The diffusion growth discussed in the last chapter undoubtedly dominates the growth of cloud particles in the initial stage of cloud formation. But the diffusion growth rate becomes very slow once a particle reaches a certain size (about 40 µm for a water droplet, and a few hundred micrometers for an ice particle), as we have discussed. In order to form larger particles, especially precipitation-sized particles, we need to look for other, faster growth mechanisms.

One such mechanism is collision and coalescence. While it seems to be intuitive now, the concept took a while to take root. It was first mentioned by Reynolds (1876), who merely suggested its possibility. It was not considered seriously by scientists until Langmuir (1948) proposed the mechanism for rain formation on top of the Bergeron–Findeisen process, and made a quantitative estimate of its efficacy. This mechanism is now accepted as one of the major precipitation formation mechanisms, especially in the warm rain process.

Since cloud droplets are free to move around in air, they can collide with each other and become larger if they also coalesce afterward. If two equal-sized drops collide and coalesce, the resulting drop will have twice the mass of either original drop. Obviously, this is a much faster growth mode than diffusion growth if it occurs. Similarly, ice crystals can collide with supercooled droplets and form rimed crystals. Upon further riming, rimed crystals can eventually grow to become graupel and hail.