How Does a Raindrop Grow?
Authors: Braham Roscoe R. 
Published: October 2006

On the basis of presently available data, combined with present-day knowledge of the physics and chemistry of cloud particle development, it is possible to make the following generalizations about the mode of precipitation in natural clouds.
1) The all-water mechanism begins to operate as soon as a parcel of cloud air is formed and continues to operate throughout the life of the cloud. The ice-crystal mechanism, on the other hand, can begin to operate only after the top of the cloud has reached levels where ice nuclei can be effective (about -15°C). Some clouds never reach this height; any precipitation from them must be through the all-water mechanism. In cold climates and at high levels in the atmosphere, the cloud bases may be very close to this critical temperature. In the tropics, approximately 25,000 feet separate the bases of low clouds from the natural ice level.
2) The number of large hygroscopic nuclei in maritime air over tropical oceans is entirely adequate to rain-out any cloud with a base below about 10,000 feet, provided the cloud duration and cloud depth is sufficient for the precipitation process to operate. Extensive trajectories over land will decrease the number of sea-salt particles, both because of sedimentation and removal in rain. Measurements show an order-of-magnitude decrease in the number of large particles as maritime air moves from the Gulf of Mexico to the vicinity of St. Louis, during the summer months. Measurements in Arizona and New Mexico show even smaller chloride concentrations, presumably because of the long overland trajectories required in reaching these areas. The maritime particles lost in overland trajectories apparently are more than replaced by particles of land origin. The latter are usually of mixed composition and are less favorable for the formation of outsized solution droplets.
3) Ice nuclei, required for the formation of ice crystals and for droplet freezing, are rather rare at temperatures higher than about -10°C. This, of course, accounts for the fact that natural clouds undergo extensive undercooling. Because of the scarcity of suitable nuclei, precipitation through the ice phase usually is not found in clouds warmer than about -15° to -20°C. Natural cirrus clouds might provide ice nuclei for precipitation at somewhat higher temperatures, but this possibility has not been extensively studied.
4) Precipitation in tropical clouds invariably first develops through the all-water mechanism; points discussed in paragraphs 1, 2, and 3 above all work toward this end. Tropical clouds which reach to heights above about 25,000 feet also develop precipitation through snow pellets.
The data for mid-latitude clouds are conflicting. Some measurements suggest that summer clouds in the central United States and in the semiarid Southwest develop rain largely through the all-water process; existing theories support such a suggestion. However, flight measurements indicate that there is considerably more ice and snow in the clouds than can be accounted for by present theory; as a consequence, one must be careful in ruling out the ice mechanism in these areas. It appears to me, however, that the ice particles in these clouds are best accounted for through the hypothesis of freezing of drops which have grown to fairly large size through diffusion of vapor. Thus, the ice would be only incidental to the precipitation development.
Winter clouds in the central United States and almost all of the clouds of northern United States and Canada appear to precipitate largely through the ice-crystal mechanism. The relatively cold cloud bases and the continental sources of air masses in these regions appear to retard the warm-rain mechanism to the point where the ice mechanism dominates. But here again, a great deal of research must be completed before a firm conclusion can be drawn.