Cloud 9

[pirwany]

What is the story behind being "on Cloud 9" when someone is very happy?

[Clanad]

Our dependable researchers at World Wide Words offer this:
"...the usual explanation of its origin, is that it comes from the US Weather Bureau (as early as the 1930's).

The story is that this organisation describes (or once described) clouds by an arithmetic sequence. Level Nine was the very highest cumulonimbus, which can reach 30,000 or 40,000 feet and appear as glorious white mountains in the sky. So if you were on cloud nine you were at the very peak of existence.

The term has always had close associations with the euphoria that is induced by certain chemicals � alcohol in its earlier days but more recently crack cocaine � so perhaps we shouldn�t ask for too great a level of exactness in counting. And the cloud here is an obvious reference to some drug-induced dreamy floating sensation. But I suspect, without having anything so restricting as evidence, that seven was chosen because it�s a traditional lucky number and that today�s more usual nine appears for similar reasons � for example it also turns up in dressed to the nines and the whole nine yards."

[Quizmonster]

Just to add a little to C's fine answer above, in the International Cloud Atlas, there were ten cloud types and Cloud 9 was the white, fluffy, cottonwool-like cumulo-nimbus...the sort angels are usually portrayed playing harps on! In that way, being "on cloud nine" meant floating free, in a sort of paradise, on a soft white cushion with not a care in the world.