- Abstract 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. An analytical model originally developed for heating applications was adapted to the description of such a nocturnal, long wave radiative cooling apparatus. The accuracy of the model was verified on the basis of experimental data from three types of radiators tested at the experimental facilities of the Center for Desert Architecture and Urban Planning at Sede-Boqer, Israel, under a variety of operating regimes. The model allows accurate prediction of outlet temperatures, taking into account the design characteristics of the specific radiator, the environmental conditions and the pattern of operation of the system. Analysis of flat-plate solar collectors converted to radiative cooling of buildings suggests that under typical environmental conditions, they may be less efficient for this type of application than radiators having no fins.