Precipitation in the form of rain or snow occurs when particles of water or ice are large enough to reach the ground. The chief difference between a cloud drop and a rain drop is size. A typical rain drop has a volume that is more than a million times that of a cloud drop. Thus it takes many cloud droplets to make up a single raindrop. Raindrops can be produced by the collision and merging of cloud droplets. Collisions take place because the terminal velocity of a water drop increases as its size increases, over the normal size range of cloud droplets and raindrops. Large droplets fall faster then collide with and merge with smaller ones. When two rain droplets merge, "coalescence" has taken place. As a result of coalescence, the large drops can grow fairly rapidly.
Raindrops are also produced by the melting of ice crystals, snowflakes, and other frozen particles. When ice crystals exist in the presence of "supercooled" water droplets in subfreezing air, the crystals grow as the droplets evaporate. There is a pressure force driving the water molecules from the water to the ice, resulting in a rapid growth of ice crystals in the presence of liquid cloud droplets.
As ice crystals grow, the heavier ones fall. As a result, collisions and merging occur. A snowflake can be made up of a group of crystals stuck together. When such a particle falls through a layer of air whose temperature is above freezing, the crystals melt and raindrops are produced. In mountainous areas during the winter, valley locations often experience rain while snow falls at higher elevations.