Why does warm air rise during convection?
Why does wood float on water?
Wood is less dense than water, and it displaces a volume of water that has greater mass than the mass of the wood. It floats.
Warm air or warm gas is LESS dense than cold gas, and it tends to rise above the cold air. As it rises it gets cooler, and there is net movement between the colder temperature gas and the warmer gas below: hence convection occurs.