Overview

Heat transfer is the transfer of heat from one place to another by movement of fluids. When an object is at a different temperature from its surroundings or another object, transfer of thermal energy, also known as heat flow, or heat exchange, occurs in such a way that the body and the surroundings reach thermal equilibrium; this means that they are at the same temperature. Heat transfer always occurs from a higher-temperature object to a cooler-temperature one as described by the second law of thermodynamics or the Clausius statement. Where there is a temperature difference between objects in proximity, heat transfer between them can never be stopped; it can only be decreased.^[1]

The radiation of the sun in which the planet is incessantly plunged, pentrates the air, the earth, and the waters; its elements are divided, change direction in every way, andd, penetrating the mass of the globe, would raise its temperature more and more, if the heat acquired were not exactly balanced by that which escapes in rays from all points of the surface and expands through the sky.The Analytical Theory of Heat, J. Fourier^[2]