WEATHER MODIFICATION

the deliberate or the inadvertent alternation of atmospheric conditions by human activity, sufficient to modify the weather on local or regional scales. the deliberate or inadvertent alteration of atmospheric conditions by human activity, sufficient to modify the weather on local or regional scales. Day-to-day weather constitutes a major element of the environment and an important factor in human well-being and activity. Agriculture, animal husbandry, transportation, and public health and safety are all greatly influenced by weather. It is not, therefore, surprising that one of humanity's oldest environment-related interests has been to manage the weather purposefully. Technically uncomplicated measures to improve the nighttime microclimate of plants by temporary covers (orchard heaters), large-vaned fans to mix the cold air that clings to the ground with the warmer air a few metres above, and water sprays to release latent heat into the air when the water droplets freeze and thereby limit the fall of temperature are just a few of the methods of weather control that have met with some success. Since 1946 a technique known as cloud seeding has been employed to augment precipitation. It involves the use of aircraft or ground generators to introduce condensation nuclei (usually silver iodide particles) into clouds. Sometimes the condensation is induced by cooling the cloud with solid carbon dioxide particles dropped from aircraft. Both of these techniques attempt to promote the natural process of rain formation, but they are more complicated and less effective than was thought in the early years of rainmaking that followed World War II. Modest increases of precipitation have been widely reported from operations in various parts of the world, but several experiments designed to increase rainfall have actually resulted in reduced fall, indicating that unknown factors are involved in rain formation. The knowledge gained from cloud-seeding projects, however, has proved valuable in other applications. One example is the suppression or mitigation of hail with silver iodide particles. Rockets or artillery shells are used to disperse the silver iodide in the central parts of cumulonimbus clouds, where the supercooled droplets necessary for hail formation predominate. By increasing the number of hail particles, their size-and hence their destructiveness-is reduced. The introduction of silver iodide into subfreezing levels of a cloud of liquid water is also used to disperse fog at airports. Industrialization and the rapid growth of large cities have produced measurable changes in local climate and may give rise to more widespread effects in the future. Cities warm the atmosphere over them, affect wind flow, and release pollutants into the air. The resultant updrafts and abundance of condensation nuclei may increase rainfall and winter fog. Moreover, the introduction of billions of tons of carbon dioxide into the atmosphere each year from the burning of fossil fuels (e.g., oil, natural gas, and coal) may in time raise the Earth's average temperature, causing the greenhouse effect (q.v.). It is thought that this potentially troublesome buildup of carbon dioxide in the atmosphere is intensified by the deforestation of the Amazon Basin and various other areas, since substantial photosynthetic biomass is lost by such activity. Additional reading National Research Council (U.S.) Committee on Atmospheric Sciences, Weather and Climate Modification: Problems and Prospects, 2 vol. (1966), an authoritative survey, supplemented by Weather and Climate Modification: Problems and Progress (1973); Georg Breuer, Weather Modification: Prospects and Problems (1979; originally published in German, 1976), a nontechnical examination of the subject; W.R.D. Sewell et al., Modifying the Weather: A Social Assessment (1973), a collection of symposium papers; William A. Thomas (ed.), Legal and Scientific Uncertainties of Weather Modification (1977); Robert G. Fleagle et al., Weather Modification in the Public Interest (1974), a review of weather modification research and activities and related public policy issues; G. Brant Foote and Charles A. Knight (eds.), Hail: A Review of Hail Science and Hail Suppression (1977); Arnett S. Dennis, Weather Modification by Cloud Seeding (1980); and W.N. Hess (ed.), Weather and Climate Modification (1974), a comprehensive account of all aspects of weather modification. Appropriate articles can be found in The Journal of Weather Modification (annual). Louis J. Battan The Editors of the Encyclopdia Britannica