Convective heat transfer occurs within a fluid itself and it is carried out by transfer of one fraction of the fluid to the remaining portion. Hence unlike conduction, transfer of molecules occurs during convection. Since movement of particles constitutes convection, it is the macro form of heat transfer. Also convection is only possible in fluids where the particles can moved easily and the rate of convective heat transfer depends on the rate of flow to a great extend. In solids the particles are rigid and cannot move, so heat is not transferred by the process of convection in solids.