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Principles of Cloud Seeding

The large and mesoscale dynamics determining the characteristics of the cloud systems down to the small scale microphysics determining the nucleation and growth characteristics of water droplets and ice particles all form part of the chain of events of precipitation development. Precipitation mechanisms involve various microphysical processes that proceed simultaneously but at different rates, with one process becoming more dominant because of its greater efficiency under given atmospheric conditions. It is useful to group precipitation mechanisms into those that involve the formation of ice particles and those that do not.

The cold-cloud mechanism postulates the nucleation of ice particles in supercooled clouds followed by their growth by vapor diffusion into snow particles. Under favorable conditions they may aggregate as snow or rime to form low-density graupel or snow pellets. This process is important in clouds of all types where temperatures are colder than about -15ºC, including the upper parts of cumulonimbus clouds.

The collision-coalescence process or the warm rain process occurs in relatively warm clouds with tops warmer than -15ºC and with bases warmer than +15ºC by the collision between water droplets. To produce the large amount of collisions required to form a raindrop that would eventually fall to the ground, some cloud droplets must be larger than others. Larger drops may form on larger condensation nuclei, such as salt particles, or through random collisions of droplets. Recent studies show that convective clouds that ingest polluted cloud condensation nuclei (CCN) suppress precipitation in the warm layer due to the large concentration of small droplets and will precipitate more slowly than a similar cloud ingesting clean air, which forms small concentrations of larger droplets that coalesce faster into raindrops.


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