Steve's Atmospheric Optics
Irisations, Iridescence, Coronae, and Diffraction
Light diffracts and interferes off of water droplets in some types of clouds. This is called iridescence or irisation.
If it is close to the sun and has a circular structure, it has the special name of corona. I notice irisations frequently in
the thinnest and most delicate of clouds. These effects can be difficult to see even when you shield the sun well. However,
using levels adjustments in photo editors makes them quite vivid.
Corona showing two orders of green light.
A wider view of the same corona at a different time.
Iridescence bisected by a jet contrail.
A contrail casting its shadow on a nearby iridescent cloud.
Iridescence from a THAAD missile test during morning twilight. The heights involved may classify this as a manmade Noctilucent cloud.
An aureole is the intense forward scattering "glare" adjacent to the sun. It's usually very hard to see since the sun's intensity
overwhelms it. Here, shortly after a rainstorm, the sun dipped below the edge of the cloud bank, leaving the aureole visible.
Here is an image of the exhaust from a Minotaur rocket showing spectacular iridescence--4-14-06
These are 3 photos from a display of intensely irridescent clouds that changed shapes and colors for over an hour: 1-27-07
Thin distributions of tiny water droplets of extremely uniform diameter can produce airy diffraction rings around the moon as shown
here. The size of the droplets are calculated directly from the angular extent of the first minimum. In this case,
the first minimum spans about 2-deg giving a 20um droplet diameter. The diagonal streaks are fast-moving clouds. The bright
disk is the n=0 diffraction order (called the aureole), and the moon is only about 1/4 of that size.