What is the difference between sun halos and sundogs?

Sundogs are sun halos - there are loads of similar effects caused by the same processes, known collectively as atmospheric optics.

The sun halos you have probably seen are like huge circular rainbows around the sun, or perhaps an arc directly overhead. The former is called a 22-degrees halo, and the latter a circumzenithal arc - but there are many more halos that can form.

Check out www.atoptics.co.uk for a more comprehensive guide to these, along with great explanations and pictures from around the world.

Sundogs, also known as parhelia, are just one of these effects, caused by light passing through ice crystals in clouds (or at ground level) and being refracted to split up the colours - just like light going through a prism. The collective glints of millions of crystals bending the sunlight combine to make all these fascinating and beautiful phenomena, which can be seen from anywhere in the world at least part of the year.