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 This paper analyzes the parameters affecting the performance of such a radiator, specifically designed for nocturnal radiative cooling A cheap, simple and flexible design for a cooling radiator was suggested as a result of the analysis, and tested at the experimental facilities of the Center for Desert Architecture at Sede-Boqer, Israel. The mean nightly cooling output of the radiator - due to the combined effect of radiation and convection - was over 90 watts/m(2) 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.