The data set obtained during the First International Satellite Land Surface Climatology Project (ISLSCP) Field Experiment (FIFE) allowed an assessment of several expressions originally proposed to estimate daily downward longwave radiation Rld under clear sky. These expressions with the original values of the constants were found to yield instantaneous Rld values with a standard error of estimate of the order of 10--20 W m-2, but with some bias, when applied to the instantaneous values of the input data set obtained under clear sky. Calibration of the constants for the FIFE area and for the instantaneous Rld estimation under clear sky improved the results with a standard error smaller than 10 W m-2 and practically without bias. Inclusion of the cloudiness and cloud type information into the formulation allowed the expansion of its usage for both clear and cloudy sky conditions; the resulting standard error of the Rld estimate was found to be around 15--17 W m-2 if only the cloudiness data were utilized, and around 12 W m-2 if both cloudiness and cloud types were considered. These results were some 5 and 10 W m-2, respectively, better than the error in the estimation of Rld without cloudiness information. ¿ American Geophysical Union 1993