This week I received some wonderful pictures from Patrick McKinnon, showing rainbow-like cloud features over Seattle. Take a look at them!
And over the past year, others have sent me similar pics. What are they? They have lots of colors like rainbows, but they aren't rainbows. And besides there is no rain with them!
What you are seeing is an example of iridescence, with the colors produced by a process called diffraction.
Iridescence is associated with thin clouds of relatively uniform, small, cloud droplets or ice crystals. The colors are generally in the pastel range. This phenomenon is most obvious in cirrocumulus, altocumulus, and lenticular clouds (lens-shaped clouds formed by flow over mountains and in their lee).
The source of the color is the same as that produces the colors in soap bubbles and oil slicks on the road--diffraction.
Diffraction depends on the wavelike nature of light. Sunlight is made up of all wavelengths of the visible spectrum and in diffraction, the colors are separated by light interacting with itself--called constructive and destructive interference. In a future blog I will go into this mechanism in more detail.