Physics Photo of the Week

October 8, 2010

Crepuscular Rays
Sunrise on October 5, 2010 was a treat with very prominent crepuscular rays - the shadows formed by the clouds low on the horizon.  The shadows from the low clouds are visible on the air molecules in the sky.  To put it another way, the sunlight penetrating the air is scattered by the air molecules (as well as by dust and haze). 

Crepuscular means "twilight" from the Latin word for twilight: crepusculum.  A number of animals are called "crepuscular" in that they are active during twilight such as deer and firefly beetles.  This is in contrast to diurnal (daylight active) or nocturnal (nighttime active) animals.  Thus crepuscular rays are literally "twilight rays".

Compare this image with previous Physics Photo of the Week images for crepuscular rays (May 7, 2010) and (December 16, 2006).  In today's image the Sun had just appeared at the horizon (the converging point of the shadows behind the clouds), and the sunrise sky color is much redder than the previous images.  This is predominantly on account of the Sun being much lower in the sky in this week's image.  The sunlight must travel through much more atmosphere.  The blue light has been preferentially scattered out of the direct path of sunlight leaving the red light to penetrate.

On rare occasions we can look opposite the Sun and see the crepuscular shadows trailing off in the opposite direction (see PPOW for November 10, 2006 for images contributed by Alissa Whalen and Paul O'Malley).  Those crepuscular shadows looking away from the Sun are called "anticrepuscular rays".

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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