What is the origin of the phrase "On Cloud Nine" ?

I don't know, but I'd also be interested in knowing the answer. I guess the number is pretty random, because in German it's "cloud seven".

Cloud Nine is of unknown origin or significance. It is an Americanism dating to the 1950s--other than that we don't know anything of its origin.
Various tales have been attached to the phrase due to the number nine. But are certainly false because in the phrase's early days other numbers were often used, most commonly seven, sometimes thirty-nine. A 1930s quote uses Cloud Eight to refer to being drunk (which hints that the phrase may be considerably older than the 1950s). The probable meaning is simply a generalization of "out there" with no specific intent.

(reference from http://www.wordorigins.org/wordorc.htm#cloud%20nine)

"CLOUD NINE – “The expression ‘up on cloud nine’ to describe a feeling of euphoric exaltation is based on actual terminology used by the U.S. Weather Bureau. Clouds are divided into classes and each class is divided into nine types. ‘Cloud nine’ is the cumulonimbus cloud that you often see building up in the sky in a hot summer afternoon. It may reach 30,000 to 40,000 feet, so if one is up on ‘cloud nine,’ one is high indeed. The popularity of ‘cloud nine’ as a catch phrase, though, may be credited to the ‘Johnny Dollar’ radio show of the 1950s. There was one recurring episode, like Fibber McGee’s famous opening of the closet door. Every time the hero was knocked unconscious – which was often – he was transported to ‘cloud nine.’ There Johnny could start talking again.” From “Morris Dictionary of Word and Phrase Origins” by William and Mary Morris (HarperCollins Publishers, New York, 1997)." -Phrase finder