'Tis the season for circumhorizontal arcs! Any day with cirrus is a candidate around mid-day. Here is my third CHA of the season, ~11:30 on 1June08. A lucky 22 deg. halo segment helps to demonstrate the relative postions of the two halos. The CHA is fun to watch, because it lengthens or shortens as the clouds pass, or hides behind cumulus.

I think what you actually have there is a Circumhorizon Arc (CHA) = the almost horizontal one at the bottom PLUS a Circumzenith Arc (CZA) at the top = the 'upside down rainbow'. Amazing to see both phenomena together, I've never seen both at the same time. Thanks for posting them. GWW

@ Granny Weatherwitch:
Isn't the Circumzenithal Arc supposed to be above the sun? This looks more like the lower part of a circumscribed or 22deg halo.

GWW, I did not mention it in my post, but the entire 22 deg halo was visible before I could get my camera out. So I feel pretty certain that what I did manage to capture was not the CZA. Sorry if I mislead you by not providing enough info. All that aside, thanks for looking and commenting.

(Note for Andrew - no need to apologise for the 'confusion' it was a good excuse to dig further into the mystery - always good for the old brainbox !! ) There are many beautiful and amazing atmospheric optical phenomena and GWW is still learning about them after over half a century of skywatching ! The most common ones around The British Isles are rainbows, probably because the exact conditions of extensive sheets of ice crystals at high altitudes needed for the various types of Arcs & Circles are less common here. Also, a lot of the time, high ice-clouds which do exist are hidden by lower layers of cumulus or stratus varieties. It seems that arcs form much more often in desert areas (including Antarctica, which has precipitation data putting it in that caterory, strange as that may seem). I have found references to 13 different names for types of arc, although it may be that some of them refer to the same thing. Plus, of course, any incomplete part of a circular halo also fits the mathematical definition of 'arc' as well. Things get confusing, such as when a Kern Arc seems to have the potential to appear as a faint circle, and so naming it as an arc gets us all muddled up about it ! The visible part of any incomplete circular phenomenon appears as an arc, of course. It is very hard to tell from a photo, having no indication of scale, whether the upper arc in the photo above is a CZA. All we can use to judge it are the brighter colours than those of haloes; and the fact that, although it could be part of a 46 degree halo - those are quite rare. The more common, smaller halo can be seen around the sun at the top of the picture. From its name, a CZA would be formed around the zenith point, which would be above the sun; but I'd think that would mean that any part of it above the sun from an observer's point of view would appear as an upward curve and not look like "an upside-down rainbow". CZA's are often alled upside-down rainbows. The CHA is curved in the other direction, but looks very much flatter because it is part of a much larger circle close to the horizon.
Granny Weatherwitch is no expert on optical phenomena. I can only refer you to the amazing and wonderful website pages of Les Cowley at http://www.atoptics.co.uk/halo/cza.htm, http://www.atoptics.co.uk/halosim.htm (downloadable PC halo simulator), & http://www.atoptics.co.uk/links.htm (links page which includes one to the Cloud Appreciation Society). I don't know if Les is a CAS Member; but, if not, maybe Ian & Gavin could create an Honourary Membership in recognition of their awesome website ? It even has a booklist for atmospheric optics ! This includes "Light and Colour in the Outdoors" by Dutchman M.G.J.Minnaert, which was written in 1940 and is now in reprint. I have a 1959 copy and it is an amazing book.
Another excellent site about sky optics is by photographer Harald Edens at:
http://www.weatherscapes.com/gallery.php?cat=optics&subcat=light_ice&expand=light_ice To quote the site-owner "If you ever saw something strange in the sky and wondered what it was, chances are that you'll find the answer here." Link & Enjoy ! GWW

Dear Andrew and Granny Weatherwitch,
Apart from being below the sun, another clear indication that the upper arc in the picture is not a CZA is that it has its red part up and its blue part down. For a CZA this is the other way around. See http://www.atoptics.co.uk/halo/cza.htm

And, finally: a CZA can only form when the sun is less than 32.3 degrees high (http://www.atoptics.co.uk/halo/czasalt.htm); a CHA only when the sun is more than 58 degrees high (http://www.atoptics.co.uk/halo/cha2.htm). This means that both phenomena occurring together is impossible. Which is a true pity!