Sunday Morning's Fire Rainbow

A viewer sent in this awesome shot of what looks like a rainbow-colored cloud today:

Fire Rainbow sent in by Rick Hadie 7/28

This is called a "fire rainbow".  These are actually pretty rare even though the physics behind the formation is relatively simple.  First, they only form via ice-based clouds which are very high up in the atmosphere.  That's why the clouds in front of the shiny colors are not also glowing--because they are only made up of rain droplets in the mid levels of the atmosphere.

When the sun shines on the ice crystal a certain way, it actually diffracts the color rather than refracts. You know Pink Floyd's Dark Side of the Moon album cover? Well, that light is refracting and shows all the colors in the visible spectrum in a linear manner after the light passes through it (like a rainbow). 

Example of Refraction

With ice crystals, however, the light diffracts and actually scatters.  This is because the light passes through a perfectly symmetrical item, like an ice crystal, rather than an asymmetrical prism (like a water droplet) in which light refracts into a rainbow.

Example of Diffraction

Since there's nothing on the other side of the cirrus cloud to have the light shine upon, the cloud itself illuminates and shows a spectacular display.  - MT