When you claim that you are "on cloud nine," what you really want to express is that you are delighted." You have a sense of being in control of everything.

Example:
The entire team was on cloud nine after winning the championship.

Nobody is exactly clear where the term originated from. Some people think that the original phrase was "on cloud seven," which is remarkably close to the phrase "seventh heaven." Some say that nine was selected because it is a magical number. The cumulonimbi, or thunderstorm cloud, is what weather experts refer to as "cloud nine." This cloud frequently reaches at height of 40.000 feet and being "on" one of these clouds is likely being on top of the world!