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A circumhorizon arc is seen over Nanaimo on June 30, 2017. (Courtesy Karen Butler)
The colours in the sky never cease to amaze me; from cool sunrises to brilliant sunsets, to rainbows and halos around the sun.
Karen Butler of Nanaimo snapped a few pictures of bright colours dancing across the clouds a few days ago, the result of a circumhorizon arc.
A circumhorizon arc is a halo of sorts, and it appears when the sun is very high in the sky – higher than 58°. It’s typically oriented parallel to the horizon. Often it’s only visible in fragments when cirrus clouds aloft are lit with a rainbow of colours. The colours are the result of sunlight passing through the ice crystals that make up the cirrus cloud. The crystals themselves act like a prism, essentially splitting the sunlight into the spectrum of colours.
A good time to see these in the mid-latitudes is around noon during mid-summer. The halo will appear beneath the sun and twice as far away from it.
So how rare is a circumhorizon arc? Well, that depends on where you are. In many parts of the United States it’s not considered a rare occurrence, as this can be seen several times per summer. The farther north you go, the less frequently they appear. Turns out, it’s incredibly rare to see a circumhorizon arc in much of Europe, and if you are north of Copenhagen, you’ll never see one!
Karen says she saw this in the sky over Nanaimo on June 30th. “What I saw lasted about 5 minutes and was constantly moving and changing,” she says. I bet it was brilliant, Karen. Thanks so much for sharing your photos!
BTW -- Sometimes people refer to this as a fire rainbow. The term is not recognized, meteorologically speaking, as this phenomenon has nothing to do with fire, nor is it actually a rainbow (you don’t need rain to make this happen).