Rainbow Clouds Seen Around the Region

I received about a dozen emails yesterday (Tuesday) from folks seeing the most beautiful, multi-color clouds stretching horizontally above the horizon:  let me show you a few examples:

Stuart Lochner sent me this one from near Naval Station, Everett

Kim Rochat forwarded this one from Camano Island

Sherry Scherer sent this from Stevens Pass

And another one from Stevens Pass, from Chelan Robbins

Notice that in all these pictures, red is on the top and that the color bands are oriented relatively parallel to the horizon...this will be important.

Yesterday, the sky was full of ice crystals associated with cirrus and cirrostratus clouds.   You could see the thin clouds on the visible satellite image.

Or even from the Space Needle Cam during the early afternoon:

So what was happening here?  

 I believe we had a circumhorizontal arc, in which the sun's light was refracted (bent) by ice crystals in the thin upper clouds.   To get the arc, the ice crystals need to be in the form of "plates" that float downward with a similar horizontal orientation (drop a small paper square and you will see that this can happen).     The sun's rays are bent (or refracted) as they pass through the ice plates (see image), passing through the narrow edge and passing out of the broad lower section.  

Different wavelengths of light are bent differently (something called dispersion), resulting in a rainbow of light, with red at the top.  A familiar example of dispersion occurs in a prism.

To produce this rainbow arc, the sun must be at least 58° above the horizon (any less and the light will pass overhead and not be visible).   Today, the sun got to 64° at noon, so there there was a limited time window to see the arc.  Due to our northern latitude, the circumpolar arc is only viewable from early May through early August.    So keep watching during the next few months--there is a good chance this phenomenon will occur again.