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Cloud seeding, a form of weather modification, is the attempt to change the amount or type of precipitation that falls from clouds, by dispersing substances into the air that serve as cloud condensation or ice nuclei. The usual intent is to increase precipitation, but hail suppression is also widely practiced. Silver iodide and dry ice are the most commonly used substances in cloud seeding.How cloud seeding worksThe most common chemicals used for cloud seeding include silver iodide and dry ice (frozen carbon dioxide). The expansion of liquid propane into a gas is being used on a smaller scale. The use of hygroscopic materials, such as salt, is increasing in popularity because of some promising research results.Seeding of clouds requires that they contain supercooled liquid water—that is, liquid water colder than zero degrees Celsius. Introduction of a substance such as silver iodide, which has a crystalline structure similar to that of ice, will induce freezing (heterogeneous nucleation). Dry ice or propane expansion cools the air to such an extent that ice crystals can nucleate spontaneously from the vapor phase. Unlike seeding with silver iodide, this spontaneous nucleation does not require any existing droplets or particles because it produces extremely high vapor supersaturations near the seeding substance. However, the existing droplets are needed for the ice crystals to grow into large enough particles to precipitate out.In mid-latitude clouds, the usual seeding strategy has been predicated upon the fact that the equilibrium vapor pressure is lower over water than over ice. When ice particles form in supercooled clouds, this fact allows the ice particles to grow at the expense of liquid droplets. If there is sufficient growth, the particles become heavy enough to fall as snow (or, if melting occurs, rain) from clouds that otherwise would produce no precipitation. This process is known as "static" seeding.Seeding of warm-season or tropical cumuliform (convective) clouds seeks to exploit the latent heat released by freezing. This strategy of "dynamic" seeding assumes that the additional latent heat adds buoyancy, strengthens updrafts, ensures more low-level convergence, and ultimately causes rapid growth of properly selected clouds.Cloud seeding chemicals may be dispersed by aircraft or by dispersion devices located on the ground (generators). For release by aircraft, silver iodide flares are ignited and dispersed as an aircraft flies through a cloud. When released by devices on the ground, the fine particles are downwind and upwards by air currents after release.While cloud seeding has shown to be effective in altering cloud structure and size, and converting cloud water to ice particles, it is more controversial whether cloud seeding increases the amount of precipitation at the ground. Part of the problem is that it is difficult to discern how much precipitation would have occurred had the cloud not been "seeded." In other words, it is hard to discern additional precipitation from seeding from the natural precipitation variability, which is frequently much greater in magnitude. Nevertheless, there is more credible scientific evidence for the effectiveness of winter cloud seeding over mountains (to produce snow) than there is for seeding warm-season cumuliform (convective) clouds. This statement is supported by the professional societies Weather Modification Association, World Meteorological Organization, and American Meteorological Society (AMS). The AMS further states that there is statistical evidence for seasonal precipitation increases of about 10% with winter seeding.The National Center for Atmospheric Research (NCAR), an institution in Boulder, Colorado, has made some statistical analysis of seeded and unseeded clouds in an attempt to understand the differences between them. They have conducted seeding research in several countries that include Mali, Saudi Arabia, Mexico, South Africa, Thailand, Italy, and Argentina.
Dearest Cloud & Cotton Wool Experts (cos that's what clouds are made from aren't they ?)How does seeding a cloud make it rain ?What is the 'seeding stuff ' ?.....and how does it convince a cloud to do it's cloud wee wee ?Is it feasible then that one day we'll be able to take it one step further and actually make clouds ?...cos that would solve a few drought issues wouldn't it ?
but the moisture has to first be there in order to seed the clouds.