A schematic picture showing how horizontal flows are initiated by warm air rising and cold air sinking. There are two parts to the figure. Part (a) shows a cross section through a room. On the left wall of the room is a radiator above which warm air is rising. At the opposite, right, end of the room is an open fridge, adjacent to which cold air is sinking to the floor. Part (b) shows the same cross section some time later. The rising and sinking air has set up a clockwise circulation pattern around the room.