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The modern equivalent of a rain dance, cloud seeding is an attempt to increase precipitation in an arid region by flying airplanes over clouds and dropping stuff into them. Sometimes it even works. For some odd reason, this seemingly inoffensive process is often described by the euphemism "weather modification."

The first thing you need to know is how rain forms naturally. Cloud droplets form around condensation nuclei whose size and composition varies widely with location. In tropical regions, a "warm rain" process often occurs in which the cloud's temperature never drops below the freezing point of water. Large, hygroscopic condensation nuclei allow for large rain droplets to which smaller ones accrete by the process of coalescence. Once the droplets get big enough, they fall to the ground as rain.

In more temperate locations, however, cloud temperatures are below freezing and the cloud consists of a combination of tiny ice crystals and supercooled water droplets. Only certain types of particles are suitable for ice nuclei, and they're often in short supply. But once ice crystals begin to form, they continue to grow until reaching a large enough size to fall, typically melting into rain if the air beneath is warm enough, thus originating a "cold rain."

Three types of cloud seeding have been researched extensively: static, dynamic, and hygroscopic.

Static seeding:
This is the most primitive method of seeding a supercooled cloud. The basic idea is to increase the concentration of available ice nuclei, and consequently of ice crystals. Early experiments often used dry ice, which has since been replaced by silver iodide, whose crystal structure is closer to that of ice. The substance is often packed in flares which are attached to the plane's wing and ignited at the appropriate moment, dispersing ice-forming dust throughout the desired cloud region.

The problem is that the conditions necessary for static cloud seeding to occur are very strict, in fact stricter than can be reliably forecasted! So while this process has been successful on occasion, its failures have been just as numerous, and current forecasting limitations make it economically unfeasible. But a more versatile technique for seeding supercooled clouds has been developed:

Dynamic seeding:
The same silver iodide is used here, but a lot more of it is dumped into the cloud, ideally in the most convective parts. An overabundance of ice nuclei assures that most of the cloud will freeze, and the latent heat released in the phase transition boosts updrafts, hence increasing the cloud's circulation. Lots of graupel forms in the cloud's upper reaches, and it stays aloft longer and falls slower than water due to its lower density. Downdraft is delayed, the graupel grows more than it would otherwise, and the cloud releases more of its stored water to thirsty farms and lakes below.
Hygroscopic seeding:
This is the method by which warm clouds are seeded. Typically, fine-ground salt crystals are released into the cloud's base, and provide the necessary hygroscopic nuclei for coalescence to occur. Hygroscopic seeding doesn't seem to have any effect in clouds that have formed over the sea, presumably because they already have as much salt as can produce a beneficial effect on precipitation. The procedure works better with clouds of continental origin.

In addition to its obvious rainfall-enhancing applications, cloud seeding has also been put to use dispersing fogs that reduce visibility around airports. Early seeding of clouds likely to produce destructive hail may be effective in coercing those clouds into releasing their water in non-hazardous form.

Though it has been researched for several decades, cloud seeding has yet to go into widespread use. The technology seems to be less easily implemented than its early proponents had claimed.