Iridescent Clouds Over Aiguille de la Tsa
Credit & Copyright: Ute Esser (U. Heidelberg)

Explanation: Before the sun rose over the mountains, iridescent colors danced across the sky. The unexpected light show was caused by a batch of iridescent clouds, and captured on film in early September in Arolla, Wallis, Switzerland. The peak in the foreground of the above image is Aiguille de la Tsa. Iridescent clouds contain patches of water droplets of nearly identical size that can therefore diffract sunlight in a nearly uniform manner. Different colors will be deflected by different amounts and so come to the observer from slightly different directions. Iridescent clouds are best seen outside the glare of the direct Sun although they can occasionally be seen to encircle the Sun.

Iridescent Clouds
Credit & Copyright: Arne Danielsen

Explanation: Why would clouds appear to be different colors? A relatively rare phenomenon known as iridescent clouds can show unusual colors vividly or a whole spectrum of colors simultaneously. These clouds are formed of small water droplets of nearly uniform size. When the Sun is in the right position and mostly hidden by thick clouds, these thinner clouds significantly diffract sunlight in a nearly coherent manner, with different colors being deflected by different amounts. Therefore, different colors will come to the observer from slightly different directions. Many clouds start with uniform regions that could show iridescence but quickly become too thick, too mixed, or too far from the Sun to exhibit striking colors. Pictured above, iridescent clouds were photographed over Norway two months ago.

Corona or iridescence? Both. Near the sun there is a coronal ring structure. Elsewhere the cloud droplets sizes vary and produce more chaotic colour patches. Imaged by Peg Staudenmaier ( Metaldreams ) on 15th June 2003. ©2003 Peg Staudenmaier

Provided and copyright by:
Shaun Lowe, Nova Scotia

[Edited on Oct 15, 2003 by Amitabha]