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Yet some of that behavior may be modified. Early man tried to influence the weather by using a variety of techniques, including sacrificing virgins or staging rain dances. Renaissance Europeans, noting that rains seemed to follow battles, theorized that the waters were shaken loose from the clouds by the noise of cannon.
Most historians date scientific rainmaking from 1946, when General Electric Researcher Vincent Schaefer put frozen carbon dioxide, dry ice, into a freezer and watched as a miniature cloud formed and snowflakes began falling. Several months later, working with Irving Langmuir, he tried his experiment on a larger scale. From a plane, he dropped dry ice into a cloud. The ice pellets triggered the formation of ice crystals, which melted into rain as they descended through the warmer layers of the atmosphere. Today, cloud seeding with dry ice or with silver iodide (which provides nuclei for the formation of ice crystals) is frequently carried out by Government and private rainmakers. The process has been used with some success to trigger rains over drought-parched Western and Midwest farm land. Cloud seeding has also been used in attempts to disarm hurricanes. In 1969 scientists seeded the clouds in the wall of the eye, or inner ring, of Hurricane Debbie; the wall expanded upward and outward, and its wind speeds decreased.
In another attempt to influence the weather, the U.S. Department of Defense spent $21.6 million on a seven-year program of cloud seeding to induce rain over parts of Southeast Asia during the Viet Nam War. Major purpose: to turn the Ho Chi Minh Trail, by which the North Vietnamese forces were being supplied, into an impassable quagmire. Its success: questionable; data produced by military authorities are insufficient to support their claim that they caused as much as 18 centimeters (7 in.) of rain to fall in one area during June 1971. The Soviets seem to have had better luck in their efforts to control hail by seeding clouds using rockets and artillery shells. In one region near the Bulgarian border, hail damage to crops was nine times greater in an unseeded area than it was in a section where clouds were seeded by Oblako rockets.
Even more elaborate weather-modification ideas have been proposed. Krafft Ehricke, a German-born rocket engineer, has revived an earlier suggestion that huge orbiting mirrors be used to reflect sunlight onto the dark side of the earth, preventing crop freezes and perhaps raising average temperatures enough to forestall the new ice age that some climatologists believe lies ahead (TIME, Jan. 31). Others have suggested paving large areas of desert with blacktop, which would absorb the sun's heat and warm the air above them, causing strong updrafts that could draw moist air in from nearby oceans.