Published: 2007-12-09 11:55:00
Updated: 2010-05-10 16:19:26
Posted December 9, 2007
Updated May 10, 2010
By Mike Moss
Question: Today around 3:30 pm, my kids and I saw a rainbow around the sun. I've never seen this before. What is it, is it common, etc.? P..S: It wasn't immediately around the sun, but out a ways. – Lori Holleman
Answer from Mike Moss: Based on the time you gave, I took a look at weather observations and found that the first cloud layer that covered most of the sky was about 25,000 feet up. That means it was a cirriform cloud that was composed principally of ice crystals.
Your observation of a well formed circle a ways out from the sun, and containing some rainbow colors, makes it sound very much like a 22-degree halo, which is formed when light shines through a fairly thin layer of clouds that happen to be composed of well-formed hexagonal ice crystals that are poorly aligned (i.e. they are not all oriented in a similar direction).
Refraction of the light through these crystals produces the halo, and the fact that differing wavelengths of light are refracted to a slightly different angle separates the red end of the spectrum from the blue. In some cases, otherwise similar ice crystals are shaped in a way that leads them to mostly be oriented in the same direction. When that happens, you will not see a circle around the sun but instead other phenomena such as a circumzenithal arc or parhelia (also called sun dogs). Other times the crystals may not be well-formed or the cloud may be too thick to transmit sufficient light, in which case no halo will be visible.
You can read more about how all these interesting displays occur, and see a lot of nice photos as well, here.
By the way, although we don't always notice them, because sometimes they are faint, partial or occur at night, halos are not especially uncommon. For our area they probably occur around 80-90 times per year.