Radiation, that is loss of heat, is going on concurrently with gain, and the rate of loss varies with the temperature according to a law recently discovered, the loss being much greater at high temperatures in proportion to the 4th power of the absolute temperature.

Then, again, the whole heat intercepted by a planet does not reach its surface unless it has no atmosphere.

When it has one, much is reflected or absorbed according to complex laws dependent on the density and composition of the atmosphere.

Then, again, the heat that reaches the actual surface is partly reflected and partly absorbed, according to the nature of that surface--land or water, desert or forest or snow-clad--that part which is absorbed being the chief agent in raising the temperature of the surface and of the air in contact with it.

Very important too is the loss of heat by radiation from these various heated surfaces at different rates; while the atmosphere itself sends back to the surface an ever varying portion of both this radiant and reflected heat according to distinct laws.