Origin of 'cloud nine' or 'cloud 9'?

Just saw this term in another thread. I know it means elation, but why?

Some thoughts

Ah, Thanks.

It’s a great site…check out the whole nine yards for a version I had never seen before.

I didn’t look at the title of the site initially, but now that I have I think I’ll have a deeper look.

I think I asked about the whole nine yards once… and was told it was related to American football. The article on the site is long… I’ll read it at work. While eating a warm runny scotch egg.

Search this site for threads about this phrase. Earliest print cites seem to suggest some US military origin. If it came from football, why aren’t the earliest cites by sports writers and such?