How High is Cloud Nine?

Cloud 9 isn't the furthest in the air, cloud 0 is.
   According to the International Cloud Atlas scale, Cloud 0 (the Cirrus) is the highest type of cloud. It can be as high as 40,000 feet (12,000 metres) in the air.
   Cloud 9, the massive thundercloud (Cumulonimbus) is at the bottom of the scale because a single cloud can cover the whole range from as low as a couple hundred feet to the very edge of the stratosphere (about 50,000 feet or 15,000 metres).
   As with the origins of most phrases, its unlikely that 'cloud nine' can not be attached to just one specific source. There have also been recorded, clouds seven, eight and thirty nine, so it's more likely that people settled on 'nine' because it's considered a lucky number (phrases like 'the whole nine yards' or 'the cat with nine lives' commonly use reference of the number nine, a trinity of trinities). Also, the idea of riding on a big, surging cloud definately seems appealing to the mind.
   The Internatioal cloud Atlas was published in 1896 after The International Meteorological Conference established a cloud committee to agree on an international system for the naming and identification of the various clouds.
   The ten categories, were based on the pioneering work of Luke Howard, an english chemist who published his essay on The modification of clouds in 1802, influnced by his experience of freak weather conditions as a child.
   Clouds are suspended collections of tiny water droplets or ice crystals in the Earth's atmosphere. These droplets/crystals are formed within the condensed state of water vapour and they surround particles of things like smoke or salt, called condensation nuclei.

   Cirrus clouds are the only clouds in the sky completely made of ice crystals. They are mostly formed by the condensation trails of high flying jets and they help to regulate the earth's temperature.

  When air traffic was stopped after 9/11, daily temperatre variation grew by up to 3°C over the 48 hour period as the cirrus protection began to deteriorate; letting in more sunlight at day, and letting out more heat at night.