Newton's law of cooling states that
the rate of heat loss of a body is proportional to the difference in temperatures between the body and its surroundings. As such, it is equivalent to a statement that the
heat transfer coefficient, which mediates between heat losses and temperature differences, is a constant. This condition is generally true in
thermal conduction (where it is guaranteed by Fourier's law), but it is often only approximately true in conditions of
convective heat transfer, where a number of physical processes make effective heat transfer coefficients somewhat dependent on temperature differences. Finally, in the case of heat transfer by
thermal radiation, Newton's law of cooling is not true.