Circumzenithal Arc Above Pangasinan, Philippines
October 01, 2011
The photo above showing a colorful circumzenithal arc (CZA) was observed above Pangasinan, the Philippines on April 12, 2011. CZAs are frequently mistaken for upside down rainbows, but these arcs aren't nearly as broad as rainbows and they're not seen opposite the Sun. Furthermore, raindrops have nothing to do with their formation. CZAs are caused by refraction of sunlight in the ice crystals that compose cirrus clouds. They result when light shines through horizontally-oriented ice crystals -- sunlight enters through the upper horizontal basal face and exits through one of the basal side faces. Red colors shape the outside of this arc with blue on the inside.