Circumhorizontal Arc Fire Rainbow — Rocky Mountain USA
The circumhorizontal arc is one of the sky's rarest optical phenomena: a horizontal band of vivid spectral colours appearing in thin cirrus clouds when the sun is at an elevation above 58 degrees, a condition occurring only in summer months in mid-latitudes. The colours are more vivid and extensive than any rainbow — wide bands of red, orange, yellow, green, and blue stretching across large sections of sky, appearing to float as horizontal stripes in the cirrus. The Rocky Mountain region of the USA sees the most reliable conditions: the high summer sun, the thin high-altitude cirrus clouds that form over the mountains, and the low humidity that provides transparency. The spectacle lasts from minutes to over an hour when conditions are right.
About this spectacle
Standing beneath a circumhorizontal arc is one of the most startling sky experiences available to a casual observer. Unlike a rainbow arching overhead, this phenomenon appears as a nearly horizontal band of pure spectral colour suspended within thin cirrus clouds — wide stripes of red, orange, yellow, green, and blue that seem painted directly into the sky rather than projected through water. The colours are saturated well beyond what any rainbow achieves. Rocky Mountain National Park offers reliably good summer conditions: high-altitude cirrus clouds drift over the peaks, the summer sun climbs well above the 58-degree elevation threshold in the early afternoon, and the dry mountain air keeps the atmosphere clear and transparent. Visitors scanning the sky on a midsummer afternoon may suddenly notice a glowing horizontal fire in the high clouds. The display can last anywhere from a few minutes to over an hour. No equipment is needed — just a clear day, thin cirrus, and a willingness to look up.
When to go
May — Oct, peak Jun — Aug
Getting there
Nearest airport: DEN. Nearest city: Fort Collins.
Booking options
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