Why This Is Not a Rainbow

Yesterday's spectacular display is an example of a halo, not a rainbow

Yesterday's spectacular display of colors in the noontime sky caught the attention of many people in the Salt Lake Valley and was widely covered by the media.  Upon further reflection, I'm thinking this was a classic circumhorizontal arc rather than an infralateral arc.  In either case, the phenomenon is an example of a halo and not a rainbow, despite the rainbow-like color spectrum.

Rainbows are produced by the interaction of visible light with water droplets, typically rain drops, although it's fairly easy to make a rainbow using the mist a garden hose if the sun is behind you.  As the light moves through these droplets, it is bent and reflected in a way that yields the classic red-to-violet color spectrum.

Halos are produced by the interaction of visible light with ice crystals.  In the photo above, the fibrous nature of the cloud that contains the halo is an indication it is comprised of ice crystals.  In contrast, clouds that are comprised primarily of water droplets usually have a smoother, harder appearance, as is the case for the alto-stratus cloud further to the left (east) in the photo.  Typically the cloud needs to be thin to produce a halo, as is the case above.  In addition, the ice crystals must have smooth faces – which means they are usually hexagonal plates, hexagonal prisms, or needles (click here for further discussion).