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Precipitation Amount: Showers

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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.

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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

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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.

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Rainbow

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’.

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Cumulus

If you’ve never spotted a Cumulus cloud, then you need to get out more. This has to be one of the easiest types to add to your cloud collection (which explains why it only earns one star). Cumulus clouds are the cotton-wool puffs, with flat bases, that drift lazily across the sky on a sunny day. Generally forming a few hours after daybreak, they tend to dissipate before sundown, for they form on thermals – invisible columns of air rising from the ground as it is warmed by the sun.

Most forms of Cumulus produce no rain or snow, and so are known as fair-weather clouds. But in unstable air, their bright, crisp cauliflower mounds can build upwards so that they develop from the small humilis species, through mediocris to the largest form, Cumulus congestus. With its ominous, shadowy base, this cloud is no longer fair-weather. Congestus can produce brief but sizeable showers, and can keep growing into fierce Cumulonimbus storm clouds.

The little ones, by contrast, are scary only when they take the form of David Hasselhoff.

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