Lets say you put your rad in the bottom of a small chimney.
The hot air above the rad will start rise and suck in cool air through the bottom. The draft in the chimney=velocity of the air depends on the ambient temperature at the top and the temp of the hot air comming out.
Bigger difference gives better draft.
I think the height of the chimney also matters but i can't recall it's significance. You need some kind of chimney though so the hot air can accelerate, undisturbed, and produce the wanted draft.