GUYS! YOU ASKED: "But teacher, "why cloud 9, not 7?"
Here it goes: Whenever a phrase includes a number, like the whole nine yards,or on cloud 9 then attempts to find its derivation usually focus on the number. 'On cloud nine' is no exception. A quite interesting explanation is that the phrase derives from Buddhism and that Cloud Nine is one of the stages of the progress to enlightenment of a Bodhisattva (one destined to become a Buddha).
Anyway, early examples of 'cloud' expressions include clouds seven, eight, nine and even thirty-nine.
The early favourite was 'cloud seven' and many of the oldest citations use that form, as in this piece from The Dictionary of American Slang, 1960, which was the first printed definition of the term

"Cloud seven - completely happy, perfectly satisfied; in a euphoric state."

This early preference for seven as the significant number may have been influenced by the existing phrase 'seventh heaven'.

So now, back to you: Which cloud do you prefer: 7 or 9?
Kisses!! Marcela