How many different types of rainbow are there?

Bolton Credit: Peter Hunter

The phrase "sunshine and showers" is a favourite amongst weather presenters, and we seem to have our fair share across the UK throughout the year. As well as bringing downpours one minute and glorious sunshine the next they are often also responsible for rainbows, which come in more forms than you might think...

Double rainbow from Glastonbury Tor Credit: Michelle Cowbourne

To see a rainbow it must be sunny behind you and raining in front of you. As sunlight passes through raindrops, some is reflected and split, creating the arc of colour from red through to violet. Rainbows are best viewed early and late in the day when the sun is low in the sky. They're also unique to each person as we see them from a slightly different place to someone else.

Although not always visible, most rainbows have a second arc above them which is fainter and more subtle in colour. Look closely and you'll see that the colours in the second arc are flipped, merging from violet to red!

All rainbows are complete circles but because we usually view them from the ground we only see part of them. Get yourself up a mountain or in a plane and you'll have a different perspective entirely.

A full circle rainbow Credit: Colin Leonhardt

It doesn't even have to be raining for a rainbow to form. Fogbows are a type of rainbow formed as sunlight passes through the tiny water droplets that make up mist and fog. The arc is the same shape but the colours are much more subtle because the light is spread out much more, broadening and smudging the colours together.

A fogbow, or 'white rainbow' Credit: Calum MacColl

Other rainbows that don't appear to have as many colours include:

  • Moonbows - rare and faint rainbows formed from the brightest light of a full moon.

  • Monochrome, or red rainbows, which form late in the day when the sun is very low and most of the light has been scattered, leaving only the red part of the spectrum.

Monochrome or 'red rainbow' Credit: Lorraine Horton

Some rainbows aren't even the right way around! A Circumzenithal arc is formed when sunlight passes through ice crystals in high level cirrus cloud. The precise angle the light hits the ice, and the position we end up looking at it from, creates an upside-down arc instead.

Circumzenithal arc, or upside-down rainbow Credit: Brocken Inaglory

There are many other optical effects that have rainbow characteristics, most of which also involve ice crystals and cirrus cloud. A Parhelion, or sun dog, occurs when light passes through hexagonal ice crystals, usually in cold weather and when the sun is low in the sky. The result is an area of light either side of the sun that has a faint colouration to it.

A parhelion or sun dog

A similar effect can be seen around the sun when there's a thin veil of cirrus cloud in front of it. A halo of light forms which sometimes has a very subtle rainbow colour. Possibly the most intriguing of all optical phenomena is the Brocken Spectre; an eerie shadow of a human form in the mist surrounded by a rainbow.

A Brocken Spectre in the mist Credit: Richard Webb

Next time you're up a hill or mountain, above a layer of mist or fog, have a look down onto it. Under the right conditions, with the sun behind you, your shadow will be cast onto the mist way off in the distance and appear gigantic. The sunlight may also be reflected by the tiny water droplets within the mist to split the light into a rainbow around your shadow.