WARMER fluid is replaced by colder fluid creating convection current. The force which induces these convection is called a buoyancy force which is due to the presence of a density gradient WITHIN the fluid and a body force.For free convection Nusselt number is a function of Prandlt number and Grashof Number.For fluid over a heated horizontal plate, Grashof number is MAXIMUM so Nusselt number will ALSO be maximum i.e. convection dominates. So minimum temperature difference will be REQUIRED fluid over a horizontal plate.