Is the CHA rare?

Circumhorizon Arc imaged at Norfolk Virginia by Rob Bruner August 25 '08. ©Rob Bruner, shown with permission.

Circumhorizon arcs spread huge and colourful beneath the summer sun.

They are often said to be rare. But that depends, among other things, on where you are.

The sun must be higher than 58� to form it, any lower and the rays that would have contributed are internally reflected inside the lower face of the plate crystals and add their brightness instead to the parhelic circle.

Manchester in northern England has only 45 days around midsummer when the sun gets sufficiently high that it is possible to see it - sunshine and favourable cirrus cloud permitting. Mancunians will admit that those are big qualifications! At Edinburgh in Scotland the sun does not ever get high enough. Contrast Norfolk Virginia which has 155 days or Houston's 192.

The circumhorizon arc is not that rare in much of the US but remains an exquisite sight for all that.