04 April 2009

Cloud seeding

The invention: Technique for inducing rainfall by distributing dry ice or silver nitrate into reluctant rainclouds. The people behind the invention: Vincent Joseph Schaefer (1906-1993), an American chemist and meteorologist Irving Langmuir (1881-1957), an American physicist and chemist who won the 1932 Nobel Prize in Chemistry Bernard Vonnegut (1914-1997), an American physical chemist and meteorologist Praying for Rain Beginning in 1943, an intense interest in the study of clouds developed into the practice of weather “modification.” Working for the General Electric Research Laboratory, Nobel laureate Irving Langmuir and his assistant researcher and technician, Vincent Joseph Schaefer, began an intensive study of precipitation and its causes. Past research and study had indicated two possible ways that clouds produce rain. The first possibility is called “coalescing,” a process by which tiny droplets of water vapor in a cloud merge after bumping into one another and become heavier and fatter until they drop to earth. The second possibility is the “Bergeron process” of droplet growth, named after the Swedish meteorologist Tor Bergeron. Bergeron’s process relates to supercooled clouds, or clouds that are at or below freezing temperatures and yet still contain both ice crystals and liquid water droplets. The size of the water droplets allows the droplets to remain liquid despite freezing temperatures; while small droplets can remain liquid only down to 4 degrees Celsius, larger droplets may not freeze until reaching -15 degrees Celsius. Precipitation occurs when the ice crystals become heavy enough to fall. If the temperature at some point below the cloud is warm enough, it will melt the ice crystals before they reach the earth, producing rain. If the temperature remains at the freezing point, the ice crystals retain their form and fall as snow. Schaefer used a deep-freezing unit in order to observe water droplets in pure cloud form. In order to observe the droplets better, Schaefer lined the chest with black velvet and concentrated a beam of light inside. The first agent he introduced inside the supercooled freezer was his own breath. When that failed to form the desired ice crystals, he proceeded to try other agents. His hope was to form ice crystals that would then cause the moisture in the surrounding air to condense into more ice crystals, which would produce a miniature snowfall. He eventually achieved success when he tossed a handful of dry ice inside and was rewarded with the long-awaited snow. The freezer was set at the freezing point of water, 0 degrees Celsius, but not all the particles were ice crystals, so when the dry ice was introduced all the stray water droplets froze instantly, producing ice crystals, or snowflakes. Planting the First Seeds On November 13, 1946, Schaefer took to the air over Mount Greylock with several pounds of dry ice in order to repeat the experiment in nature. After he had finished sprinkling, or seeding, a supercooled cloud, he instructed the pilot to fly underneath the cloud he had just seeded. Schaefer was greeted by the sight of snow. By the time it reached the ground, it had melted into the first-ever human-made rainfall. Independently of Schaefer and Langmuir, another General Electric scientist, Bernard Vonnegut, was also seeking a way to cause rain. He found that silver iodide crystals, which have the same size and shape as ice crystals, could “fool” water droplets into condensing on them. When a certain chemical mixture containing silver iodide is heated on a special burner called a “generator,” silver iodide crystals appear in the smoke of the mixture. Vonnegut’s discovery allowed seeding to occur in a way very different from seeding with dry ice, but with the same result. Using Vonnegut’s process, the seeding is done from the ground. The generators are placed outside and the chemicals are mixed. As the smoke wafts upward, it carries the newly formed silver iodide crystals with it into the clouds. The results of the scientific experiments by Langmuir, Vonnegut, and Schaefer were alternately hailed and rejected as legitimate. Critics argue that the process of seeding is too complex and would have to require more than just the addition of dry ice or silver nitrate in order to produce rain. One of the major problems surrounding the question of weather modification by cloud seeding is the scarcity of knowledge about the earth’s atmosphere. Ajourney begun about fifty years ago is still a long way from being completed. Impact Although the actual statistical and other proofs needed to support cloud seeding are lacking, the discovery in 1946 by the General Electric employees set off a wave of interest and demand for information that far surpassed the interest generated by the discovery of nuclear fission shortly before. The possibility of ending drought and, in the process, hunger excited many people. The discovery also prompted both legitimate and false “rainmakers” who used the information gathered by Schaefer, Langmuir, and Vonnegut to set up cloud-seeding businesses.Weather modification, in its current stage of development, cannot be used to end worldwide drought. It does, however, have beneficial results in some cases on the crops of smaller farms that have been affected by drought. In order to understand the advances made in weather modification, new instruments are needed to record accurately the results of further experimentation. The storm of interest—both favorable and nonfavorable—generated by the discoveries of Schaefer, Langmuir, and Vonnegut has had and will continue to have far-reaching effects on many aspects of society.