|Two showings of Europe's rare halo
(Upper: Circumhorizon arc near Solden, Austria (47°N) July 10, '09 by Jeff White.
Lower image: CHA at the foot of the Carpathian Mountains in Ukraine (~49°N) by Irek Lochert June 14, 2009. ©Jeff White, Irek Lochert, shown with permission.
Circumhorizon arcs - never, ever, 'fire rainbows' if you want street cred in atmospheric optics circles - are generally considered to be rare. But that depends on where you are.
Horizontal plate crystals in high cirrus form them. Solar rays enter a side face and leave through the large lower hexagonal face. The ray path is only possible when the sun is higher than ~58° and this limits the place and time of year when it might be seen.
Solden in Austria (upper image) is has a viewing window from May to Mid- August.with only about 290 hours when the sun is high enough. Paris (220 hours), London (140 hrs), Holland and northern Germany have even smaller windows. North of Copenhagen's Tivoli Gardens a solar CHA is never seen.
In contrast, most of the United States is at lower latitudes, the sun is higher for longer and skies are less cloudy. Boston might get a CHA between mid April and late August (430 hrs). Houston has 740 hours of possible visibility.
A CHA is a rare and treasured sight in mid to north Europe. It is a relatively common but hopefully still appreciated apparition over most of the USA.
For visibility dates and windows see graphs here.