National Geographic : 1972 Apr
Wispy wake of the jet era, a contrail created by an airliner's exhaust grows into a cirrus cloud over the Colorado Rockies. Meteo rologists fear such man-induced effects may disturb the earth's weather system. may need many of these balloons, especially in the tropics, and hundreds of new buoys scattered in remote parts of the ocean. And we will need several more geostationary sat ellites spaced strategically around the Equa tor. Japan and France are both considering launching such spacecraft. Dr. Robert M. White, head of the National Oceanic and Atmospheric Administration, estimates that this expanded weather obser vation system will save our nation more than a billion dollars a year. I believe Dr. White's estimates are conservative. I have seen quite genuine optimism for success among the scientists with whom I have worked for eight years in GARP's planning meetings. While scientists in many countries seek to understand and predict the weather, others search for ways to tame, or at least alter, the severe weather conditions-the storms and droughts-that plague mankind. In western Texas last spring a group of farmers sorely beset with drought offered a private rainmaker $10,000 if he could pro duce five inches of rain within 30 days. Day by day the rainmaker chased clouds across Stonewall County in an old pickup truck with a pot of charcoal and chemicals glow ing and smoking in the back (pages 530-31). At the end of the month: failure. Less than three-fourths of an inch of rain had fallen. In ancient times the unsuccessful rainmaker might have lost his head; in this case he lost only his contract. Downpour Follows Airborne Seeding A few weeks later Government scientists sought to ease a severe drought in southern Florida. Flying into the tops of clouds with a four-engine research plane, they dropped scores of burning flares that spewed out tril lions of microscopic crystals of silver iodide. In the six hours after this seeding, five inches of rain poured on the grateful citizens of the Miami and Everglades areas.* The drought, while not broken, had been sub stantially reduced. These two examples characterize the his tory of rainmaking-a story of frequent fail ure and occasional resounding success that may or may not have been related to the deliberate interventions of man. One early failure came in the 1890's, when Congress provided $9,000 for tests of rain making by sending explosives aloft with kites and balloons and by firing cannons. Perhaps the legislators had read the Greek writer Plu tarch, who had noted 18 centuries before that "extraordinary rains pretty generally fall after great battles." But the experiments in 1891 and 1892 produced more smoke than rain. Since that time a more scientific approach has been developed. We know that if air con taining a good deal of water vapor is very clean, it will often drop in temperature below the level at which condensation normally takes place, yet without producing clouds. Such air is called supersaturated. Similarly, water droplets in nature are often "super cooled" well below freezing without turning to ice, if there is an absence of nuclei. A cloud droplet cannot form without a condensation nucleus. A mote of dust, a salt *South Florida's water problems were described in "The Imperiled Everglades," by Fred Ward, NATIONAL GEOGRAPHIC, January 1972.