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Nucleation creates the initial ice embryo, either within liquid or with assistance from a suitable interface. Deposition, riming and ice-crystal aggregation enlarge already existing ice. A list of efficient ice-growth processes does not by itself explain how the first crystals formed. Small-particle nucleation probabilities depend on thermodynamic driving, interfaces, volume and exposure time.

Ice particles collide and stick to form a larger aggregate, such as a snowflake. Temperature, habit and collision speed affect sticking. This differs from vapour deposition onto ice and from riming, in which collected supercooled liquid freezes on the ice particle. ^[ref]

Warm-rain growth: condensation first creates cloud droplets, but its approximate $r^2\propto t$ growth narrows relative size differences and becomes inefficient at making large raindrops quickly. A broadened size spectrum lets larger, faster-falling drops collect smaller ones by collision-coalescence. The cloud-droplet collision kernel combines collection efficiency and differential settling. Turbulence, sufficient cloud depth and liquid-water content determine whether collector drops grow rapidly enough. Collisions must actually lead to merging rather than bypass, bounce or breakup.

Ice-phase growth: in a mixed-phase cloud, equilibrium vapour pressure over ice is lower than over supercooled liquid. Ice can grow by deposition while droplets evaporate: the Wegener-Bergeron-Findeisen process. Ice particles then grow further by riming of supercooled droplets and by ice-crystal aggregation. They may fall as snow or melt into rain. Deposition, riming and aggregation are distinct processes; depositing vapour alone need not complete precipitation growth.

Precipitation efficiency depends on the supply of suitable collector drops or ice, residence time, updraft speed, supersaturation and collection/sticking efficiencies. Competition among too many small particles can inhibit growth, and evaporation or sublimation below the cloud can prevent condensate from reaching the surface. Both pathways require growth and fall-out to compete successfully with these losses. ^[ref]

Warm-rain pathway: condensation first grows activated droplets, but its relative size growth slows. A sufficiently broad size spectrum creates different fall speeds; large drops collect smaller ones by collision-coalescence. Collection efficiency, turbulence, liquid-water supply and residence time control the bottleneck. Rain reaching the ground also requires survival against subcloud evaporation.

Ice pathway: below freezing, saturation vapour pressure over ice is lower than over supercooled liquid. In mixed-phase air, ice can grow by deposition while droplets evaporate: the Wegener-Bergeron-Findeisen process. Ice also collects droplets by riming and combines by aggregation. Adequate ice nuclei, sustained supersaturation and residence time are critical; particles must become heavy enough to fall, with snow sometimes melting to rain. Condensation or deposition alone is not generally sufficient for rapid precipitation.