I’ve been meaning to post this for quite some time; I’ve had it floating around on my phone; I took it while on a walk on a cold, spring day.  I thought it quite remarkable, but I didn’t know what it was.  Despite the church in the foreground, I was fairly certain its caused could be chalked up to physical, rather than supernatural mechanisms.

Fortunately, Wikipedia was already on the case!  The main ingredient necessary for this phenomenon is hexagonal ice crystals.  When aligned perpendicular to sunlight, visible rays passing though two (non-adjacent, non-opposing) faces will deflect light approximately 22 degrees.

Since the index of refraction of the ice is actually a function of wavelength, the longer wavelength red light will deflected slightly less than the higher frequency blue light.  Hence, in the picture, you can see the inner part of the halo being redder, and the outer edge as bluer.  This is not unlike the color-dependent refraction responsible for producing the colors in a rainbow.  However, unlike a rainbow, there is no internal reflection of light necessary; hence, the halo appears on the same side of the sky as the sun, where a rainbow appears on the opposite side of the sky.

So, even if it’s too cold for a rainbow, that doesn’t mean there isn’t awesome stuff going on overhead; just be careful not to look too closely to the sun!