Hydrometeors are liquid and ice particles that form in the atmosphere. Hydrometeor sizes range from small cloud droplets and ice crystals to large hailstones. Precipitation occurs when hydrometeors are large and heavy enough to fall to the Earth’s surface. Virga occurs when hydrometeors fall from a cloud, but evaporate before reaching the Earth’s surface.
Precipitation particles are much larger than cloud particles, as illustrated in Fig. 7.1. One “typical” raindrop holds as much water as a million “typical” cloud droplets. How do such large precipitation particles form?
The microphysics of cloud- and precipitation-particle formation is affected by super-saturation, nucleation, diffusion, and collision.
Supersaturation indicates the amount of excess water vapor available to form rain and snow.
Nucleation is the formation of new liquid or solid hydrometeors as water vapor attaches to tiny dust particles carried in the air. These particles are called cloud condensation nuclei or ice nuclei.
Diffusion is the somewhat random migration of water-vapor molecules through the air toward existing hydrometeors. Conduction of heat away from the droplet is also necessary to compensate the latent heating of condensation or deposition.
Collision between two hydrometeors allows them to combine into larger particles. These processes affect liquid water and ice differently.