Physics Photo of the Week
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
was a large cumulous cloud in the west casting the shadow towards the
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
In order to make the
shadow in the sky highly visible on a photograph, the photo above has
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
published weekly during the academic year on Fridays by the Warren
Wilson College Physics
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 firstname.lastname@example.org.
here to see all Physics Photo of the Week for 2005
to see all Physics Photo of the Week for 2004.