WMOD00544 - Laserfiche WebLink

Home Browse Search

conditions, they are related and a change in one will affect another. It also is known that such favorable seeding conditions are not ever-constant properties but appear at intervals during the lifetime of the cloud or weather system. It became imperative early in Skywater's research that the characteristics of the windows be defined. Since the life of a cumulus cloud is brief, frequently less than 90 minutes, there was need for develop- ment of a technique by which these seed- ing opportunities could be quickly and accurately recognized. Similarly, there was a need for methods of accurate deliv- ery of seeding agents. These problems had to be resolved, at least partially, be- fore a basic operational technology could exist. Although improvement is needed, these techniques and methods have been developed. I HYGROSCOPIC AND ICE- PHASE SEEDING AGENTS THESE AGENTS WORK on different principles but are effective within their individual windows. Summer cumulus clouds over the Great Plains and the large winter (orographic) systems moving over Western mountains respond to quite differ- ent seeding principles. Each, however, has particular conditions in which precipi- tation can be enhanced. Research in the Great Plains has deter- mined that if other factors are compatible, silver iodide can be effective in summer cumulus clouds in the ice-phase process if cloud-top temperatures are between -100 C and _230 C (140 F and _90 F). It ap- pears that the temperature at the base of the cloud is less important but should be warmer than _50 C (230 F). Other factors are involved, but tempera- ture at the top of the cloud which relates 11 to the cloud depth is the most important criterion for determining if cold cumulus clouds can be seeded to increase pre- cipitation. The water content of warm summer clouds appears to be a major factor favoring hygroscopic seeding. It appears that the cloud must 'contain at least 4 grams of water per cubic meter of air before the hygroscopic seeds can produce precipita- tion. This amount can occur in a 10,000- foot-thick cloud in the Northern Great Plains and a 5,000-foot-thick cloud in the tropics. An agent, such as common salt or a solution of ammonium nitrate and urea are presently used as the seeding agent. I ,~ J I 1.1 The updrafts (or convection) and cloud droplet size are also important. Once a cloud's droplets grow to 0.1 mm in size, the coalescence process within the cloud should be rather efficient and not require stimulation. Droplets of less than 0.1 mm, however, can be encouraged to grow to the efficient level. I i , I j , ,A silver Iodide generator In operation.