the primary and secondary rainbows and looks to be bent at an odd angle. Like the usual rainbows, this rare optical effect is formed by the reflection and refraction of sunlight shining from behind the viewer onto a shower of raindrops up ahead. But in the case of the reflection bow, the sunlight that forms it has been reflected up off a surface such as a smooth body of water behind the viewer. Thanks to the reflection, the bow appears at a different angle from the other rainbows since it is being formed as if by light shining from a Sun beneath, rather than above, the horizon.

Should you are ever lucky enough to spot a reflection rainbow – and you’ll have to be rainbow spotting with a large body of water behind you – you’ll find that a reflection bow always meets its regular-rainbow counterpart at exactly the level of the horizon.

Supernumerary bows

Supernumerary bows are repeating fringes of colours that can sometimes appear along the edge of a rainbow. They form at the inside of the primary rainbow. Where an outer, secondary bow is also present, faint supernumeraries can sometimes be seen along its outer edge. The fringes are caused by the interference of light waves emerging from the raindrops leading to brighter and darker bands. Supernumerary bows only appear bright like this when the raindrops are quite small and of consistent size. They are predominantly coloured purple, pink and green

Rainbow wheel

When a rainbow appears as if pierced with rays of light, the effect is called a rainbow wheel. This is because the rays can occasionally appear right around the whole arc of the bow. They are forms of anti-crepuscular rays, shadows cast from tall clouds in front of the Sun, which is shining from behind the viewer.

These rays of light and shadow appear to converge as they recede towards the horizon. This is just perspective – they are in fact parallel to each other – and it means they seem to fan out from the ‘anti-solar point’, which is in the opposite direction from that of the Sun. A rainbow is always centred on the anti-solar point too, and so the rays appear within it like spokes of a bicycle wheel. A very hippy-looking bicycle wheel.

Supralateral arc

The high, up-turned bow of the ice-crystal optical effect called a circumzenital arc is on rare occasions joined by a fainter and much rarer light effect. Known as a supralateral arc, this is the far broader bow than that circumzenithal arc, and it curves downwards rather than up. It only tends to appear about once a year over mainland Europe, according Claudia and Wolfgang Hinz of the German halo observation network Arbeitskreis Meteore, compared to the average of six occurrences of circumzenithal arcs.

The supralateral arc has the potential to be a very broad arc of colour stretching across a lot of the sky, and so a particularly dramatic example will have ends that extend down into people’s normal field of vision. But it will only be that broad when just the right sort of ice crystals are distributed over enough of the sky. More typically, just a central segment of the arc appears up high in the sky above, positioned at a tangent to a circumzenithal arc. You are only likely therefore to spot a supralateral if you choose to pay attention to what happens up in the sky high above your head. The rare supralateral arc, like its circumzenithal arc sister, is a treasure hiding in plain sight from all but those with their heads in the clouds.

Tangent arc

Some halo phenomena like the tangent arc vary in shape depending on the elevation of the Sun. Tangent arcs tend to form about once a month in the skies over temperate regions, so they are relatively common. They can appear either above or below the Sun, always positioned so that the centre of the arc is just touching that most common of halo phenomena, the 22-degree halo. So if a 22-degree halo around the Sun is visible at the same time the tangent arc will be touching it tangentially.

When the Sun is low on the horizon the tangent arc is shaped like a ‘V’. As the Sun climbs higher in the sky it has a more flattened shape, its edges curving more and more around the Sun.

Tangent arcs below the Sun are usually only seen from elevated positions like a mountainside or an aircraft. When the angle of the Sun above the horizon is 30° or higher, the ends of the upper and lower tangent arcs will have curved around the Sun so much that they appear to join together to form a continuous oval shape. These joined upper and lower tangent arcs are then known as a ‘circumscribed halo’.

Are you all paying attention at the back of the class?

Parhelic circle

A parhelic circle is a white band light that runs parallel to the horizon. Caused by sunlight passing through, and reflecting off, the sides of tiny hexagonal plate-shaped ice crystals, the light effect always appears at the same height in the sky as the Sun. Though it usually only appears in patches of the sky, it can occasionally extend 360° around the whole sky. When the Sun is high, this appears as a broad ring centred on the zenith. When the sun is lower, it appears more as more of a horizontal band encircling the sky. The parhelic circle tends to be a very fleeting optical effect, rarely lasting for long. It is most frequently observed as one or more extensions running between the Sun and the spots of light known as parhelia, or sun dogs, that give the effect its name.

Circumhorizon arc

When you see what looks like a rainbow that has been flattened into a horizontal line, you have likely spotted a ‘circumhorizon arc’. The optical effect is caused by ice crystals in the sky acting as tiny prisms that refract and reflect the light, and it appears as a bright, flat band of colours parallel to the horizon. Ranging from red at the top to indigo at the base, it appears well below a Sun that is high in the sky. In fact, a circumhorizon arc can only form when the Sun is very high – at an angle of more than 58 degrees above the horizon. This means that it is a summer optical effect, and one that can only form in some parts of the world. Even in mid-summer, the Sun only rises high enough in the sky for a circumhorizon arc to be visible in latitudes below 55 degrees. As a result, this ‘flat rainbow’ that has nothing to do with rain is common in Nairobi, infrequent in Seattle, and unheard of in Copenhagen.

Anti-crepuscular rays

When you spot those lines of light and shade across the sky, known as crepuscular rays, you should turn around and have a check in the opposite direction just in case you manage to see their far subtler counterparts, ‘anti-crepuscular rays’. Both forms are the shadows cast by clouds that have been rendered visible by atmospheric haze. This might be dust, pollen, pollution or just a subtle layer of water droplets, and it scatters the sunlight enough to reveal the path of the Sun’s rays through the atmosphere.

Anti-crepuscular rays appear when tall clouds in front of the rising or setting Sun cast long shadows across the sky which can be seen receding off into the distance when facing away from the Sun. Just like the more familiar crespuscular rays, these shadows appear to converge towards a point near the horizon. The shadows are in fact parallel, and this just the effect of perspective.

On rare occasions, you can spot crepuscular and anti-crepuscular rays at the same time. Looking towards the sunrise of sunset, the crepuscular rays appear to fan outwards from the low Sun. They look wider apart as they pass overhead. Then they converge again towards a point on the horizon opposite the Sun, where they recede into the distance as anti-crepuscular rays. It is impossible to represent this in a photograph. You sort of have to be there – and you should count yourself lucky if you ever are.

Circumzenithal arc

The circumzenithal arc is a halo phenomenon that appears like a multicoloured smile in the sky. Photographs of it look as if some fool’s got a rainbow snap upside down, but this bow of colours actually appears in a totally different part of the sky from rainbows. On the 25 or so times a year that it appears (in Western Europe), it forms high up in the sky, like the fragment closest to the Sun of a circle around the zenith (the point directly above you).

Whenever you notice the spots of light on either side of the Sun called Sun dogs, always look directly up because you might also be able to add this most beautiful of all halo phenomena to your collection of cloud optical effects, for it is produced by the same cloud ice crystals. It appears as sunlight is refracted by the ice crystals of thin layers of high clouds, such as Cirrus, Cirrostratus and Cirrocumulus, or the ground-level ice-crystal cloud, diamond dust.

Sun pillar

A sun pillar is a vertical streak of light that appears above or below a low Sun that is shining through ice-crystal clouds, such as Cirrus, Cirrostratus and Cirrocumulus, or the ground-level ice-crystal fog, diamond dust. At night, they are known as ‘Moon pillars’.

These halo phenomena, which appear on about 25 days of the year (in Western Europe), are due to sunlight reflecting off the surface of ice crystals. They are akin to ‘glitter paths’ that shine on the rippled surface of the sea. The pillar extending above the Sun appears brightest when the Sun is just below the horizon.

Most halo phenomena look best when the clouds’ crystals are optically pure, regularly shaped and neatly aligned, but this is not the case for Sun pillars. The light needs only to glance off a surface, so the crystals can be rough, irregular and jumbled. The poor man’s halo phenomena, they often appear when cloud crystals aren’t quite right for the more refined arcs, rings and spots of light to form.


Cloudbows are like rainbows, but with much paler colours. In fact, they often don’t show any discernible colours at all – looking like albino rainbows, or the ghosts of rainbows past. They appear as the sunlight is reflected and refracted by the tiny droplets that make up low and mid-level clouds, rather than the much larger raindrops that produce rainbows.

In order to see a cloudbow, you have to be looking towards cloud droplets on to which the Sun is shining from directly behind your line of vision. Such a viewpoint is possible only from above clouds, which is why cloudbows are usually seen from a plane, or a mountaintop. These are the same conditions in which to see the multicoloured ring around your shadow, called a glory. If you see one, look for the other.

CloudSpotters wanting to add an albino rainbow to their collection, without taking to the air, should seek ‘fogbows’. These are exactly the same, but appear in fog or mist.

Cloud/fogbow colours are pale or absent because, at less than 0.1mm across, cloud droplets diffract sunlight more efficiently than larger raindrops, causing the colours to overlap and blur into each other more than those of a rainbow.


CloudSpotters must gain some elevation to add a glory to their collection of cloud optical effects, for this striking phenomenon is seen only with the Sun directly behind you, as it casts your shadow on to a layer of cloud. The glory, which looks like a series of rainbow rings around the shadow, is produced by cloud droplets reflecting, refracting and diffracting sunlight, although the exact optics are still not fully understood.

One of the easiest places to spot a glory is from that great cloudspotting location, the window of a plane. It can sometimes appear around the plane’s shadow, cast on to a nearby layer of cloud or fog. When the cloud’s some distance away, the shadow is absent and you just see the coloured rings.

The most eerie form of glory, the ‘Brocken spectre’, is when the rings appear around your own shadow as you look at cloud from a mountain ridge. The perspective can make the legs of your shadow flare out. What with the multicoloured halo, it looks like a ghost from the 1970s.

Crepuscular rays

Even if they don’t know the name, most CloudSpotters will have plenty of opportunities to add crepuscular rays to their collection of cloud optical effects. They’re the familiar Sunbeams that appear to burst from behind a Cumulus cloud, or shine down through a hole in a Stratocumulus.

Crepuscular rays appear when the path of sunlight is made visible by tiny atmospheric particles, too scarce to appear as cloud, but plentiful enough to scatter the light noticeably. Like fingers through a torch beam, the cloud shadows give edges to the rays. In spite of being almost parallel, these rays seem to radiate outwards from behind the cloud. This is just the same perspective effect as railway tracks seeming to widen as they get nearer.

Whenever you notice crepuscular rays from a low Sun, look to the opposite horizon for the far less obvious ‘anti-crepuscular rays’. Appearing to emanate from a point directly opposite the Sun, these are the shadows cast by clouds behind you, like the shadow of someone shuffling behind you in a dusty cinema. Perspective makes them appear to converge in the distance. Few people ever notice anti-crepuscular rays – except vampire CloudSpotters, eager for the arrival of night.

Sun dog

Also known as ‘mock suns’ and ‘parhelia’, sun dogs are large spots of light that can appear on one or both sides of the Sun, and at the same level as it. The second most frequent halo phenomenon after the 22-degree halo, sun dogs appear on about 70 days of the year (over Western Europe). They are formed as sunlight is refracted through the ice crystals of thin layers of high clouds, such as Cirrus, Cirrostratus and Cirrocumulus, or those of the ground-level ice-crystal cloud, diamond dust. The distance from the Sun of both spots of light is equivalent to the outstretched span of a hand held up at arm’s length. Sun dogs are brightest when the Sun is low, and visible only when it’s below about 40˚ from the horizon. If the cloud isn’t in the right parts of the sky, just one sun dog appears.

CloudSpotters should learn to recognise the sky that tends to produce sun dogs, as well as the other halo phenomena. Paying careful attention not to stare into the Sun, as it can damage your eyes, scan the blue for light effects when the sky first pales with the subtle veil of ice-crystal clouds. Once these high clouds have become white enough to be noticed by the riffraff, they’re generally too thick to produce sun dogs.


Iridescence is the beautiful effect of bands of pastel colours that can appear when sunlight or moonlight passes through thin cloud. Also known as ‘irisation’, these mother-of-pearl colours are caused by the light being diffracted as it passes through the cloud. It is the same process that gives rise to coloured discs around the Sun or Moon, called coronae. The light waves are dispersed as they pass around the cloud’s tiny droplets or ice crystals, with different wavelengths being spread out by different amounts. Each wavelength can also produce an interference pattern of light and dark fringes. All this means that the sunlight is separated into alternating fringes of colour.

Any thin cloud with droplets or ice crystals small and uniform enough can produce iridescent colours. Best viewed through Sunglasses, the bands of pastel hues are reminiscent of those on oil slicks, and can be seen over thin Stratocumulus, Altocumulus, Cirrocumulus and Altostratus. Nacreous clouds in the stratosphere, high above most weather clouds, show the most intense iridescence of all.

The colours can also appear at the edges of clouds thick enough to block much of the sunlight, such as lenticularis. To claim that every cloud has a silver lining is therefore wrong – some have tutti-frutti coloured ones.


Look towards the Sun shining through thin cloud and you might find that it is surrounded by a corona. This is a bluish-white disc of light with a ruddy outer edge, often surrounded by rings of iridescent colours. CloudSpotters should be careful to protect their eyes by blocking the Sun with a hand. Coronae can be seen less painfully when clouds drift in front of a bright Moon.

Closely related to cloud iridescence, coronae are caused when the light is diffracted as it passes around a cloud’s particles. Only if these are all very small and the cloud layer is thin will the colours of the corona appear distinct around the central bright disc. The smaller the cloud droplets, the larger the corona.

CloudSpotters should take care not to confuse a corona with a 22-degree halo. Not only is the corona much smaller (the outer edge usually being less than 5˚ from the Sun or Moon – the width of three fingers, at arm’s length), it also has a bright central disc, or ‘aureole’, while the halo is just a ring of light. Nor should they confuse it with a glory, which appears in the opposite direction, looking away from the Sun.

Coronae can also be seen around car headlights viewed through a windscreen misted up with condensation. Anyone claiming credit for this is a terrible cheat.

22-degree halo

The 22-degree halo is the most frequent of the many halo phenomena that can appear as sunlight is refracted through the ice crystals of thin layers of high clouds, such as Cirrus, Cirrostratus and Cirrocumulus, or the ground-level ice-crystal cloud, diamond dust. Appearing on about 100 days of the year (in Western Europe), the 22-degree halo looks like a large ring around the sun or moon. Its inner edge generally has a reddish tinge to it, with the region of the sky between halo and sun appearing darker than that just outside the halo. When cloud cover is less than extensive, only parts of the halo appear.

CloudSpotters observing a 22-degree halo for the first time will be surprised at how much larger it appears compared with photographs. This can present challenges when trying to photograph one on your phone. The distance from sun or moon to the edge of the ring is equivalent to the outstretched span of a hand held up at arm’s length.

Though it’s worth two stars, a 22-degree halo is not worth screwing up your eyesight for, so take especial care never to stare directly at the sun. Be sure to photograph from the shade of a tree, lamppost or building in order to block out the sun itself, and reveal the part of the sky showing the halo.


We all love rainbows, but don’t expect to win any awards for spotting one because they’re just too easy to notice. How ironic that rainbows, appearing on average about ten times a year (over Western Europe), are actually less frequent than some halo phenomena (caused by cloud ice crystals, rather than raindrops) that most people never even notice.

To see a rainbow, look towards a rain shower with your back to the sun, which must be no higher than 42˚ above the horizon (unless you are looking down onto rain, say from a mountain or high building). Passing into each raindrop and reflecting off the back inner surface, the sunlight is refracted as it enters and leaves each drop. The paths of its constituent wavelengths are bent by different amounts, separating out the colours. Convection clouds like Cumulus and Cumulonimbus are the best sort for making rainbows, as they’re more likely to produce showers when the sky around is clear, allowing direct sunlight to shine on them.

Besides the primary bow, a larger, fainter secondary bow can appear – the darker sky between the two being known as ‘Alexander’s dark band’. Within the primary bow, there are sometimes faint coloured fringes, called ‘supernumerary bows’.