August 16, 2011
Photographer: Randell Teodoro (top); David Lee Tiller (bottom); David's Web site
Summary Authors: David Lee Tiller; Randell Teodoro; Jim Foster
The photos above show examples of iridescence in mid-level clouds. Iridescence or irisation occurs in clouds composed on water droplets having similar sizes. They're most frequently observed in altocumulus or altostratus type clouds -- in clouds not undergoing substantial vertical growth. Diffraction processes are responsible for their formation and their metallic coloration. Note that the reds are found on the outside of the blues.
Midday iridescence can be spotted more readily if you're wearing sunglasses. However, always use extreme caution when looking near the Sun. Roof peaks were used in both pictures to block out the Sun. Top photo taken on June 13, 2011 from Manila, The Philippines. Bottom photo taken on July 9, 2011 from Nashua, New Hampshire, U.S.
Photo details: Top - Canon S95 camera; set to fastest shutter speed and f/8.0. Auto-exposure compensation. Bottom - Canon Powershot G11 camera; 30. 5 mm focal length: f/8.0 aperture; 0.001 second (1/2000) exposure; 160 ISO equivalent.