In cloud physics, the process producing precipitation through collision between liquid particles (cloud droplets, drizzle drops, and raindrops). The warm rain process includes growth by collision–coalescence and limitations to growth by drop breakup. Precipitation produced by the warm rain process occurs in clouds having sufficient liquid water, updraft, and lifetime to sustain collision–coalescence growth to drizzle drop or raindrop sizes. Since warm base (>10°C) convective clouds of about 2 km depth typically have these features, the warm rain process is found to be active in both shallow and deep convection in the Tropics and midlatitudes. The major role of the warm rain process in thunderstorms is to transfer condensed water, in the form of cloud droplets, to precipitable water, in the the form of drizzle droplets and raindrops, by the collision–coalescence process. The warm rain process can also produce supercooled raindrops that freeze and become graupel, necessary for the rapid glaciation of convective tops by production of secondary ice crystals. This has been called the coalescence freezing mechanism. See Hallett–Mossop process.