On the diurnal variation of surface temperature

The global short-wave radiation is the main external factor which determines the diurnal temperature variation at the surface of the earth. Therefore, there should be a close connection between the diurnal variation of the global radiation, which is normally quasi-symmetrical relative to noon, and that of the surface temperature, which on the contrary has a very characteristic non-symmetrical appearance. It is assumed that the surface temperature can be represented with sufficient approximation by a small number of terms of a Fourier series, and that the energy loss by various effects, such as evaporation, is proportional to the surface temperature. In doing so it can easily be shown that each harmonic term of the temperature variation is determined by the respective term of the diurnal variation of the global radiation; the changes in amplitude and phase angle of each term being functions of the heat capacity and thermometric conductivity of the soil and the coefficient for the rate of loss of energy by non-conduction. The formulae arrived at are successfully tested against observed data published by Franssila. In doing so the first four terms of the Fourier series are used.