When you say that you are on cloud nine, what you mean is that you are extremely happy; you feel as if you are on top of the world.
Example: The entire team was on cloud nine after winning the championship.
No one is really sure where the expression comes from. Some believe that the original expression was `on cloud seven’, very similar to the expression `seventh heaven’. Others believe that nine was chosen because it is considered a mystical number. According to weathermen, `cloud nine’ refers to the `cumulonimbus’ or the thunderstorm cloud. This cloud often rises to forty thousand feet; and when you are `on’ such a cloud, you are literally on top of the world!