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III. RESULTS OF CLOUD SEEDING TO MODIFY PRECIPITATION 611 it was not possible to evaluate accurately opera- tional projects that had been carried out over flat regions of the United States because of lack of sufficiently good controls. Most information on the results of convective cloud seeding has come from randomized exper- iments. Experiments with dry ice and AgI have been carried out in many countries, including Australia, Canada, the United States, and the Soviet Union. The experiments taken together have shown beyond any reasonable doubt that certain cumulus clouds exist which can be in- duced to yield precipitation by seeding with gla- ciogenic agents, but which would not precipitate otherwise, and that some precipitating convec- tive clouds yield more rain when seeded than they would naturally. The important question remains, however, as to whether these increases from individual clouds actually result in rainfall increases on the ground or whether they are compensated by decreases in rainfall from other clouds. The most widely accepted positive results from the seeding of convective clouds have come from Israel. Following 10 yr of random- ized experiments, Israeli scientists concluded that the effects of silver iodide seeding from air- craft upon cumulus moving inland from the Mediterranean Sea during winter were positive and economically important. Various analyses have indicated increases in seasonal precipita- tion ranging from 10 to more than 20%, with the largest indicated increases occurring in the mid- dle of predesignated target areas in northern Is- rael. The clouds treated are relatively small, with cloud base temperatures of about + 50C and cloud top temperatures seldom colder than - 30oC. There is some evidence that maximum effects occur on days when the median cloud top temperature is in the vicinity of -IS to - 20oC. Possible explanations for this finding have been sought in terms of crystal habits and the proba- bility of rapid riming of dendritic (star-shaped) crystals, which tend to predominate at about the -150C level. Randomized experiments on area-wide pre- cipitation conducted in the United States in- clude projects in Arizona, Florida (FACE), Mis- souri (Whitetop), North Dakota, and South Dakota (Rapid Project). The Arizona projects, conducted by the University of Arizona between 1956 and 1964, produced no definite answers about rainfall on the ground. The Florida Area Cumulus Experiment (FACE) was intended to produce area-wide rainfall increases through dy- namic effects, but also proved to be inconclu- sive. The other projects provided evidence of both rainfall increases and decreases, the appar- ent increases occurring in the Dakotas and, pos- sibly, in Missouri on days with moderate con- vectivf: activity, and the apparent decreases in Missouri and, possibly, in South Dakota on days with intense convective activity. The indicated effects are substantial. Increases of 100% or more on days with moderate convection are in- dicated, but on Whitetop such effects were over- whelmed by decreases on days with storms hav- ing tops above about 12 km. The results of the randomized experiments just quoted and of certain others argue against indiscriminate seeding of large convective storms to increase rainfall. It is now generally agreed that it is not productive to seed very tall cumulonimbus clouds, which already possess strong updrafts and often eject large amounts of condensate into their anvils. Instead, most weather modification operators favor treatment of cumulus congestus clouds before or just after they reach the precipitation state in an attempt to increase their precipitation efficiency, and possibly, stimulate further cloud growth by dy- namic effects. The microphysical effects of glaciogenic seed- ing on supercooled convective clouds were ex- plored in detail in southeastern Montana dur- ing the High Plains Cooperative Program (HIPLEX). The HIPLEX-I randomized experi- ment was conducted there in 1979-1980. The treatment consisted of dropping dry ice into the top of a cloud at the rate of 0.1 kg/km during a single aircraft pass over the cloud. The results clearly showed the formation of ice crystal cur- tains within 2 min and their subsequent spread- ing. Some of the artificially produced ice crys- tals grew to about 500 /-tm in diameter and began to aggregate. This behavior differed from that typical of natural ice particles in such clouds, which is to grow into graupel particles by rim- ing. While the small clouds treated had such short lifetimes that both the natural and artificial precipitation processes were cut short in most of them, the HIPLEX-l experiment confirmed that precipitation can be initiated by glaciogenic seeding. It also yielded quantitative data on ice crystal yields and the rate at which the artificial precipitation process proceeds. Recently re- ported dry ice seeding trials on winter convec- tive clouds on SCPP also point to aggregation of ice crystals as an important precipitation mecha- nism in seeded clouds.