- Roof ponds cooled by nocturnal long wave radiation have often been proposed as a cheap and effective means of providing thermal comfort in buildings in hot-arid locations. Many of the schemes incorporate flat-plate radiators through which the water is circulated at night to be cooled. The present paper analyzes the parameters affecting the performance of radiators specifically designed for nocturnal radiative cooling. Two cheap, simple and flexible designs for a cooling radiator were suggested as a result of the analysis, and tested at the experimental facilities of the Center for Desert Architecture and Urban Planning at Sede-Boqer, Israel. The mean nightly cooling output of the radiators—due to the combined effect of radiation and convection—was over 90 watts/m2 under typical desert meteorological conditions. The analytical model adapted for this application allows accurate calculation of the fluid temperature at the outlet of the radiator, as a function of the properties of the radiator, the meteorological conditions and the operating parameters of the cooling system.