Physics Photo of the Week

April 6, 2012

Ice Halo
Ice halos are one of my favorite topics.  It's always been very intriguing how certain clouds in the sky could form a distinct ring around the Sun or the Moon.  Thanks to explanations by Robert Greenler (Rainbows, Halos, and Glories, Cambridge Univ. Press, 1980) the halo is caused by ice crystals that are found in high-level cirrus clouds.

The ice crystals responsible for ice halos are small hexagonal prisms (the same shape as an ordinary wooden hexagonal pencil, but much smaller.  These hexagonal needle-like ice crystals are falling in the upper atmosphere, but the air friction in falling (like snowflakes) causes them to fall relatively slowly.  Another major phenomenon of the air friction is that all the crystals are horizontal.  The horizontal orientation is the most stable when falling through the air.  Think of confetti falling: the confetti tends to fall in a horizontal orientation.  Even though the ice needles are all horizontal, they are oriented in all horizontal directions, and at all rotations along the long axis of the crystal.  The hexagonal shape of the crystals refracts and bends light from the Sun or Moon similar to a triangular glass prism.

The picture above shows various orientations of the hexagon prisms (rotated about the long axis) and the paths of sunlight affected by the prisms.  Sunlight enters from the left in each prism.  The various orientations of the prisms cause the light to emerge from another side having been deflected an angle D from the original direction.  The paths are calculated from the laws of refraction.  In the left orientation, the light is deflected the least.  The other two orientations create a larger deflection.  The minimum deflection angle creates the halo at an angular size of 22 degrees from the Sun or Moon - the smallest D angle from the left orientation). 

Whenever the sky is somewhat hazy, take a look at the Sun (shield your eyes by standing in the shadow of a tree or building) and you may see many more of these wonderful phenomena.  This week's photo was taken last Sunday afternoon on April 1, 2012.

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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