Physics Photo of the Week

February 4, 2011

Sun Pillar
Sunrise on January 23, 2011 was preceded by this majestic pillar - the vertical column of scattered sunlight that is in-line with the Sun.  The Sun lies below the horizon.

Sun pillars are very simple to explain.  They are formed by flat plates of ice crystals that form the thin cirrus clouds.  When the flat plates fall in the air, they fall to maximize the air resistance, which is horizontal.  The millions of tiny flat-plate crystals reflects light from the Sun.  Each tiny crystal could be thought of as a tiny mirror that reflects an image of the Sun, but the plates are too small and so numerous that the images blend into a smear. 

The small hexagonal plates are also responsible for parhelia - sun dogs - seen at about 22 degrees on either side of the Sun.  Later, after the Sun had risen, a parahelion is visible (future PPOW). 

Cirrus clouds almost always consist of ice crystals.  Whether the crystals form needles (PPOW for January 28, 2011), hexagonal plates, or snow stars depends on the temperature and humidity.  Often cirrus contain both plates and needles as evidenced by the presence of both parhelia and halo (PPOW for December 3, 2010).  Usually cirrus clouds show neither a halo nor parhelia.  Such cirrus clouds must mostly consist of snow stars.

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 

All photos and discussions are copyright by Donald Collins or by the person credited for the photo and/or discussion.  These photos and discussions may be used for private individual use or educational use.  Any commercial use without written permission of the photoprovider is forbidden.

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