A friend of mine who is on holiday in Cape Cod recently posted this nice picture on Facebook and remarked that they seem to have flat rainbows in that part of the world. Spotting unusual rainbows and halos is one of my hobbies (see earlier posts here, here, here and here), but this was one I did not recognise. I guessed from the appearance, and the fact that it seems to be caused by a cirrus cloud, that it was a halo (caused by ice crystals) rather than a rainbow (caused by water droplets). As usual, the very splendid atmospheric optics web site had the answer: it is a circumhorizon arc. The same web site also has a good explanation of why you are far more likely to see this particular halo in Massachusetts than you are in the south of England, and why you are never going to see one in Scotland: essentially, the Sun has to be very high in the Sky.