Answer: Cumulonimbus

While it’s a phrase that doesn’t get much use these days, over the last century when someone said they were “on cloud nine” everyone understood that to mean they were extremely happy. But how, exactly, did the ninth cloud become the place where elated people went to lounge?

Although, as with most word and phrase etymologies, it becomes increasingly more difficult to trace origins as you move back through time, the origin of the phrase “cloud nine” is commonly attributed to a scientific text. The 1986 edition of the International Cloud Atlas listed ten types of clouds, including the Cumulonimbus cloud. One of the defining characteristics of the Cumulonimbus-type cloud is that it grows vertically and reaches altitudes of 60,000 feet. The International Cloud Atlas listed the towering and fluffy-topped Cumulonimbus as, you guessed it, cloud nine.