Atmospheric Optics is an excellent resource for rainbows and similar phenomena. Their page on 22-degree halos says:
22º radius halos are visible all over the world and throughout the year. Look out for them (eye care!) whenever the sky is wisped or hazed with thin cirrus clouds. These clouds are cold and contain ice crystals in even the hottest climes.
The halo is large. Stretch out the fingers of your hand at arms length. The tips of the thumb and little finger then subtend roughly 20°. Place your thumb over the the sun and the halo will be near the little finger tip. The halo is always the same diameter regardless of its position in the sky. Sometimes only parts of the complete circle are visible.
Much smaller coloured rings around the sun or moon are a corona produced by water droplets rather than ice crystals.
Lots more at Atmospheric Optics!
More rainbows, sundogs, etc. on Michigan in Pictures – seriously cool stuff in here folks!