When air is warmed, it expands and becomes less dense. As the air becomes less dense, its air pressure decreases. This occurs because molecules in warm air have greater kinetic energy (energy of motion) than in cold air. As the molecules move faster they spread out. As the molecules move farther apart the volume of the air increases and its density decreases. When air is cooled, its molecules move closer together. The air contracts and becomes denser. As air becomes denser, it contains more mass per unit area and exerts greater pressure. Therefore, cooler, denser parcels of air have a higher air pressure than warmer, less dense parcels of air. As air warms it becomes relatively less dense than the cooler surrounding air and rises as a result.