Pileus With Iridescence
June 27, 2013
Photographer: Geert Sassen; Geert's Web site
Summary Authors: Geert Sassen; Jim Foster
The photo above shows an iridescent pileus cloud as observed near sunset from Udon Thani, Thailand. The silky appearing pileus cloud was high enough to be illuminated by sunlight though the Sun itself was concealed from view by building cumulonimbus clouds. As storm clouds rise, they force a mass of air ahead of them. If the humid air layer just above the cumulonimbus cloud is close to saturation, this added lift (velocities may reach over 40 ft or 12 m a second) may cause the air column to become fully saturated, forming a smooth, rounded cloud. In this case, the cloud droplets were of the right size, and the Sun properly positioned, to cause iridescence. Photo taken on May 26, 2013.
Photo details: Camera Model: Canon PowerShot SX30 IS; Focal Length: 26.868mm; Aperture: f/4.5; Exposure Time: 0.0008 s (1/1250); ISO equiv: 160.