Hot air rises.
Cold air descends as a result of the hot air rising (something has to come in to fill the void)
Secondly. As a gas heats up, it expands, so the same mass takes up more space. Therefore a volume of hot air "weighs" less than the same volume of cold air. Since the air is in somewhat of a column, the "weight" of the air pressing on the air at the bottom of the column is more in cold air than it is in warm air.