Anyway, Dave, the chimney top has to be higher than the highest part of the house that is directly behind or infront of the chimney top.
When the chimney top is above the ridge line, the wind will go over the top of the chimney creating a vacume if you will and cause the smoke to draw, but if the ridge line is higher, the wind or environment will roll or rumble over the chimney top and cause a down draft of sorts.
Its like when I was in the Navy, at sea we dumped our garbage off the fantail or back of the ship. Due to the forward motion of the vessel the water behind the vessell was "dead". The trash would stay there and float with the ship. But if a man fell over board along side the ship and when the fantail came to him, this water would suck him into this "dead" area and take him to the screws.
The wind does the same thing, it does not have to be wind, just movement of the air molecules over the chimney top.