UK Rare Arc

Rare for some - A circumhorizon arc shining in wisps of cirrus. Sighted by Sally Elder 8th June '14 over the Solent, Southern England. A rare sight indeed in England, almost impossible in Scotland yet common in the USA.
All images ©Sally Elder, shown with permission

The circumhorizon halo can sometimes be mistaken for cloud iridescence.

The CHA has ordered spectral colours ranging from red on top through yellows and greens to blues at base.

Iridescent cloud colours are almost never so ordered nor so pure.

Rare or Common?

There is an Internet myth that the arc is a very rare even a once in a lifetime sight. Nonsense.

But ease of seeing does depend on location, or more precisely, geographic latitude.

The sun must be higher than 58°. No problem in summer in most of the USA where it might be seen several times.

Northern Europe is different. In southern England, weather permitting, it might be glimpsed near noon for a few weeks around the solstice. In Scotland abandon hope unless looking for a lunar circumhorizon arc.

Sun rays enters side faces of plate crystals and leave through the lower near horizontal face.

This ray path is only possible when the sun is higher than ~58°. At ~58° the emergent ray grazes the ice crystal lower face. When the sun is lower the ray is instead totally internally reflected from the lower face and we see no circumhorizon arc.

The circumhorizon arc is one of the largest and most colourful of the ice halos. As here, it is often seen only as a fragment filling small cirrus clouds with its spectral colours.