In a macroscopic context, where you talk about air pressures, densities, volumes convection is pretty easy to understand; air heats up, it expands, is less dense than its surroundings and hence rises.
But the question is how does this work at the molecular level?
Hot molecules weigh the same as cold molecules hence why does a hot molecule rise?
You will not find an explanation in terms of individual molecules because
there is not one.