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.. 58, J.j/' .("clouds?The answer is simple. Many res~arch s.cientists, ,". and a large segment of the general pubhc,- beheve that ./ . . cloud seeding can provide a measure of relief from some weather-related adversities at reasonable cost and risks, under certain favorable meteorological conditions. In the simplest of all seeding experiments-Dry Ice' seeding of supercooled stratus-a cloud of water drops is converted into a shower of snow crystals. The evidence is clear, dramatic, and incontrovertible. Of course, the amount of snow produced from seeding a thin stratus cloud is trifling. But it is easy for the nonspecialist to imagin!l a similar transforma.tion taking place on a much larger scale when large clouds are seeded. Even a small percentage increase in precipitation can have enormous value. We can sympathize with a farmer who sees the 10 to 50 cents per acre charge for commercial seeding services as a good bet compared with the $100 to $200 he has invested in seed, fertilizer, and labor. But the issues of cloud seeding are much larger than the actions of individuals willing to invest modest sums for something which they perceive as probably beneficial or which, at worst, can do no harm. Several research experiments have shown that precipitation can be in- creased or decreased by seeding, depending on the types of clouds present and the manner in which they are seeded. At first the cases of precipitation decrease were dismissed as due to experimenter bias or lack of skill. But as .our basic understanding has expanded, we have come to appreciate that nature has certain rather narrow ranges of meteorological conditions suitable for seeding. These conditions may not always coincide with periods of water need, and they may be accessible only by certain seeding techniques. (Grant and Elliott (1974) discuss the evidence for seedable. conditions in winter orographic clouds. ) Moreover, seeding effects that may be viewed as beneficial by some may be viewed as undesirable by others. Users of cloud seeding, and public officials who press for weather modification, seldom confront the possibilities of seeding disadvantages in a realistic manner. So-called cost-benefit analyses, which have been carried out, consider only the direct value of water pro- duced and ignore all the related side benefits and costs, as well as costs of alternative solutions to the water problem. . Lest this detailing of some of the issues surrounding cloud seeding be interpreted as an argument against seeding research, let me repeat my view that weather modification offers prospects for immense social benefits. A viable seeding technology is not yet available. It takes little imagination to visualize tpe need for, and opportunities for, statisticians in the field of weather modification. 2. SCIENTIFIC BACKGROUND The most promising of past cloud seeding experiments are those aimed at converting supercooled droplets into .'- Journal of the American Statistical Association, March 1979 ice crystals through the addition of artificial ice nuclei, most commonly minute particles of silver iodide. Meteorologists have developed two distincJ~~different stra.tegies for exploiting the possibilities in clouaseeding. One strategy rests upon the observation that in certain situations, precipitation mechanisms operate at less than maximum efficiency because of' a shortage of. natural nuclei. In these situations it may be desirable to supple- ment the natural nuclei by cloud seeding. But one must be c:areful not to introduce so many nuclei that precipita- tion is actually decreased as a result of overseeding. This seeding strategy is aimed at exploiting a preexisting, but underutilized, precipitation potential within a cloud by increasing the number of primary precipitation embryos when they are deficient. We call this t.he "optimum nuclleus concentration" strategy, and in it cloud dynamics plays a more or less passive role. This strategy is hard to apply when nature provides a mixture of cloud types and sizes with differing degrees of embryo deficiencies or when cloud-top temperatures are variable and not properly forecast. To illustrate this problem, consider the fact that the activity of both .natural and artificial ice nuclei normally increases one order of magnitude for every four-degree Centigrade de- crease in temperature. In the atmosphere a four-degree decrease in temperature corresponds to an increase in elev:a.tion of only about 2,000 feet. Thus, one cloud may be nucleus-deficient while another only slightly taller and colder may have all the ice crystals it can use effec:tively. Many researchers believe that in seeding an entire field of clouds, say by ground generators or air- planes flying along the upwind edge of a target, there is substantial risk that both favorable and unfavorable clouds will be seeded. The second seeding strategy applies mainly to cumulus clouds. It attempts to increase a cloud's potential for rain by causing the cloud to grow larger than it would have grown without seeding. This strategy is based on the fact that ice formation releases heat of fusion. If supercooled water in a cloud updraft can be turned to ice, the additional heat might alter the updraft sufficiently to allow it to reach substantially greater heights, pro- ducing a larger cloud than would have occurred naturally. Although the heavy seeding required to use this strategy is likely to overseed a cloud initially, it is argued that .because the total storm is invigorated, and processes more water vapor, the end result is more rain on the ground. This strategy has been called "dynamic seeding" or "l~laciation for dynamic effects." The theoretical' bases for both seeding strategies are incomplete. Cloud physicists generally believe that theory is best developed, and natural 'cloud conditions most favorable, for application of the optimum nucleus strategy to winter orographic storms for increasing snow- pack on high mountain slopes. Techniques for seeding summer convective clouds for rain augmentation are still being tested. Seeding to reduce hail and to moderate hurriicane winds is much more controversial. Interested' . ->";, ~_-~2;;.~".~.- ~'-:;:.!_:~"2<.~~);'_.-~':i.>,~".";'::" . "'-'.>':_.t~ ~i;::~'::k..-