Thanks for your thoughts gentlemen.
I certainly have several descriptions of skies that can be used.
I have a very detailed description of weather and weather phenomena in my keyword hierarchy.
I also have colours.
I also have an attributes and characteristics hierarchy that can be used to describe anything, including skies.
My difficulty is whether 'sky' itself is a useful keyword in itself. (Mixed thoughts on this forum so far - see above.)
If one thinks of how one would classify 'dog' this is easy: dog is a canine, canine is a placental, placental is a mammal, mammal is an animal.
Can one classify 'sky' in any useful way or is this something that truly stands alone?