Answer:

Generally warmer substances, air for example, rises above cooler. The reason is that the heat causes expansion and the air becomes less dense. The cooler air then falls through the warmer layer, pushing it upward.
It is more correct to say that warm air is pushed upward, rather than saying warm air "rises".

First answer by Sikuan. Last edit by Sikuan. Contributor trust: 108 [recommend contributor recommended]. Question popularity: 2 [recommend question].