LENINGRAD -- The Soviet Union could launch a "weather war" against the United States and, because of the whimsy of weather patterns, Americans wouldn't even know it. The notion sounds like fantasy, but scientists say it is true. The Soviets are not the only ones meddling with Mother Nature. They are so afraid of America's capability to wage war with the weather that they quietly signed a treaty with the United States some years ago that banned hostile manipulation of the weather. The trouble with the treaty is that it is totally unverifiable. For the record, no U.S. intelligence agency has any evidence that the Soviets are engaged in even minor skirmishes using the weather. The only country that has ever used weather as a weapon is the United States. From 1967 to 1972, the Pentagon conducted a $21.6 million rainmaking operation designed to make the Ho Chi Minh Trail slippery. Mistakes were made. Once, seven inches of rain was dumped on U.S. Special Forces camp in two hours. It is possible that the American cloud seeding compounded a deluge in August 1971 in North Vietnam that resulted in flooding that killed thousands of people. We have reviewed a dozen secret Central Intelligence Agency reports about Soviet weather modification research done in the last three decades. The research is conducted at the "Hydrometeorological Institute" in Leningrad and in a network of more than 100 similar institutes. Almost all of the Soviet research has a quasi-legitimate agricultural use, but the CIA is well aware that the same techniques could be used to deliberately disrupt the climate of other countries. The Soviet belief that bigger is better is reflected in their grandiose weather plans, including a scheme to melt the Arctic ice and moderate temperatures in the northern Soviet republics. One civilian Soviet scientist told the CIA that he knew of as many as 300 secret weather modification experiments done in the Arctic. One plan, by engineer P.M. Borisov, was to pump water from the Atlantic to the Arctic Ocean. Borisov believed that all the ice in the Arctic Ocean could be melted within three years, never to return. Part of his blueprint was a dam across the Bering Strait. Complaints of more levelheaded scientists dampened Soviet enthusiasm for the scheme. Temporarily, the Soviets have settled for melting icepacks across huge areas of their country by covering them with black substance or another to draw the sun. The Soviets are proficient in seeding clouds to meddle with the amount and type of precipitation, but they are not content to use the standard seeding method of dropping chemicals from airplanes. They use rockets shot from the ground. "The use of aircraft was judged to be inadequate and anti-aircraft artillery shells and rockets were developed for this purpose," says one CIA report. The Soviets "claim that the shells fragment completely and it is rare to find a piece as large as one gram on the ground. The danger of the rockets is minimized by the use of parachutes and the rocket sites are established in relatively unpopulated areas."