Where did the idiom to be on cloud nine originate?

The idiom "on cloud nine" (meaning to be in a carefree kind of bliss) appears to have a rather muddled past. The basic idea of having "one's head in the clouds" (meaning rather out of touch with reality for some reason) has been in use in slang since around the 1930s and has evolved over the decades. However, the cloud number has changed along with it in accordance to the different ideas in which the idiom has been used. It is during this time you get related idioms such as "seventh heaven".

The use of the specific "cloud nine" seems to have been solidified through music. Both The Temptations in 1969 and George Harrison in 1987 titled albums "Cloud Nine" (in the former's case, the album also contained a song of that title with the lyrics, "I'm feeling fine on Cloud Nine."). The use of "cloud nine" has apparently stuck from that point.