Explanation:
Heat transfer by convection involves gases and liquids as a result of density differences when they are heated.
In the hot air balloon, the air closer to the heater is warmed. This air becomes lighter and it rises. This in turn pushes the balloon up. The cold air away from the heater is denser and begins to sink.
This exchange of hot and cold air sets up convection cells within the balloon and this how heat transfer is attained.
