Hot air rises because its less dense than cold air. The reason so has to do with the kinetic particle theory. As the temperature increases, the average speed of the molecules increases, and thus they collide with more kinetic energy and spread out more. Thus there are less particles per unit volume, decreasing the density.
Additional-Dark2919 t1_j67sfki wrote
Reply to Eli5 : What does hot air rise and cold fall? And why they higher I get in the atmosphere, the colder it get? by hopitlong21
Hot air rises because its less dense than cold air. The reason so has to do with the kinetic particle theory. As the temperature increases, the average speed of the molecules increases, and thus they collide with more kinetic energy and spread out more. Thus there are less particles per unit volume, decreasing the density.