Physics Photo of the Week

March 13, 2009

Sun Pillar
Sometimes before dawn or after sunset, the reflection from special clouds forms a pillar of light called a Sun Pillar.  This was photographed on Wednesday, March 4, 2009 before sunrise. 

Besides the pillar, notice the gold touch on the base of many of the cloud strata.  the Sun is below the horizon, but at the elevation of the cloud layers, the Sun is above the horizon.  Because the sunlight has traveled through a much longer path through the Earth's atmosphere, the short wavelengths have been scattered out.  Only the long red wavelenghts remain, so the base of the clouds, as well as the sun pillar are magnificently reddened.

The sun pillar itself is caused by ice crystals in the clouds.  Atmospheric ice crystals can take many forms, but the forms responsible for ice pillars are thin hexagonal plates.  The plates fall through the atmosphere from where they form in the cloud.  When falling, they tend to lie horizontal and thus act as tiny mirrors to the sunlight that is reaching them.  Millions of tiny horizontal mirrors (hexagonal ice crystals) reflect the light toward the observer - similar to the elongation of the Moon's reflection from ripples on a lake.

A sun pillar was featured on Physics Photo of the Week on September 16, 2005.  The 2005 sun pillar was the most recent sun pillar before this week's.  This indicates that sun pillars are a relatively rare occurance.

See PPOW for January 26, 2007 for close-up photos of ice crystals showing hexagonal structures.

Due to Spring Break at Warren Wilson College, there will be no Physics Photo of the Week next week.  The next Physics Photo of the Week will be published on March 27, 2009.

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: