June 23, 2011

Iridescent Clouds

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.


  1. Here is an awesome resource for learning about these sorts of things: Atmospheric Optics

    I saw those rainbow-looking things over Seattle too, and they made me smile. :-)

    Also, I think on Monday night, we might have seen the first (very small) noctilucent clouds of 2011. We always look from the community garden; that's the best patch of wide-open sky around here.

  2. Cliff, I may be wrong, but I believe that the photos are examples of a circumhorizon arc. The website posted by JewelyaZ describes the difference between the arc and iridescent clouds. In Spokane we had a circumhorizon arc event 5 years ago. See: http://www.wrh.noaa.gov/otx/photo_gallery/circumhorizon.php

  3. Ron...Such arc clouds can have iridescence...they are not mutually exclusive...cliff

  4. That's a circumhorizontal arc. JewelyaZ got the AtmoOptics site link before me but here's another link directly to the page: http://www.atoptics.co.uk/droplets/irid1.htm

    I hate to disagree with such a learned professor, but atmospheric optics is a big hobby for me, and I believe it's a different mechanism causing the effect.

  5. Emily..I was not commenting on whether this was a circumhorizontal arc or not. You can iridescence with that feature as well...you often do...cliff

  6. Sundogs, that's the "technical term"

  7. As I was whipping up my 'cap' this morning, it crossed my mind . But....Doc....I SOOOO enjoyed hearing your expaination and seeing the maps with it.


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