The correct answer is - convection current.
There is a non-stop cycle that goes on on our planet, where we have a situation where the hot air rises above, and the cold air sinks bellow. It is a very simple process, that appears because of the different density of the hot and cold air masses. The hot air masses are less dense, and lighter, while the cold air masses are more dense, and heavier. Because of this, the cold air always has the tendency to go down, as it goes down it squeezes and pushes up the hot air, thus we get the convection current cycle.
