This saying dates back to the early 1800s. It was during this era that scientists, especially an amateur meteorologist Luke Howard, first began organizing clouds. By observing cloud appearance and height above ground level, they categorized all clouds into ten basic cloud types that we still use today: cumulus, stratus, stratocumulus, nimbostratus, cumulonimbus, altostratus, altocumulus, cirrus, cirrostratus, and cirrocumulus.
To make recording cloud observations easier to remember, Howard also assigned a number from 0 to 9 to each of these ten cloud groups. According to their code abbreviations, zero represented the lowest clouds (stratus), and nine, the tallest clouds (cumulonimbus, or thunderstorm clouds). It’s from this that the expression “ cloud nine” was born!
Today’s meteorologists still learn these 0 to 9 cloud codes in school, but rarely use this terminology in broadcast. And because the numbers aren’t used in television forecasts today, most don’t even know they exist.
Now you have where the term “cloud nine” originated.