Storm clouds over rainmakers

US rainmakers are having to face the unpleasant fact that more than three decades of research have failed to produce hard evidence that they can substantially modify the weather.

They are under pressure from within their own ranks and from the federal government to stop trying to increase rainfall, suppress hail, or moderate hurricane winds. Instead, they are urged to concentrate on gaining a thorough basic knowledge of the weather phenomena involved. This kind of research has been neglected in an eagerness to see what cloud seeding could do.

Aware of the lack of rainmaking progress, the Reagan administration wants to eliminate weather modification funds from the National Oceanic and Atmospheric Administration (NOAA) budget. Congress doesn't want to go that far. Both the House and Senate are expected to take up legislation this month which would restore some of that funding. The House would give NOAA $4 million while the Senate would provide $5 million.

However, such funds would be earmarked for basic research, not for rainmaking field operations. A member of the Senate Commerce Committee staff explains: ''It's time to get back to fundamentals - try to gain better understanding of the physics of weather modification - and be a little more modest in our claims (of what cloud seeding can do).''

This is what leading weather modification scientists have been saying themselves. For example, Roscoe R. Braham Jr. of the University of Chicago, who has worked in this field since the 1950s, has said he thinks ''that our limited understanding of clouds and cloud systems has been the pacing factor in our progress.'' He explains, ''In a sense, we were trying for a 'home run' at a time when a 'bunt single' might have advanced our cause somewhat faster.''

He, too, has urged fellow researchers to ''declare a moratorium on experiments in which the primary objective is to verify some change in precipitation at ground level.''

One reason that rainmaking researchers have tried for a home run has been that success has often seemed almost within their grasp. Four years ago, the Weather Modification Advisory Board, with former diplomat Harland Cleveland as chairman, told the secretary of commerce that some weather modification ''is scientifically feasible and within sight.'' It foresaw a 10 to 20 percent reduction in the force of hurricane winds, reduction by up to 60 percent of hail in some storms, and an increase of 10 to 30 percent in precipitation through cloud seeding in favorable cases.

But, in a little noticed caveat, the board also warned, ''The experimental evidence for cloud-seeding has not yet reached the levels of objectivity, repeatability, and predictability required to establish new knowledge and techniques.''

One of the most promising of recent cloud-seeding tests - the Florida Cumulus Area Experiment (FACE) - has given this warning new prominence. FACE aimed to increase rain by seeding selected clouds in a designated target area. The seeding triggered freezing of cloud droplets which released heat as they froze. This heat strengthened the clouds' buoyancy - invigorating individual clouds and encouraging them to merge into larger cloud systems.

When the first six-year FACE project ended in 1976, project scientists said they believed it showed rainfall gains of up to 70 percent. Deputy project director Robert Sax said he expected the overall rainmaking potential of the technique to lie between 20 and 70 percent additional precipitation.

FACE then ran a second series of field tests carefully designed to confirm this expectation. Although the final report on those tests is not yet available, FACE scientists have already acknowledged that their earlier hopes were not confirmed.

Braham had warned that ''experiments . . . that try to relate seeding to precipitation on the ground while ignoring the chain of intermediate processes are not likely to be satisfactory.'' While FACE did emphasize cloud physics, many factors were not taken into account. Project director William Woodley has been quoted in Science magazine as acknowledging that ''the whole process seems to be far more complex than we thought. . . .'' He notes further that ''a black box experiment (where you don't know what is going on inside) in the 1980s cannot be justified. We can do better than that.''

Cloud seeders can dissipate cold fogs and decks of low-lying stratus clouds. But, as the American Meteorological Society (AMS) explains in an official policy statement on weather modification, there is little they can do beyond that with any certainty.

The strongest evidence so far for a positive effect has come from Israeli experiments with winter cumulus clouds. These indicate rainfall gains of about 50 percent. Here again, independent confirmation is needed.

Meanwhile, there is also concern about possible harmful effects of seeding. Rain may be suppressed instead of increased. Also, there seem to be effects, still poorly understood, well beyond the seeding target area. ''Public as well as scientific concern requires that a better physical understanding of the nature and magnitude of this possible extended seeding effect be obtained,'' the AMS warns.