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Ask Eric: What Created This Colorful Show Over Boston?

Hi Eric,

I looked out the window from my work and saw this rainbow-like phenomenon in the sky.  It looks like a combination of a rainbow and a cloud. What exactly is this?                   - Jean Galluccio

Hey Jean! We received a bunch of photos of this colorful patch in the sky on Wednesday, so you're not along in wondering what caused it! It's not a rainbow, and it's definitely not a 'fire rainbow.' I assure you the cloud wasn't bursting into flames! Rainbows are something you will see when the sun is at your back, relatively low in the sky, and small water droplets are present in the air. We had none of that working for us on Wednesday.

This particular case is what we call a 'circumhorizontal arc.' In actuality, it's a gigantic halo that you'll only see when the sun is very high in the sky (at least 58º). This also means there is a season for circumhorizontal arcs in this part of the country - namely mid-spring through mid-autumn. Once you reach a latitude of either 55ºN or 55ºS, they are impossible to spot because the sun never reaches high enough up in the sky. And when you do spot a circumhorizontal arc, you will find that it is always parallel to the horizon.

Circumhorizontal Arc seen over the Greenway in Boston. Photo via Anthony Ruggiero.

Even though it is a part of a huge halo, you will only see the pieces that cross cirrus clouds, which are made up of hexagon shaped ice crystals. They pick up the halo and scatter the visible spectrum of light into individual colors (wavelengths).

They're similar to iridescence, which are also patches of color in the sky. But there's a big difference - circumhorizontal arcs will only be seen in cirrus clouds. You may spots patches of color (iridescence) in a number of different cloud types, including lenticulars, altocumulus, and cirrocumulus.

A beautiful sight!

Photo credit: Kathleen O'Keeffe
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