Pileus Cloud Over Boca Raton, Florida

October 12, 2010

: Mary Beth Radford
Summary Author: Mary Beth Radford; Dottie Edwards; Jim Foster

As friends and I arrived for a vacation in southern Florida, we noticed these beautiful sky colors and initially thought we were seeing a rainbow. Upon further investigation, however, we ruled out a rainbow and knew it was something we had never seen before. This is a pileus cloud, or cap cloud, displaying lovely iridescence. Pileus form when building cumulus clouds rapidly ascend; at velocities of 40 ft (about 12 m) per second or more. As the cloud rises, it pushes a mass of air ahead of it. When the air in the vicinity of the building cumulus cloud is near saturation, the boost from the rising currents may saturate it thoroughly. This creates a cloud that's markedly smoother in comparison to the puffier cumulus clouds beneath it.

Iridescence often occurs in clouds comprised of minute water droplets. These droplets deflect sunlight, which results in overlapping colors and thus the metallic or pastel shades. If the cloud droplets happen to be close in size throughout the cloud, the iridescent colors are more apt to be brighter and purer. Photo taken on August 14, 2010.

Photo Details: Camera: Canon PowerShot SX120 IS; Focal Length: 31.5mm; Aperture: f/4.5; Exposure Time: 0.0008 s (1/1250); ISO equiv: 80.