<P> However, by definition, the validity of Newton's law of Cooling requires that the rate of heat loss from convection be a linear function of ("proportional to") the temperature difference that drives heat transfer, and in convective cooling this is sometimes not the case . In general, convection is not linearly dependent on temperature gradients, and in some cases is strongly nonlinear . In these cases, Newton's law does not apply . </P> <P> In a body of fluid that is heated from underneath its container, conduction and convection can be considered to compete for dominance . If heat conduction is too great, fluid moving down by convection is heated by conduction so fast that its downward movement will be stopped due to its buoyancy, while fluid moving up by convection is cooled by conduction so fast that its driving buoyancy will diminish . On the other hand, if heat conduction is very low, a large temperature gradient may be formed and convection might be very strong . </P> <P> The Rayleigh number (R a (\ displaystyle Ra)) is the product of the Grashof and Prandtl numbers . It is a measure which determines the relative strength of conduction and convection . </P> <Dl> <Dd> R a = G r P r = g Δ ρ L 3 μ α = g β Δ T L 3 ν α (\ displaystyle Ra = GrPr = (\ frac (g \ Delta \ rho L ^ (3)) (\ mu \ alpha)) = (\ frac (g \ beta \ Delta TL ^ (3)) (\ nu \ alpha))) </Dd> </Dl>

Temperature is the transfer of thermal energy from one object to another