History and Problems inWeather Modification Copyright 2002 by Ronald B. Standler Table of Contents Introduction Basic Technology Early History of Weather Modification at General Electric Problems with Experiments American Meteorology Society's Policy on Weather Modification Ethical Issues Support of Basic Scientific Research Bibliography About the author Introduction Weather modification is the effort of man to change naturally occurring weather, for the benefit of someone. The best-known kind of weather modification is cloud seeding, with the goal of producing rain or snow, suppressing hail (which can ruin crops), or weakening hurricanes. This essay is a companion to my earlier essay, Weather Modification Law in the USA, which concentrates on a discussion and analysis of court cases in the USA involving weather modification, and contains a detailed review of tort law in the USA that applies to weather modification. This essay is intended only to present general information about an interesting topic in law and is not advice for your specific problem. See my disclaimer. I cite articles and books in the (Author, year, page) format; complete bibliographic data is given below. Basic Technology It is a common misconception that pure water freezes at a temperature of zero celsius (32 degrees Fahrenheit). Zero celsius is actually the temperature at which ice melts. Water freezes at a temperature between 0 and -39 celsius, depending on the type of nuclei (i.e., contaminants) present. Liquid water with a temperature of less than 0 celsius is called "supercooled water". In November 1946, Dr. Bernard Vonnegut discovered that microscopic crystals of silver iodide (AgI) nucleate water vapor to form ice crystals. Vonnegut choose AgI crystals because there is nearly the same distance between molecules in the crystal lattice for both ice and AgI, which makes AgI the optimum material to nucleate ice. (Vonnegut, 1947) Vonnegut's discovery is a classic example of doing the right thing for the right reason at the very beginning of new technology, as a result of scientific knowledge. (Usually, progress is made by a series of small improvements on past practices, as a result of bumbling and guesswork. In contrast to the usual way, Vonnegut used his scientific knowledge to make a giant leap that has persisted as the state-of-the-art for more than fifty years.) Vonnegut not only discovered the ice-nucleating properties of AgI, but he also invented a practical way of generating tiny AgI particles to serve as nuclei for ice crystals. Vonnegut dissolved a mixture of AgI and another iodide in acetone, sprayed the solution through a nozzle to make droplets, then burned the droplets. (Vonnegut, 1949; Vonnegut & Maynard, 1952) In this way, one gram of AgI can produce 1016 nuclei for ice crystals. More than fifty years later, Vonnegut's method continues to be the common way to seed clouds. Release of AgI into an existing supercooled cloud (i.e., air temperature between -39 and -5 celsius) can convert water vapor to ice crystals, which is called sublimation. The ice crystals nucleated by the AgI will grow and local water droplets will shrink. The latent heat released by converting water vapor (or liquid water) to ice will increase vertical air motion inside the cloud and aid the convective growth of the cloud. Raindrops or snowflakes will grow larger by falling through a taller cloud. Also, moist air from evaporated moisture in the soil will be sucked into the base of the cloud by convection (i.e., updraft), thus increasing the total amount of water in the cloud. Perhaps 30 minutes after the AgI release, snow may fall below the cloud. Depending on the temperature and humidity below the cloud, the snow may change to rain, or even evaporate, before reaching the ground. AgI is the most common ice nucleus used in cloud seeding, but it is not the only material used. Substances with temperatures less than -40 celsius (e.g., solid CO2 ["dry ice"] pellets, liquid CO2, liquid propane, liquid nitrogen, etc.) can be dropped from airplanes into the tops of clouds, to induce formation of ice crystals. seeding warm clouds Clouds that do not contain appreciable amounts of supercooled water are known as "warm clouds". In warm clouds, most of the liquid water droplets will have temperatures greater than zero celsius. Seeding warm clouds with AgI or dry ice makes no sense, because the air temperature is too high. Langmuir (1948, p. 170 of Collected Works) proposed that using an airplane to dump liquid water into warm clouds might initiate a chain reaction that would produce rain below the cloud. Later, Langmuir (1951, p. 196 of Collected Works) proposed an alternative technique for seeding warm clouds: using an airplane to dump hygroscopic materials (e.g., NaCl) in the form of a dust particles, with each particle having an approximate diameter of 25 µm, into such clouds to convert water vapor to a droplet of liquid water. These new droplets fall and either...