In general, mist starts at the surface. We've all sat at some time waiting for it to burn off. Or at least start showing a bit of blue in the overhead. It's not cloud. On a Metar it's referred to as MIST. Vis is always in excess of 1,000 mtrs. Less than that is likely to be considered fog.
There are scenarios where the distinction between mist and cloud can be a bit blurred. Here's one example. Potentially scary for a non-instrument pilot.
File:Heavy mist.jpg - Wikipedia, the free encyclopedia