Pileus Cloud and Shadow

August 04, 2004


Provided and copyright by: Andrew Boyle
Summary authors & editors: Andrew Boyle; Jim Foster

This unaltered photo showing a pileus cloud was taken on June 25, 2004 from New Smyrna Beach, Florida. As afternoon thunderstorms were quickly building, and the Sun was beginning to set, the ingredients were in place for a spectacular light show -- if the timing was exactly right. When the Sun was in just the proper position, a pileus cloud began forming, and its shadow was cast upwards at the same time the pileus was "catching fire."

These clouds are observed to ascend quite rapidly -- rising at velocities of 40 feet (about 12 m) per second and sometimes even faster. If the layer of air above a rising cumulus cloud is nearly saturated, the additional lift from below is often sufficient to saturate it completely, forming a smooth and rounded "cap" cloud. Iridescence frequently accompanies these clouds, as is the case on this photo. Also see the EPOD for February 1, 2004.

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