<P> Further classification can be made depending on the smoothness and undulations of the solid surfaces . Not all surfaces are smooth, though a bulk of the available information deals with smooth surfaces . Wavy irregular surfaces are commonly encountered in heat transfer devices which include solar collectors, regenerative heat exchangers and underground energy storage systems . They have a significant role to play in the heat transfer processes in these applications . Since they bring in an added complexity due to the undulations in the surfaces, they need to be tackled with mathematical finesse through elegant simplification techniques . Also they do affect the flow and heat transfer characteristics, thereby behaving differently from straight smooth surfaces . </P> <P> For a visual experience of natural convection, a glass filled with hot water and some red food dye may be placed inside a fish tank with cold, clear water . The convection currents of the red liquid may be seen to rise and fall in different regions, then eventually settle, illustrating the process as heat gradients are dissipated . </P> <P> Convection - cooling is sometimes loosely assumed to be described by Newton's law of cooling . </P> <P> Newton's law states that the rate of heat loss of a body is proportional to the difference in temperatures between the body and its surroundings while under the effects of a breeze . The constant of proportionality is the heat transfer coefficient . The law applies when the coefficient is independent, or relatively independent, of the temperature difference between object and environment . </P>

State the direction of heat transfer by way of convection