7: Precipitation Processes
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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.
- 7.2: Nucleation of Liquid Droplets
- Nucleation (the creation of new droplets) in clean air is called homogeneous nucleation. We will show that homogeneous nucleation is virtually impossible in the real atmosphere and can be neglected. Nucleation of cloud droplets by water vapor condensing on tiny dust particles in the air is called heterogeneous nucleation. Even with heterogeneous nucleation, there is a barrier to droplet formation that must first be overcome.
- 7.3: Nucleation of Ice Crystals
- Ice cannot survive at temperatures above 0°C at normal atmospheric pressures. Below that temperature, ice crystals can exist in equilibrium with air that is supersaturated with respect to ice. There is a thermodynamic barrier to ice formation, analogous to the barrier for droplet growth. This barrier can be overcome with either very cold temperatures (colder than –40°C), high supersaturation in the air, or by the presence of ice nuclei.
- 7.4: Liquid Droplet Growth by Diffusion
- In a supersaturated environment, condensation onto a growing droplet removes water vapor from the adjacent air. This lowers the humidity near the droplet, creating a humidity gradient down which water vapor can diffuse.
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