Physics Photo of the Week

November 12, 2010

Smile in the clouds
This bright rainbow "smile" appeared in my yard almost three years ago (Dec. 29, 2007).  It is not a rainbow, but a "circumzenithal arc" and is formed by ice crystals in the wispy cirrus clouds at high altitudes.  As this specimen shows, circumzenithal arcs are very brightly colored.

Like the more familiar circumscribed halo around the Sun or the Moon, the circle around the zenith is formed by refraction of the millions of tiny prisms of ice crystals.  In this case the ice crystals are thin hexagonal plates.  The Sun was fairly low in the sky, perhaps only about 22 degrees above the horizon.  The colorful arc is centered on th
e zenith; its angular radius from the zenith is the same as the Sun's angle above the horizon.

Understanding this rare display is much simpler
than the halo that circumscribes the Sun or Moon.  The diagram at right shows a hexagonal ice crystal.  The plate crystal is falling slowly in the upper atmosphere.  The friction of the air makes the plate fall in a horizontal orientation.  Any other orientation is not stable.  The light from the low Sun orientation enters the top face, is refracted by the 90 degree prism formed by the top face and one of the side faces.  When the Sun is about 22 degrees above the horizontal the incoming angle to the flat face of the prism and the outgoing angle are the same.  The spectrum of color is displayed due to the prism effect of the hexagonal ice plate.

Physics Photo of the Week is published weekly during the academic year on Fridays by the Warren Wilson College Physics Department.  These photos feature interesting phenomena in the world around us.  Students, faculty, and others are invited to submit digital (or film) photographs for publication and explanation.  Atmospheric phenomena are especially welcome.  Please send any photos to 

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