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We couldn't shake the mental picture of a gardener in the sky, digging little holes in the clouds and dropping in seeds, but we were pretty sure that wasn't how cloud seeding worked. We set out to learn more by doing a little rain dance, then typing "cloud seeding" into the Yahoo! search box. Here's what we learned. Cloud seeding is a form of weather modification. It can be used to disperse fog, suppress hail, or control winds, but is most often used to increase precipitation. In order to understand the process, however, a basic understanding of clouds and how precipitation
is formed is needed. As warm air rises from the Earth, it begins to cool and forms tiny droplets of water that condense into cloud droplets. Cloud droplets are formed around particles of dust, salt, or soil (called cloud condensation nuclei) that are always present in the atmosphere. These cloud droplets group together into clouds, which can form precipitation in one of two ways. In warm temperatures, the droplets in the clouds merge with many other droplets and become heavy enough to fall to the Earth as rain. (It takes millions of cloud droplets to form a single raindrop.) In colder temperatures, the droplets of water form ice crystals. Other droplets freeze onto these ice crystals, which grow larger and heavier until they fall to the ground as rain, snow, or hail. Cloud
seeding is actually a very complex process. In the simplest terms, it introduces other particles into a cloud to serve as cloud condensation nuclei and aid in the formation of precipitation. There are three types of cloud seeding: static mode, dynamic mode, and hygroscopic seeding. Static mode cloud seeding seeks to increase rainfall by adding ice crystals (usually in the form of silver iodide or dry ice) to cold clouds. Dynamic mode cloud seeding increases rainfall by enhancing "vertical air currents in clouds and thereby vertically process more water through the clouds." Basically, in this method of seeding,
a much larger number of ice crystals are added to the cloud than in the static mode. In hygroscopic seeding, salt crystals are released into a cloud. These particles grow until they are large enough to cause precipitation to form. Clouds can be seeded from above with the help of airplanes that drop pyrotechnics, or from the ground by using artillery or ground-to-air rockets The science of weather seeding is not without controversy. Many question the validity of the results, since they are difficult to prove. Others
feel that science shouldn't interfere with nature. Still others claim that increasing rain in one area decreases it in another, in effect, "stealing" rain from other lands that may be in need. Many states have also passed legislation that attempts to regulate cloud seeding and other forms of weather modification.
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