(...) There's a continuous spectrum between e.g. altocumulus and cirrocumulus; sometimes it's really clear as to whether a particular cloud formation is one or the other, sometimes it isn't.
(...)
The cloud formation with iridescence which I photographed (above) was quite definitely altocumulus though of an unusually thin and transparent kind.
Exactly!
Steven's #1 photo is a good example of those "thin Ac", which are hard to define whether they are Ac, Ci or even Cc
Here some photos of similar cloud forms which I found in my latest archives:
July, 03
July, 26
August, 15
All of them, in my opinion, are Altocumulus
