When you say that you are on cloud nine, what you mean is that you are extremely happy. You feel as if you are no top of the world.
• 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, Could nine refers to the cumulonimbus or the thunderstorm could. This cloud often rises to forty thousand feet; and when you are on such a cloud, you are literally on top of the world.
COURTESY : The Hindu (The National News-Paper) - India