What was that ‘rainbow’? Alice explains the ‘circumhorizon arc’

By Alice Enevoldsen
Special to West Seattle Blog

Today a beautiful blue-green iridescent cloud stretched straight across the sky, contrasting impressively with the white clouds behind it. As we watched, more rainbow-spectrum colors showed up in a second partial arc a little further south. The colors persisted for hours, though will be gone by the time you read this article.

(Added Tuesday: West Seattleite Don Brubeck‘s photo of how it looked @ Mt. Rainier NP)
We see many rainbow phenomena regularly here in West Seattle: Rainbows, secondary bow rainbows, supernumeraries, sun pillars, halos, heiligenschein, crepuscular (and anti-crepuscular) rays, and sundogs. This was a new one to me: The circumhorizon arc.

The circumhorizon arc is only visible when the Sun is higher than 58° in the sky. This can be true in Seattle only around noon from early May through mid-August. Like other halo-related visual effects, the arc is formed by sunlight refracting through ice crystals. As you can see in the photos, this arc is visible where there are streaky, filamental cirrus clouds. Those clouds are high enough in the atmosphere that even on hot days ice crystals can form.

Although this is not a common event, the circumhorizon arc is not classified as rare for our latitude. Look for it again throughout the summer around noon whenever you see cirrus-type clouds in the sky. This is the same event as a fire rainbow, but “fire rainbow” is not an accepted term, as it is misleading.

Before I go, let me mention noctilucent clouds, because the section of circumhorizon arc we saw today was almost exactly the same color, and now is the time to watch for them. Noctilucent clouds are also visible only in summer, but about 30 minutes to two and a half hours after sunset. They’re high enough in the atmosphere that they are still bathed in sunlight even as the sky darkens through twilight, giving them an ethereal blue glow in the dark sky.

We report a fair number of great new visual cloud and sky events around June. With the Sun reaching its highest altitude (nearly 66° off the horizon here) at solar noon on the summer solstice, the days and weeks around now are when we have the opportunity to see effects that require the Sun to be high.

You can read about many common sun/atmosphere effects we see in West Seattle in this Skies over West Seattle article from two years ago. I would love to discuss this effect and any others at the solstice sunset watch on Saturday.

Who is Alice?

Alice is many things and works and volunteers for a few notable organizations, but the suggestions and opinions put forth in this article are her own and no one else’s. You can find more about astronomy at alicesastroinfo.com.

CE June 15, 2015 (5:54 pm)

Clouds? I saw dozens and dozens of “contrails” laid down and then spread out all over the sky today.

heather June 15, 2015 (8:06 pm)

Cool!

Mike June 15, 2015 (8:34 pm)

Cool, my daughter was telling me she say this today during recess. I didn’t know how, but now I do. Glad the boy at recess showed her so she could enjoy it too.

Karin June 15, 2015 (10:39 pm)

Alice and Jason, fascinating report and the images make it so clear. You two make a good team, both in explaining this phenomenon and in raising Julie’s incredibly adorable granddaughter. :)

Andi June 16, 2015 (9:20 am)

WAKE UP SHEEPLE. Those are chemtrails left by the gov’t makin’ us all follow their orders like, uh… what’s the word I’m looking for…

WSB June 16, 2015 (9:51 am)

https://en.wikipedia.org/wiki/Chemtrail_conspiracy_theory
.
P.S. Note the cool view from Mount Rainier, just added after I spotted Don Brubeck’s share on the WSB Flickr group page …

CE June 16, 2015 (12:41 pm)

Wow, can’t believe the WSB is putting up links to wikipedia, as if that means anything. Next thing you know there will be a link to Snopes. The term “conspiracy theory” has been weaponized in order to marginalize anyone who has an idea that falls outside of the group-think our media and education steer us towards. I’m glad Andi can think outside of the box, and he shouldn’t be criticized for it. “The greatest obstacle to discovery is not ignorance; it is the illusion of knowledge”

WSB June 16, 2015 (12:52 pm)

Wikipedia does not deserve its semi-bad rep. It shows all its sources and you can judge those sources if you want. And as for Snopes – they’re gold standard, dating back more than a decade, particularly back when somebody would send every member of their mailing list some sort of suspicious chain e-mail … they carefully and meticulously traced it back to its origins, and again, showed their work along the way. (Including when the suspicious-seeming claim turned out to be true!) We try to show our work too – with links when available. – TR

Alice June 16, 2015 (3:58 pm)

Mike — yup, I was confused at first, and would definitely have been more confused if someone had been describing it to me. Luckily a few minutes of observation cleared up the basics for me, and then some optics research narrowed in on which exact halo effect this was.

Thanks WSB!

I’m so glad you got the Mt. Rainier shot too. Lovely, isn’t it?

VBD June 16, 2015 (6:19 pm)

I love spotting these iridescent clouds and arcs. I first studied them in an atmospheric physics class 30 years ago. Still fun to find!

@ CE what is it that you don’t like about the scientific description of the clouds? There’s little mystery to all of it. The science is pretty conclusive. There is no need to make up a pretend explanation and say it’s the only possible answer. Just have and open mind. As you said, “The greatest obstacle to discovery is not ignorance; it is the illusion of knowledge”

Wolf June 16, 2015 (9:01 pm)

Yes – Chemtrails! thank you Andi.

Tony June 16, 2015 (10:35 pm)

WSB – In the spirit of balanced reporting/commenting perhaps you might also want to consider adding the wiki link for Solar Radiation Management (SRM) ?