Fly ash released from a coal-fired power plant was detected as active ice nuclei. Observational evidence is presented that the plume aerosol inadvertently seeded supercooled fog and induced local snowfall. Snow crystals were replicated and examined with an electron microscope. Individual ice nuclei were analyzed with an X-ray energy spectrometer. Most ice nuclei contained Si, Al, S, Ca, and Fe which are major components of fly ash. It is interesting to note that plume aerosol can seed clouds at temperatures as warm as -5 ¿C and produce needle-shaped snow crystals.