from The Century Dictionary.
- Surrounding the horizon.
Sorry, no etymologies found.
Those wispy cirrus clouds are made of ice crystals, and the circumhorizontal arc is a large enough arc that the colors often cover the entire cloud.
That’s a circumhorizontal arc; they are formed by light refracting in a certain way through ice crystals shaped like horizontally oriented hexagonal plates if conditions are right, they’ll orient themselves that way as they fall through the air.