Brilliant ‘Sun halo’ graces skies over the Northeast yesterday

Anyone living along the U.S. East Coast and around the lower Great Lakes may have seen a really cool sight if they looked up into the sky yesterday, as a brilliant rainbow-coloured halo encircled the Sun.

Called a 'Sun halo', 'icebow' or 'nimbus', or even a '22° halo', this incredible site is caused by sunlight shining through thin clouds, called 'cirrus', that are 5 to 6 kilometres high in the sky.

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These wispy clouds are made entirely of tiny six-sided ice crystals, and light passing through the crystals is deflected (both reflected and refracted), and it separates out into colours, like when light passes through a prism. Because of the hexagon shape of these crystals, the strongest deflection is at an angle of 22° (thus the name), but the separated colours fan out to higher angles.

There's enough crystals in the cloud, all of them randomly oriented, that most of these deflections just cancel out, and we still see the Sun in its actual position (although a little 'washed out'). However, enough of the effect is preserved that we see the Sun encircled by a brilliant halo of light that bleeds off to rainbow colours at the edges.

It's worth noting that, even though the Sun threw off three powerful X-class solar flares over the past few days, these halos are solely due to the conditions here on Earth.

Also, although this phenomenon is related to a 'Sun dog', they aren't always the same thing. Sun dogs are the extremely bright points that can be seen on either side of the Sun, usually when it is fairly low in the sky. They're caused by the same process as a halo, and they can even be seen at the same time as a halo is happening, but the name specifically refers to the one or two 'false suns' that are seen to the side.

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This same effect can happen with the Moon as well — called a 'moon halo', 'lunar halo', 'moon ring' or 'winter halo' — and there's even an old saying that goes "a ring around the Moon means rain soon". This is because cirrus clouds are typically seen ahead of the 'warm front' of a storm system, which usually brings rain (or snow) to the area.

Did you see yesterday's halo? Let me know in the comments below!

(Photos courtesy: Desmond Boylan/Reuters, Wikimedia Commons)

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