Actually, air is constantly moving... even on calm days. Warm air is rising and cool air is sinking. You might think of it as warm air is fluffy and light, while cold air is dense and heavy. As warm air rises, it cools. As it cools, it begins to sink. As it sinks, it begins warm again… and back up it goes. Since the earth has vastly uneven terrains (mountains, valleys), the temperature of air isn’t constant. That’s why all the air doesn’t rise or sink at the same time. Thus, the air is constantly moving.