Cloud Nine

On cloud nine (a euphoric state). The phrase appears to be American. Originating from the U.S. Weather Bureau's rating of clouds. A cumulonimbus cloud is rated 9 often reaching 40,000 feet. In the 1950s a popular radio show "Johnny Dollar" in a recurring episode the hero was knocked unconscious and awoke on cloud nine.

Source: Dictionary of Word and Phrase Origins, Nigel Rees