April 08, 2011

Upside Down Rainbow

Several people sent me pictures of what they described as an upside down rainbow high in the sky around 5 PM today. Here are some examples:

Courtesy of Casey Burns

Courtesy of David Ovens

The colors in these "smiley faced" rainbows were very vivid.

What was causing this unusual occurrence? It is what is called a circumzenithal arc.

This arc are caused by the same general phenomenon that creates halos: the refraction of light in ice crystals. Circumzenithal arcs occur when the sun is relatively low in the sky (below 32 degrees above the horizon) in the presence of ice crystals with generally horizontal orientation.

Ice crystal have a hexagonal (six sided) structure and often are in the form of hexagonal plates (see figure). As they fall, they tend to have relatively horizontal orientation (not unlike leaves falling out of trees). Light passing through the top plate and then out the sides is bent (refracted) by roughly 46 degrees. The light (which initially has all colors is divided into the whole visible spectrum by this refraction--just like a prism--in a process known as dispersion)

The light of the C- arc is found about 46 degrees ABOVE the sun--with the arc (generally no more than 1/4 of circle) appearing to circle around the zenith of the sky.

Current research has not found any evidence for a pot of gold at the end of this rainbow. Apparently, it is too high for leprechauns to deal with.


  1. In Port Angeles we had some very pronounced sundogs, around 5 pm, some of the better ones I've seen. Didn't notice a rainbow though.

  2. Yes! I saw this yesterday out walking with the wife and kids. It was amazing! First, there were at least 4 areas of "rainbow effect". There was a general halo around the sun with sun dogs to each side. Each sun dog had a clear rainbow effect. At 90 degrees there was also a faint rainbow, which was directly opposite the brilliant upside down rainbow. I have never seen anything like it and am still kicking myself that I didn't have a camera. Perhaps the photos do not do it justice, but from southern Mukilteo it was even more brilliant than the photos in the post, and it lasted at least 20 minutes.

  3. Cliff, I'd like to ask that you share weather resources on the web with your weather-interested readers. I'm sure that you're familiar with these, but... My toolbar has the new WeatherSpark.com which I spend time doodling around in. Wow, data for any station spanning two days, or up to two years.

    Sure, it's over your head, so to speak, but I also look in on SpaceWeather.com regularly. Its links are a resource. I'm a bit surprised that you did not link to the linked atoptics.com site for explanation of the inverted bows in this blog. If you haven't run across this site, I think that you'd probably love it, given your interests.

    Anyway, thanks for the substantial effort that you put into this blog. This life long learner really appreciates your teaching. And thank you for taking a position on that wonky math teaching BS. You are a fine example of Good Citizen!

  4. Here's an excellent page that describes many of these rainbow-like forms: http://www.atoptics.co.uk/halo/common.htm

  5. There was even a pronounced sundog visible from South Seattle. The halo ring was most clear to the right of the sun, and there was a solid blob of light right in the middle. Fascinating to see first-hand.

  6. Matt, you're describing a circumhorizontal arc opposite the circumzenithal arc. The Atmospheric Optics site that another user linked to is the best resource I've seen for the subject. Glad you enjoyed the display! I always look up and toward (but not at) the sun to see if there's anything interesting going on. 99% of the time people are oblivious to fun optical displays.

  7. Ooh, I saw it, in Edmonds, and I even knew what it was. Never saw one before, and wouldn't have this time if I hadn't happened to look straight up!

  8. And yes, it was very vivid and pure. The sundogs caught my eye first, then the hint of a halo, then the colors at the top of the halo, then, looking up more, the circumzenithal arc, as Wikipedia calls it. I was thrilled--had only read about such things in "Light and Color in the Outdoors".


Please make sure your comments are civil. Name calling and personal attacks are not appropriate.

Thunderstorms Return to the Northwest

 Thunderstorms have been relatively rare this summer, but today will see some boomers over the Cascades and eastern Washington. In fact, the...