Here's how it's thought to have developed:
* Cloud Types: In the 19th century, meteorologists developed a system to classify clouds into different types. The highest cloud type was called "cirrus", which is a wispy, feathery cloud found at very high altitudes.
* Cloud Numbering: The "cirrus" cloud type was assigned the number "9" within this classification system.
* Elevated State: Because of the cloud's height and association with a sense of elevation, it began to be linked to a feeling of exalted happiness or being in a blissful state.
* Idiom Formation: Over time, the phrase "to be on cloud nine" evolved to describe someone experiencing extreme joy or happiness, drawing on the association of the "cirrus" cloud with height and a sense of being above it all.
While there's no definitive evidence to prove this origin story, it's the most widely accepted explanation for the phrase's origins.