Newton's law of cooling

for: the rate of heat transfer by a combination of conduction, convection, and radiation in everyday situations

is: a 'rule of thumb' which is actually very reliable so long as temperature differences are not large. [P7.3]

states: that where is used here, as in convection, as a general purpose empirical heat loss coefficient whose value has to be measured, guessed, or looked up, is the effective area of the surface from which the heat is being transferred, and is the temperature difference between that surface and the surroundings. [P7.3]