This paper reports a study on the thermal radiation components of the energy balance on the ground. The atmospheric radiation was measured and correlated to the cloudiness and to air temperature and humidity at screen level. Hence the net thermal and the net total radiation were estimated in relation to the atmospheric conditions. The long-term averages of the atmospheric, net thermal and net total radiation fluxes, as well as their day-to-day and year-to-year variabilities, were studied and are predicted with data from Lisbon.