Physics Photo of the Week

December 16, 2011


Almost a year ago we woke up to this colorful parhelion amidst an impressive sunrise.  The parhelion is the rainbow pillar in the lower left of the photo.  The Sun is currently to the right of the large old pine tree, but the Sun hasn't appeared over the edge of the mountain yet.  The clouds, however, are in the bright sunlight.

The rainbow parhelion means literally "next to the Sun"; it is also called a "sundog".  These are frequently visible when the Sun is near the horizon when the clouds consist of hexagonal platelike ice crystals.  The flat ice crystals slowly fall in the air in the horizontal orientation - the most stable configuration under air resistance.  The sunlight enters a vertical side of the hexagon and exits two sides over at an angle of 22 deg.  Not only is the sunlight bent, but it is dispersed into the rainbow of colors like a rainbow.  The hexagonal ice crystal acts exactly as a prism.  Parahelia are often seen on both sides of the Sun due to the symmetry of the crystals as was shown on the February 8, 2008 Physics Photo of the Week.

On careful inspection of today's photo, we can also notice the shadows of the clouds cast onto the sky in the  center of the photo.  I would welcome any similar photos weather phenomena from readers.  Cell phone cameras have the advantage of nearly always being on-hand to take images when they appear.

This is the final Physics Photo of the Week for 2011 due to the winter holidays at Warren Wilson College.  Look for the next publication on January 20, 2012.  Have a Merry Christmas and a Happy New Year!

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.

Click here to see the Physics Photo of the Week Archive.

Observers are invited to submit digital photos to: