I'm on cloud nine.

The International cloud atlas today consists of two volumes, containing many unknown and unusual cloud formations. Many of these clouds, I don't think evoke any interest outside meteorology. The first cloud atlas published in 1896 had only 10 clouds. (Mariners like me still study only those 10 clouds, which more than suffices our practical requirements) Among the 10 in the original atlas, the most fluffy, soft, cushiony looking one was number 9, cumulonimbus. That seems to be the source of the phrase "to be on cloud 9". That's how the story goes, although it's an unconfirmed etymology for the phrase. Its just too cute to let it go.