This paper present a parameterization of the joint distribution of size and infrared optical thickness in cumulus cloud. The model involves only two parameters, each of which has a clear physical interpretation. It is shown how knowledge of the size distribution and the mean infrared radiance transmitted by a cloud field can be used to deduce the thickness distribution of the field. One implication of this result is that the conjunction of Advanced Very High Resolution Radiometer data with size statistics, gathered for example in real time by processing high resolution images on board a satellite, can be used to predict the transfer of infrared flux by broken cloud. ¿ American Geophysical Union 1990