The effects of water vapor, aerosol, and cloudiness on the incident surface solar irradiance continuously measured in Brazil's Amazonia at six observational sites from 1992 to 1995 are examined by means of a clear-sky broadband radiative transfer model. The aerosol optical depth and precipitable water, both retrieved from Sun photometer measurements, serve as inputs to the model. Computed monthly mean values of clear-sky surface irradiance are analyzed in conjunction with the monthly mean values of all-sky surface irradiance measured on the ground. To assess the effect of cloudiness, we present the cloud radiative forcing and cloud radiative forcing ratio at the surface, both widely employed in cloud radiation budget studies. By its definition the monthly mean cloud radiative forcing (cloud radiative forcing ratio) is the difference (ratio) between surface solar irradiances under all-sky and clear-sky conditions. The analysis of the irradiances, as computed and as measured, shows that during the wet season the gaseous and cloudiness effects on the solar radiation attenuation in the atmosphere are comparable, while the aerosol influence is much smaller. The aerosol effect increases and cloudiness effect decreases in the second half of the dry season. Thus during the biomass burning period in southern Amazonia, the water vapor and aerosol effects become comparable, while the cloudiness impact is 2--3 times smaller. Both cloudiness and aerosol effects have strong seasonal variations, while the gaseous effect changes slightly throughout the year. ¿ 2000 American Geophysical Union