Near the earth’s surface, many variables have a characteristic diurnal or daily cycle, driven by the diurnal cycle of the incoming solar radiation, which is zero at night and peaks at local noon. The atmosphere is relatively transparent to the short-wave radiation from the sun and relatively opaque to the thermal radiation from the earth. As a result at the surface is warmed by a positive net radiation balance in the daytime, and cooled by a negative radiation balance at night. The surface temperature oscillates almost sinusoidally between a minimum at sunrise and a maximum in the afternoon. This is referred to as the diurnal cycle of temperature. In warm seasons, the daily net radiation balance is positive, and the daily mean temperature is determined by the daily mean surface energy balance, which involves not only the short and long-wave radiation components, but also heat transfers to the atmosphere.
Betts, A.K., (2003): Diurnal Cycle. Encyclopedia of Atmospheric Sciences, J. R. Holton J. Pyle and J. A. Curry, Eds., Academic Press, London, pp. 640-643. ISBN 0-12-227090-8