What is responsable for the difference in mass between hot and cold air?

Answer 1

Th molecules are closer together in cold air so it is denser, or the same volume is heavier. In warm air the molecules are moving about faster so take up more room. They are farther apart, less dense so warm air rises.

Answer 2

Think of air molecules bouncing around like ping pong balls in a bingo machine. At a higher temperature they move faster and collide with more energy and this spreads them apart so that there are less molecules per cubic foot. Less molecules in a given space means less mass or weight.

Answer 3

Density.