Ralph Arwood Photography

Cloud Iridescence

Sunday, October 15, 2017 1:00 AM

This thunderstorm moved toward the setting sun, giving a prolonged display of Cloud Iridescence.

Cloud iridescence occurs as colors in a cloud similar to those seen in oil films on puddles. The colors are usually pastel, but can be very vivid. When occurring near the sun, the effect can be difficult to spot as it is blocked by the sun's glare. 

Iridescent clouds are a diffraction phenomenon caused by small water droplets or small ice crystals scattering light. Larger ice crystals do not produce iridescence, but can cause halos, a different phenomenon.

If parts of clouds contain small water droplets or ice crystals of similar size, their cumulative effect is seen as colors. The cloud must be thin, so that most rays encounter only a single droplet. Iridescence is mostly seen at cloud edges or in semi-transparent clouds, while newly forming clouds produce the brightest and most colorful iridescence.