You probably wouldn’t want to be on cloud ten.
Dear Matthew Alice: When we're feeling good, why do we say we’re on cloud 9? What about clouds I through 8? Could things ever get so good that we'd be on cloud 10 or 12? — Generally on Cloud 5-1/2 or 6 in San Diego
A couple of choices here. Once again we have fistfights in phrase-origin land. Pick of the litter, according to the experts: “Cloud nine” is an extension of the idea of Dante’s ninth and highest heaven (the Primum Mobile, the celestial sphere closest to God and the angels) described in The Divine Comedy. Most far-fetched proposition: The expression is somehow related to Cloud-Cuckoo-Land, the fantasy realm from Aristophanes’ play The Birds—which has nothing to do with the number nine and really was no paradise at all, in the end. But the theory has its advocates
My fave, and the best word picture: Since the 19th Century, meteorologists have grouped clouds into ten genera based on shape and altitude, from low-level stratus through high-level cirrus. Genus nine (cloud nine) includes the huge, vertical, billowy, fair-weather, cumulus clouds. Therefore, to be on cloud nine is to be sunny and high as a kite. You probably wouldn’t want to be on cloud ten. That’s the cumulonimbus genus, generally a precursor of thunderstorms, hail, and maybe even tornadoes. And to save you the trouble of asking, “seventh heaven,” a close neighbor of cloud nine, comes from the Islamic concept of heaven divided into seven realms, each higher and more sublime than the last, the seventh being the dwelling place of God.