The results indicated increased rainfall. Cloud seeding got its start in 1946 when Dr. Vincent J. Schaefer, working at the General Electric Laboratory in New York, was involved with research to create artificial clouds in a chilled chamber. During one experiment, Schaefer thought the chamber was too warm and placed dry ice inside to cool it. Water vapor in the chamber formed a cloud around the dry ice. The ice crystals in the dry ice had provided a nucleus around which droplets of water could form inside the chamber.
Natural rainfall works much the same way.
Ice crystals are formed when cold water contacts particles of dust, salt, or sand. The ice crystals provide a nucleus around which water droplets can attach, increasing the size of the droplet. When the droplet becomes large enough, it falls as rain. Cloud seeding is thought to increase the number of these nuclei available to take greater advantage of the moisture in the cloud and form raindrops that otherwise would not have formed. Small droplets collide and coalesce until they form a drop large enough to fall.