A field campaign designed to investigate the second indirect aerosol effect (reduction of precipitation by anthropogenic aerosols which produce more numerous and smaller cloud droplets) was conducted during winter in the northern Rocky Mountains of Colorado. Combining remote sensing and in-situ mountain-top measurements it was possible to show higher concentrations of anthropogenic aerosols (~1 ¿g m-3) altered the microphysics of the lower orographic feeder cloud to the extent that the snow particle rime growth process was inhibited, or completely shut off, resulting in lower snow water equivalent precipitation rates.