Have you ever looked up in the sky and wondered "What on earth is that?" I can still remember the first time I saw a little rainbow in the sky not far from the sun. Hang on, said my inner physicist, when you see a rainbow the sun should be behind your head. So what was I looking at? It's time to say Halo to optical phenomena...
Paint the whole world
Let's start with everybody's favourite, the good old rainbow. These form when sunlight falls on water droplets, normally rain drops but sometimes spray from a waterfall, a fountain or even fog will do the trick.
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Some of the light gets reflected off the drop while some enters the drop, bounces off the back and passes back out the drop again, towards us.
Just as when white light enters a prism, sunlight entering a water drop is split into its component colours. Each colour is refracted, or bent, by different amounts depending on its wavelength - red light bends the most, orange and yellow slightly less and so on down to violet, which bends the least.
It's the differences in wavelength that dictate the order of colours in our rainbow, so we end up with red on top and violet on the bottom.
Circle of colour
You most often get rainbows in the classic weather mix of sunshine and showers. When sunlight hits the rain drops at just the right angle you see a rainbow, but only if the sun is shining behind you which is why rainbows are only seen when the sun is low in the sky.
In the early morning rainbows will appear to the west, while in the afternoon they will be seen to the east.
Explaining the physics behind the shape of a rainbow has kept many a scientist busy over the centuries. Most of us only ever see part of the rainbow arc or if you're lucky a semicircle.
A rainbow's shape is actually a circle centred on the point directly opposite the sun from you - the anti-solar point - with an angular radius of 42 degrees. For the really keen rainbows technically form a cone shape but as we only see it as a circle let's stick with that.
Pot of gold
Most of the time we can't see the full circle because the earth gets in the way. The closer the sun is to the horizon, the more of the circle we see. Right at sunset, we would see a full semicircle of a rainbow with the top of the arch 42 degrees above the horizon.
The higher the sun is in the sky, the less of a rainbow is visible above the horizon. The higher you are, say on top of a mountain or in a plane, then you might see the whole circle.
Because the position of a rainbow is centred on the anti-solar point and that is determined by your position, everyone sees their own personal rainbow. As you move, so does the bow, making it impossible to reach that pot of gold.
Of course, with a circular shape there isn't an end to the rainbow anyway as it only appears to touch the ground.
Double the luck
Sometimes more than one rainbow is visible, with a second fainter rainbow visible outside the main one. This secondary rainbow is formed by rays of light that have been reflected inside the rain drop twice - most likely due to irregular shaped water droplets.
The extra reflection means the colours in the secondary rainbow are in the opposite order to the main rainbow.
You can even see rainbows at night if the moon is shining brightly enough, although the colours in a moonbow are so pale and faint they look almost white.
Halo rainbow
Back to what I spotted in the sky many years ago, sparking my interest in meteorology. My not-a-rainbow was in fact part of a halo, formed by ice crystals high in the atmosphere.
Ice crystals reflect and refract light, rather like water droplets, and the resulting optical phenomena depends on the shape of the ice crystals.
Haloes are circular arcs around the sun or moon. They are actually pretty common and can be seen roughly one day in three in the UK. They tend to form whenever there is a thin veil of cirrostratus cloud high in the sky, but as the sky is still so bright most people don't realise clouds are moving in.
This kind of cloud often heralds an approaching warm front so spotting a halo can indicate rain is on the way.
Catch 22
The most common halo is a circle with a radius of 22 degrees, produced when light passes through two non-adjacent faces of a hexagonal ice crystal, with an included angle of 60 degrees.
These crystals normally have completely random orientations so that the light is spread in every direction and thus appears as a circle at a fixed distance from the sun.
Natural variations in the amount and nature of the cloud that is present means the circle is often incomplete or varies in strength.
When faint, the halo ring will appear white but when stronger the inner edge will be red surrounded by a yellow tint and sometimes a violet colour is spotted on the outside.
So we can get our colours of the rainbow but in the opposite order. The easiest way to tell the difference between a halo and a rainbow is by locating the sun.
Haloes lie on the same side of the sky as the sun whereas a rainbow always appears around the anti-solar point.
Sun dog
A bright spot of light that appears on one side of the sun is called a parhelion, or more commonly a sun dog, false sun or mock sun. These are the next most common halo phenomena after the 22-degree halo.
Usually only one sun dog is spotted but you might see one on each side of the sun and they are often the same size if not larger than the sun making them quite conspicuous.
Their exact position relative to the 22-degree halo depends on the sun's altitude. When the sun is low they lie on the halo, but the higher the sun the further the parhelia lie outside the 22-degree halo until the sun's altitude reaches approximately 60 degrees and they disappear.
Parhelia are produced when light is refracted through the horizontal flat plates of hexagonal ice crystals. These ice crystals often end up with their large flat surfaces lying horizontally as they fall through the air.
Because the crystals share a particular orientation the sun dog can be brilliantly coloured and is usually spotted in small patches of cirrus cloud.
Rainbows, haloes and sun dogs are just a few of the many amazing optical phenomena out there. So keep your eyes to the skies and hopefully you'll spot something beautiful (even in the rain).