Why does warm air rise
Air exhibits the tendency to rise up when it is heated. Heating up of air doesn’t sound like an activity one could undertake in a physics laboratory easily. One doesn’t need to, actually. The air around us constantly experiences heating effects from the radiations of the sun, the heat reflected from the surface of the earth, and the heat content of the gases circulating in the atmosphere. Upon heating up, air rises up.
Before venturing any further into the discussion regarding this property of the air, we need to know that the heating and the consequent rising up of the air is an overall adiabatic process, i.e., a process with net zero energy receipts. There is no energy lost or gained in an adiabatic process. Coming back to the point, let us assume that ait has acquired a considerable amount of heat from the cumulative effects of all the factors mentioned above. The heat results in the gaining of kinetic energy of the air molecules. Due to the enhanced kinetic energy, these molecules become a lot more dynamic and start moving rapidly. Soon, the molecules start buzzing and bouncing off each other, at fast velocities. This leads to the logical conclusion that the density of the packets of air becomes a lot less than it used to be when the air was cooler. Obviously, lesser density implies more buoyancy and hence, the warm parcels of air tend to rise up.
The rising of the air is not a self sustained phenomenon. As air rises up and expands, it starts to lose the acquired heat in a corresponding manner. Soon, it reaches the temperature equivalent to that of the surrounding air blanket. This is where the rising motion ceases. Quite interestingly, the air becomes cool at higher temperatures. Thus, the reverse effect stars exhibiting itself at higher altitudes when the colder air starts plummeting downwards.
The idea behind the rising up of hot air can be easily scaled up to explain many atmospheric phenomena like hurricanes, dry and wet seasons alternating one after the other, etc.