22° halo
A pale ring around the sun, red on the inside, made by sunlight bending through ice crystals.
WhenAny season; commonest with thin high cloud ahead of a frontWhereA ring around the sun or moon, about a hand span in radius at arm's length
Why it happens
Sunlight passing through the six-sided ice crystals of high cloud is bent by at least 22 degrees, so a ring of that radius lights up around the sun. Red light is bent least and sits on the inside; the outside fades to white and then to a slightly darker sky.
How to see it
Stretch your arm out and spread your hand: with your thumb on the sun, the ring passes roughly by the tip of your little finger. Halos are far more common than most people think, once every few days in many places, and nearly always mean thin ice cloud overhead.
Field note. Never stare at the sun. Hide the disc behind a hand, a roof edge or a lamp post and look at the sky around it.