Circumzenithal Arc – The Happy Halo
Of all the many halo phenomena, those arcs and spots of light caused by sunlight glinting through and off ice crystals in the atmosphere, the brightest and most dramatic turns out to be the least noticed. The circumzenithal arc forms quite often and is striking once you see it. It looks like a vibrant multicoloured smile, but it is hiding in a part of the sky where most people never look.
And that’s directly overhead. The location of this optical effect is suggested by its name: circumzenithal means ‘around the zenith’, the point in the sky straight up above you. Imagine tracing a large circle overhead with the zenith at its middle. This optical phenomenon appears as an arc of that circle, one that’s at the side nearest to the direction of the Sun.
The circumzenithal arc featured as Cloud of the Month for April was spotted by Laura Simms (Member 32,141) over Surprise, Arizona, US. The wide-angle lens of her camera makes it seem lower in the sky than it actually would have been. The ice crystals that formed Laura’s circumzenithal arc came courtesy of high ice-crystal clouds known as Cirrus fibratus. The crystals up in this cloud would have been in the shape of tiny hexagonal plates of clear ice that were oriented with their large faces almost horizontal as they gently fell through the sky like autumn leaves. They acted like microscopic atmospheric prisms of ice, bending the light as it shone through them.
If you ever see the other optical effect known as a sun dog off to one or the other side of the Sun when it’s low in the sky*, always remember to check directly above you. Sun dogs are formed by the same platelet shape and horizontal orientation of ice crystals that can produce circumzenithal arcs.
Most people never bother looking up, but cloudspotters aren’t most people. And this vibrant smile of spectral colours is the sky’s secret way of acknowledging them.
Circumzenithal arc in Cirrus fibratus clouds, with a contrail and Altocumulus also present, spotted over Surprise, Arizona, US by Laura Simms (Member 32,141). Look for the tiny hot air balloons to get a sense of scale.
