"Cloud nine" is a phrase that refers to a feeling of perfect happiness. It originated between the years 1955 and 1960 and is often used by saying someone is "on cloud nine."
Some people connect this phrase to the 1895 International Cloud Atlas, which describes the 10 cloud types. Cloud type No. 9 is called cumulonimbus, and these clouds are the biggest, puffiest and most pleasant looking. Some people suggest that the number nine is significant because it is the largest one-figure number. An example of this phrase used in a sentence is "Despite her exhaustion, Martha was on cloud nine after winning first place in the marathon."