Physics Photo of the Week

September 13, 2013

Ice Crystals
This is usually called a "Sun Halo", but it is caused by the refraction of sunlight by ice crystals (cirrus clouds) in the upper atmosphere.  Notice the color - due to prism-like dispersion of the light into its spectrum of all colors.

The ice crystals that form this 22 degree halo around the Sun or Moon resemble old-fashioned pencils - they have hexagonal cross sections.  The crystals are all oriented so that the long axis is horizontal.  The long horizontal axis of the crystals can be oriented any direction in the horizontal plane.  To create the halo, sunlight enters one side of the hexagon, emerges from another side of the hexagon at a particular angle (22 degrees - governed by the refractive index of ice and the geometry of hexagons).  At a particular point in the halo, only a few crystals whose horizontal and rotational orientation are just right contribute to the halo.  In a portion of the sky that isn't part of the halo, none of the crystals can line up correctly.  Different colors refract at slightly different angles, hence the slight separation of colors.

I was joining some friends to play tennis in Vermont when I first noticed this excellent halo.  I let them know that when I missed points (particularly serves) that I was distracted by the physical atmospheric phenomenon of ice crystals.  My interest in physics would supposedly distract me from my game.  As things turned out, my friends must have been equally distracted because my team won!  I waited until after the tennis to obtain this picture.  The halo was long lasting and even improved its contrast when I got back "home".

An excellent book on ice halos is Rainbows, halos, and glories. By Robert Greenler. Cambridge University Press (Cambridge), 1980.  Dr. Greenler's articles and his textbook have provided most of my knowledge of the physics of sun halos.


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 dcollins@warren-wilson.edu.

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