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Note that the possible height of cloud varies with location and airmass, so the highest cloud can occur in the tropics, the lower heights (with cirrus possible at 10,000 ft or 3500 m) in polar regions.
Cirrus: They are the highest of all clouds and they are composed purely of ice crystals. They occur, specifically, at an altitude of  16,500 to 45,000 feet and they can be found all over the world. Although they are known to form precipitation, none of it reaches the ground.