Iridescent Clouds
Iridescence or irisation is a diffraction phenomenon. Cloud iridescence is caused by diffraction by water droplets and tiny ice crystals and mostly occurs along the borders of a cloud. In contrast to coronae, the patterns are not concentric. Although the colours can be very bright, mostly green and pink, iridescent clouds are often overlooked because of the brightness of the sky. If you want to observe iridescent clouds, protect your eyes! Look for a place where the Sun is hidden behind a tree or any other object or use your hands to protect your eyes and use sunglasses! Never look directly at or near the Sun. Digital cameras may be damaged by the bright light (use a grey filter).
Iridescent cloud on January 2nd, 2005. Canon Powershot G6, processed with Adobe Photoshop Elements.