Smoke particles from biomass burning can generate forcing of climate by modifying cloud microphysics and reflectance of sunlight. Cloud modification, critical to an understanding of climate change, is uncertain and variable. Satellite data over the Amazon Basin and Cerrado were analyzed for cloud reflectance and droplet size and for smoke concentration. Smoke increased cloud reflectance from 0.35 to 0.45, while reducing droplet size from 14 to 9 micrometers. The regional variability of the smoke effect was correlated to the availability of water vapor. During the 3 months of biomass burning in the dry season, the smoke-cloud forcing of climate was only −2 watts per square meter in this region, much smaller than what can be inferred from model predictions.