What the devil is a circumhorizon arc?

As we made our way from base camp for an acclimatisation hike on Ojos del Salado (6,893m) in the Puna de Atacama region of the Andes last month, we witnessed a strange phenomenon in the sky.

Above the adjacent peak of Cerro El Muerto (6,488m) a solid bank of cloud flashed all the colours of the spectrum. It was something none of us had ever seen before. We have all seen rainbows many times, but these are just narrow bands in a semi-circle. Here we had a whole band of dark grey cloud giving off a colourful, luminous glow.

As we made our way up from base camp, we witnessed a strange phenomenon in the sky
As we made our way up from base camp, we witnessed a strange phenomenon in the sky

It was spooky and Edita, who has an affinity for earthquakes by virtue of having experienced many of them, was certain that it portended another one. Chile is often rocked by them, including the most powerful earthquake ever recorded – the 1960 Valdivia earthquake, which registered a frightening magnitude of 9.5.

It probably didn’t help that Cerro El Muerto means The Dead Mountain. Luckily – and I don’t get to say this very often – Edita was wrong, and the ground remained firm.

When he got home, one of our team, Peter Salenieks, contacted the Atmospheric Optics website for an explanation. It was, he learned, a circumhorizon arc – a particular variety of ice halo. It’s not a very snappy title, I know. I have no idea why some joker decided to call it that, but it was probably invented by a scientist who didn’t go out much.

Other cloud phenomena such as the Brocken Spectre – where a giant version of your shadow surrounded by a halo is cast in the clouds – have much more poetic names. Cloud rainbow would have been better, or even spectral cloud. The phenomenon deserves a better name, because what we were seeing was a natural work of art.

A circumhorizon arc over Cerro El Muerto in the Puna de Atacama region of Chile
A circumhorizon arc over Cerro El Muerto in the Puna de Atacama region of Chile

But what was it and how was it formed? Ice halos are rings of shining light in the sky caused by sunlight glinting through ice crystals in the upper atmosphere.

In the case of a circumhorizon arc, two particular cloud formations need to be present: wispy cirrus clouds in the higher atmosphere, and thicker cumulus clouds beneath. The streaks in the cirrus clouds glow spectral colours as the sun shines through and the crystals glint. The cumulus clouds block us from seeing the full halo, and only fragments of it shine through.

The cirrus clouds are formed by ice crystals in the shape of hexagonal plates, which align horizontally as they drift downwards on air currents. The sunlight enters from the side and leaves the bottom, causing the colours of light to be refracted into a full spectrum.

Circumhorizon arc, or a better name would be spectral clouds - they are a natural work of art
Circumhorizon arc, or a better name would be spectral clouds – they are a natural work of art

Cloudspotters will be interested to read a special feature on the Atoptics website explaining the phenomenon in more detail with the aid of Peter’s photos.

And if none of this makes any sense to you, don’t worry and just enjoy the photos. There is a special technique to photographing them, which involves hoping you seen one, then pointing the camera at them and pressing ‘click’. Peter’s are better than mine, but he did have a more expensive camera.

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One thought on “What the devil is a circumhorizon arc?

  • February 15, 2019 at 5:31 pm
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    Believe it or not I’ve seen this phenomenon above the Glyderau in North Wales.I was at the foot of Glyder Fach main cliff so unfortunately the mountain was probably blocking the best of the view, but a few multicoloured streams of colour were peeking out and I managed to get a photo, but due to the brightness of the sky affecting the cameras exposure level it didn’t come out looking nearly as good as it did to the human eye.

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