Sleeping Freightdog,
Clear air turbulence is a function of windshear, most commonly occuring near the jetstream, but potentially found at any altitude and occuring between currents of air moving in any direction (horizontally or vertically).
It can occur due to mountain waves or disturbed air from orographic lifting (mountains can cause severe turbulence that extends hundreds of miles downwind).
Clear air turbulence does not occur due to swamp gas or big bubbles of hot air, any more than television programs about Bigfoot make it more than a ridiculous myth.