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Physics Photo of the Week

August 26, 2005

Cloud shadow on the sky

This interesting feature of the sky from the Swannanoa Valley on Aug. 16, 2005 occured right at sunset.  The dark wedge-shaped feature is the shadow of a cloud behind the camera.  The sun was low to the horizon in the west behind the camera.  There was a large cumulous cloud in the west casting the shadow towards the east.  Usually, cloud shadows are projected on the ground as shady spots, but when the sun is low to the horizon, the cloud shadow is projected horizontally, and the "shadow" is formed on the air and other clouds.

Notice that the shadow appears to get narrower in the distance.  This is due to  perspective - exactly analogous to the apparent convergence of railroad tracks in the distance of a photograph.  The edges of the shadow are parallel, like railroad tracks, but appear to converge. 

In order to make the shadow in the sky highly visible on a photograph, the photo above has been strongly enhanced.  That is why the sky looks quite "grainy", and perhaps an un-natural color.  At left is a small version of the unenhanced photo.  The human eye is amazing in that the cloud shadow is much more visible directly than in a photograph.

On the right is a photograph of the cumulous cloud, with the sun behind it.  This is the cloud that that produced the shadow.  This cloud has almost matured into a small thunderhead, with a forming flattened "anvil" top.

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

Click here to see all Physics Photo of the Week for 2005

Click here to see all Physics Photo of the Week for 2004.