Monday as I was driving through Walla Walla, WA, USA, I noticed the beautiful mini rainbows in the clouds. I looked up the website that flickerite 'fdecomite' guided me to and found it is a circumhorizon arc.
From www.atoptics.co.uk/halo/cha2.htm = "Look for a circumhorizon arc near to noon near to the summer solstice when the sun is very high in the sky (higher than 58°). It lies well below the sun - twice as far from it (two hand spans) as the 22º halo." "The arc is a very large halo and is close to, and parallel to the horizon. Usually only fragments are visible where there happen to be cirrus clouds." "whenever the sky is wisped or hazed with thin cirrus clouds. These clouds are cold and contain ice crystals in even the hottest climes."
It has to do with refraction of light through ice crystals in the
atmosphere --and it was over 80F at the time!
Highest position on Explore #145 on Saturday, Aug 25, 2007