Physics Photo of the Week

November 10, 2006

Anti-Crepuscular Rays - Photos by Alissa Whalen and Paul O'Malley - Discussion by Donald Collins

Alissa Whalen, Administrative Assistant to the Warren Wilson MFA Program in Creative Writing, has sent me this impressive photo from the Big Sky in the prairie of Montana looking in the oppositie direction of the setting Sun.  What we see are shadows of clouds stretching out in the distance.  The clouds that create the shadows are either behind the camera or the clouds in the foreground ("foresky").  Because the Sun is on the verge of setting, the Sun's rays are parallel to the flat plains of eastern Montana.  Because the Sun is so far away, the shadows caused by the clouds are parallel to each other.  The parallel rays of shadows appear to converge similar to the convergence of railroad tracks in the distance.  In meteorology and discussions of atmospheric optics, these rays are called "anti-crepuscular rays", and are relatively rare.  A photograph from the Swannanoa Valley of a similar cloud shadow was published on PPOW over a year ago (Aug, 2005).

Paul O'Malley from the Boston, Massachusets area send me a similar photo of anti-crepuscular rays a couple of years ago.  Again the sun is directly behind the camera close to the horizon.  The camera is looking in the opposite direction from the setting sun, and shadows from the clouds (both foreground clouds and clouds behind the camera) are visible tapering off into the distance.

I thank both Alissa Whalen and Paul O'Malley for these excellent photos.  Donald F. Collins

Physics Photo of the Week is published weekly during the academic year on Fridays by the Warren Wilson College Physics Department.  These photos feature an 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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