Under the auspices of the Multi-State Power Production Pollution Study (MAP3S) a field experiment was carried out over Lake Michigan in cooperation with the Battelle, Pacific Northwest Laboratory (PNL) during November 1978 and January 1979 with an aim to delineate cloud-aerosol interactions. The PNL DC3 aircraft. was used as an observational platform and a network of ground stations collected samples of precipitation which consisted of liquid phase during the early part of November and of solid phase during the January flights.