I'm sure there are sky jockeys that can explain it in better terms than me Dem but my understanding is that once they are above the cloud levels they refer to it as being above the weather something like > 15,000 ft...
Clouds go way up above 15,000 ft; some CBs (bangers) go up to, and occasionally above, 45,000. As the name suggests, clear air turbulence (CAT) happens away from, and above, clouds and is caused by pockets of air at differing speeds mixing and causing instability in the atmosphere. Really fast moving air manifests itself as a jetstream which is effectively a river of air which can travel at speeds of up to 200 mph. In the northern hemisphere, the jetstreams almost always move from west to east. So, when flying to the States you avoid them like the plague as they wreck your ground speed but coming home you try and ride those babies for as long as you can. However, you can experience CAT as you enter, and leave jetstreams, due to the 'shear' between fast and slow moving air. Sometimes you can fly 10 hours to or from Cancun and not feel a jolt and sometimes it's like galloping a horse drawn cart over Belgian pave. Still, it's an office with a nice view however lumpy it gets!