High concentrations of small atmospheric aerosols are known to reduce the size of cloud droplets, increase cloud albedo and suppress precipitation formation. In contrast, cloud simulations suggest that even low concentrations of large soluble aerosols should promote droplets' growth and rainfall. Until now, though, no observational evidence of such microphysical effects in natural circumstance over land has been presented. By using NOAA-AVHRR retrievals on cases where salt-dust from the Aral Sea interacts with clouds we show that large salt-containing dust particles increase cloud drops to sizes that promote precipitation. These findings are in line with the findings of the microphysical models and recent results from hygroscopic cloud seeding experiments for rain enhancement.