July 25, 2010
Photographer: Mark Arzadon
Summary Author: Mark Arzadon; Jim Foster
The photo above shows an iridescent pileus cloud that formed in a developing thunderstorm near San Jacinto, Philippines, during the afternoon of June 11, 2009. June is the tail end of the rainy season in the Philippines, and cumulonimbus clouds may are frequently observed whenever or wherever atmospheric instability increases; over mountainous regions, for example. Pileus, or cap clouds, occur when a building cumulus cloud ascends at velocities of 40 feet (about 12 m) per second or more. As the cloud rises through the lower atmosphere, a mass of air is forced ahead of it, and if the layer of air above the rapidly rising cumulus cloud is nearly saturated, the additional lift from below is sometimes enough to saturate it completely. This results in an oddly smooth and rounded cloud form, and on occasion iridescence is observed.