According to the “Morris Dictionary of Word and Phrase Origins,” the phrase is based on U.S. Weather Service terminology. This theory holds that cloud types are numbered, and that “cloud nine” is the designation given to “cumulonimbus” clouds, the highest-flying clouds around, making them an apt metaphor for being “on top of the world.”
When “you’re on cloud nine” you feel wonderful!
The phrase to be on cloud nine, meaning that one was blissfully happy, started life in the United States and has been widely known there since the 1950s; itâ€™s since spread worldwide. Itâ€™s said to have been popularised by the Johnny Dollar radio show of that period, in which every time the hero was knocked unconscious he was transported to Cloud Nine. But that wasnâ€™t the origin of the phrase. Itâ€™s been around since the 1930s, though early examples show a lot of numerical variability, with the cloud sometimes being as low as number seven or eight or as high as thirty-nine, though seven and nine were most common.
These discrepancies make me suspect the usual explanation of its origin, which is that it comes from the US Weather Bureau. 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 â€” alcohol in its earlier days but more recently crack cocaine â€” so perhaps we shouldnâ€™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 itâ€™s a traditional lucky number and that todayâ€™s more usual nine appears for similar reasons â€” for example it also turns up in dressed to the nines and the whole nine yards.
Then, according to Thomas Veil on the Phrase Finder website:
The true and original origin of “Cloud 9” is derived from Buddhism.
The state of being in “Cloud 9” is the penultimate goal of the Bhodisattva.
Check on Martin Luther King’s paper on
“The Chief Characteristics and Doctrines of Mahayana Buddhism”.
“The Mahavastu, a late Hinayana work, gives a list of ten stages in the progress of the Bodhisattva, and the same number is retained, with modifications in detail, by the Mahayana authorities… In the ninth stage the seeker reaches the point when all his acts are unselfish, done without desire. Finally the Bodhisattva reaches the tenth stage in which he becomes a tathagata, a cloud of dharma.[Footnote: Radhakrishnan, op. cit., pp. 601, 602.]
I believe that the ninth cloud is also referred to as “The Bright Cloud of Great Refuge”.
Finally, we have Christi the Wordsmith, who weighs in with:
In Dante’s Paradise, the 9th level of heaven is closest to the Divine Presence, which itself dwells at the 10th and highest heaven. This notion may have enhanced the popularity of the expression on cloud nine.
So it seems that there is no single answer. Which, in my opinion, makes your question particularly fascinating.