Description is made of a non-randomized experiment which was carried out during winter 1968/9 with the purpose of increasing visibilities in a vast area of the Eastern Po Valley. 6.75 tons of 2 different types of material which generated, with 80% yields, condensation and ice-forming nuclei of 1.5 micron average diameter were aerosolized in daily batches of about 190 kg, three times per week, for three months. The seeding was carried out, on pre-determined days and independently of the atmospheric conditions, from the top of a hill 150 meters above the plain. Substantial and significant increases of average visibilities were obtained in a large area downwind from the operation site.