Physics Photo of the Week

December 9, 2011

In mid August, 2 days before the full moon, I was captivated by the rising Moon in the south east and the striking illuminated cumulus cloud.  The Sun had not yet set, so the cloud was still in daylight.  The Moon had risen about 2 hours previous because it was about 2 days before the completely full phase.  Notice the reddish tint to the cloud due to the long path the sunlight travels in the Earth's atmosphere when near the horizon.  The blues become scattered and absorbed by the atmosphere and only the longer red wavelengths survive to illuminate objects.  The sky is blue because the short blue wavelengths are scattered by the atmosphere to the camera or observer.

In the time-lapse animated sequence at right, we can see that the cumulus cloud is very dynamic.  The cloud consists of rising warm moist air soon condensing into cloud.  In this cloud, the faster higher level winds shear the upper parts of the cloud laterally - limiting the maximum altitude reached by the cloud.  In the absence of the wind shear, this cloud could have developed into a thunderstorm.

The animation spans a time of about 6 minutes compressed to about 2 seconds.  In this time, we can see that the shadow of the Earth's horizon on the cloud is propagating up the cloud as the Sun is setting. 

Also the path of the Moon is inclined from the vertical and sloping to the right (southeast).  This is due to our latitude on the Earth.  At WWC's latitude of about 35 deg, the rising angle is inclined at 35 degrees from the vertical.  When viewed from the southern hemisphere, the angle of sunrise and moonrise is inclined to the north (see PPOW for Sept 23, 2005 and PPOW for Sept. 13, 2004

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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