Holben et al. <1995> reported measurements of the aerosol spectral optical thickness and derived volume size distribution from measurements of the sky spectral radiance in the aureole (scattering angle ≤40¿). In this paper we use these and other sky data for analysis of the aerosol hemispheric backscattering ratio β for two aerosol types: smoke aerosol, prevailing during the biomass burning season in the Amazon and the Cerrado in 1993, and predominately sulfate aerosol, measured during the Sulfate/Smoke Cloud and Radiation-Atlantic field experiment in the eastern United States in 1993. The β determines the efficiency (per unit of optical thickness) of the aerosol particles to reflect radiation back to space and therefore to generate a negative radiative forcing of climate. The average value of β, using the Wiscombe and Grams <1976> definition, βWG, varied between 0.20 and 0.28 for both aerosol types, as compared to βWG=0.29 used by Charlson et al. <1992> and Penner et al. <1992> to calculate radiative forcing by sulfate and smoke aerosol, respectively. The variable βWG is an average value of β on all the illumination directions. However, high optical thicknesses occur in the Amazon and eastern United States during the period of July to September, when the solar elevation is high. For these months and the latitude range the actual average value of β is 25% lower than βWG for the same aerosol type. A combination of these two factors results in values of β, and the corresponding aerosol direct radiative forcing, that are 30--50% lower from these estimates. The climate modeling of Kiehl and Briegleb <1993> uses actual computations of the sulfate aerosol hemispherical backscattering as a function of time and geographic position for a particle radius of 0.2 μm, which corresponds very closely to the present results. The volume size distribution, used to derive β, was obtained from sky radiances for scattering angle ≤40¿, assuming spherical homogeneous particles. It can be compared with values of β derived from the whole sky almucantar radiance (scattering angle ≤140¿) that reflects the true aerosol scattering phase function. The two values of β did not differ significantly.