Irisation - December 8, 2023

This patch of Altocumulus and Altostratus clouds reveals the optical effect called Iridescence or Irisation. These clouds are excellent producers of irisation which are really small segments of large coronas. Irisation occurs when clouds like these cover the Sun and Moon. In this example the colors range from pink/magenta and light blue to green and yellow. The coloration occurs when light is diffracted by very small cloud particles that are nearly the same size. The particles can be water droplets or ice crystals. Larger ice crystals cause halos instead of irisation if conditions are right.

Depending on your computer monitor the colors may be easy or difficult to see. The sky is not blue because this photo was shot into the Sun which caused the camera to close down the lens aperture to compensate for the bright sunlight. The coloration is extensive along the cloud edge from the where the coloration is indicated to off the edge of the photo. While irisation is relatively rare it does occur often enough for it to be visible several times a year from many locations if you are in the habit of looking for it.