October 17, 2003
While to the naked eye, the clouds pictured above were blindingly bright, this underexposed photo shows their iridescent nature. Iridescent clouds can be thought of as detached coronae (see the Earth Science Picture of the Day for yesterday and for October 12) and are caused by diffraction of sunlight -- minute water droplets in clouds interfere with light from the Sun or Moon. A number of different clouds may display iridescence or irisation, but when altocumulus or altostratus clouds are in the vicinity of the Sun, it's more likely that this phenomenon will be observed. On the photo above, the Sun has already set behind the mountain and was located to the lower left of the left-most cloud.
Photo Details: f/11, 1/8000s, ISO 100, 400mm lens, cropped image.