Heat diffusion from a plane, periodic source

Summary

The solution of the heat diffusion equation with a periodic, plane source is applied to problems of the diurnal and annual range of surface temperature, and its lag on insolation. The application of diffusion theory to practical problems is much simplified by the introduction of the concept of a heat capacity for a surface source. Heat capacity may be calculated from the thermal characteristics of the diffusing substance, and the nature of the periodic source. Heat capacities so determined give diurnal surface temperature ranges fo the order revealed in nature. It is possible to account for the way in which the annual temperature range changes with latitude. The annual temperature range computed on diffusion theory alone is about 250% in excess of observed values, but the discrepancy may be accounted for by the interlatitudinal exchange of heat. The indications are that in average circumstances the air absorbs 50%–100% more heat from a plane source than the soil.