Answer: Warm air rises, cool air sinks. Some areas of Earth's surface heat up more readily than other areas. Air that touches these "hot areas" becomes warmer than the surrounding air. As the heated air expands it becomes less dense than the surrounding air and is buoyed upward as a thermal bubble that rises, expands, and cools. This rising of air is accompanied by the sinking of cooler air aloft. As the cool air sinks to the warm surface, it is heated where it in turn warms and rises thus repeating the convection cycle.