Red rainbows exist, but they're extremely rare. Here's the science and how to see one for yourself

Red rainbows – or monochrome rainbows – can be visible in the sky, but there's a good chance you’ve never seen one because they’re vanishingly rare.

'Ordinary' rainbows, as we’re sure you remember from geography or physics lessons at school (or indeed, our article on what causes a rainbow), appear when sunlight passes through droplets of water in the atmosphere – i.e. rain.

These raindrops refract light passing through them, so that light entering the water droplets as a single beam of white light exits it having been separated out into its component colours.

As a result, we appear to see a spectrum of colours on the sky – a rainbow, in other words.

What causes a red rainbow

Red rainbows form in exactly the same way as regular rainbows, but they only appear when the Sun is low in the sky.

At such times, sunlight has to travel a greater distance through the atmosphere before it reaches our eyes.

Along the way, it encounters molecules of dust and gas – and the more molecules it encounters, the more the light is scattered by them.

But not all wavelengths of light are scattered equally: shorter wavelengths of light (equating to colours like green, blue, indigo and violet) are scattered the most, while longer wavelengths (red, orange and yellow) are scattered less.

Eventually, if the Sun is low enough and its light is travelling far enough through the atmosphere, then it will be ONLY the reddest components of sunlight that make it through the atmosphere at all.

So when that red light is refracted by raindrops as we explained earlier, we see a rainbow that’s entirely red (or shades of red, pink and orange), as opposed to one made up of the full spectrum of colours.

It’s essentially the same phenomenon that causes red sunsets.

But while red sunsets are common, red rainbows are much less so.

Many people can spend a lifetime studying the skies and only ever see one or two of them.

So if you spot one, count yourself lucky – and grab a camera if you can!

Other types of rainbow

Incidentally, red rainbows aren’t the only ‘variant’ rainbow that meteorology has to offer.

Other types include double, twinned, supernumary, full circle, reflected, reflection and higher-order rainbows.

Not to mention sleetbows, fogbows, moonbows and circumhorizontal/circumzenithal arcs (more on this in our guide to daytime astronomy).