During January and August 1985 the Earth Radiation Budget Satellite from the Earth Radiation Budget Experiment (ERBE) was operated in a unique along-track mode that allows all points along the ground track to be viewed over an entire range of viewing zenith angles at essentially constant solar zenith angle. These conditions are ideal for deriving a longwave limb-darkening model. An analytic model that predicts longwave radiance as a function of atmospheric path length is presented. Model parameters derived from the along-track data demonstrate that clear desert limb darkening exhibits a greater variety of diurnal, seasonal, and scene-dependent variability than is present in the corresponding ERBE limb-darkening models. Conversion of radiance to radiant exitance using both the derived and ERBE models show that the ERBE algorithms underestimate representative monthly mean daytime regional (2.5¿¿2.5¿) desert radiant exitances by amounts ranging from 0.4 to less than 5 W/m2, a maximum difference of about 1.6%. ¿ American Geophysical Union 1989