Bloomington, Illinois Iridescence
September 23, 2007
Iridescence or irisation photos, such as the ones above, demonstrate various cloud properties such as opacity, layering, and even the proverbial "Silver Lining." Iridescence is caused by diffraction of sunlight around tiny water droplets of various sizes. The droplet sizes, and size-related differentiation, are prominent factors determining the color that's displayed in the cloud.
These two images were taken approximately 20 seconds apart show how quickly droplet size can evolve in clouds. To my eye, the iridescent cloud was brilliant white. The camera, shooting at 1/5000 sec at f/22, was able to reduce the brilliance to the point where the colors were visible.