Circumzenithal arc among cirrus clouds


Nicky Clarke
Location: Manchester, UK
Date: September 2013

This “smile in the sky” is called a Circumzenithal arc. What in the world is that? Well, cirrus clouds occur at such cold temperatures that they are made up mostly of ice crystals. These ice crystals come in different shapes, sizes, and orientations. When these crystals are shaped like plates and oriented with their horizontal faces downward, the sun’s light can enter the tops of these large faces, bending (refracted) through the ice crystal, and exiting the vertical side face with a spectrum of visible colors.

HexIcePlate(Diagram courtesy of Warren Wilson College’s Physics department)

So in order for the sunlight to enter these crystals in the cirrus clouds at the correct angle, the sun has to be less than about 32 degrees above the horizon. The word “zenith” refers to directly overhead at 90 degrees so the center of this arc (if it were a complete circle) would be directly overhead. The angle at which this arc is observed from below zenith is the same angle that the sun is above the horizon!



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