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.' e policy statement e Planned and Inadvertent Weather Modification A Policy Statement of the American Meteorological Society as adopted by the Council on 5 January 1992 1. Introduction Humans modify the weather deliberately and unin- tentionally; this is well founded in both theory and observation. Evidence accumulated over the last 40 years suggests that certain local weather conditions including fogs, low clouds, and precipitation in some areas can be altered by carefully controlled cloud seeding. Similarly, the effects of inadvertent weather modification are becoming better understood. Cities and industrial complexes affect local weather condi- tions and alter precipitation. Regional weather changes result from other human activities such as deforesta- tion and vehicle traffic on major transportation corri- dors. The focus of this statement is limited to local and regional changes and is mainly concerned with the scientific background to the problem. The Society's policy statement on global climate change is separate and has previously been presented (Bull. Amer. Me- teor. Soc., 72, 57). Early cloud seeding efforts, conducted in the 1940s and 1950s, involved attempts through field operations to increase precipitation without a sound scientific foundation. Most early scientific cloud seeding e>.<peri- ments were designed, conducted, and evaluated on the premise that statistical analyses of one or more variables (primarily precipitation) would yield statisti- cally significant and scientifically acceptable results. Data from many experimental seasons were often required to achieve sufficient numbers of test cases. . Results of such experiments have been mixed, some showing increases, some decreases, and many no statistically significant changes. Most ofthese experi- ments lacked supporting physical documentation as to how precipitation increases, if any, were a.chieved. I n addition, the lack of clear cut replication of the more successful cloud experiments has diminished their credibility and value. Thus, satisfactory determination of the capabilities of cloud seeding to produce desired effects under various conditions has not resulted from the statistical analyses. Developments have expanded our abilities to un- derstand and document precipitation processes. For example, the development of microwave radiometers Bulletin American Meteorological Society and their application to the measurement of super- cooled liquid water (SLW) has revealed the existence of low altitude SLW within winter orographic clpuds above the crestlines of mountain barriers. Aircraft- based sensing platforms often fail to detect,low-Ievel SLW because aircraft cannot safely fly sufficiently close to the mountain barriers. Vertically pointing radiometers sited at or near barrier crestlines have often observed quantities of SLW sufficient to give useful additional snowfall if successfully precipitated by cloud seeding. Multiparameter radars, which can distinguish be- tween liquid and ice-phase hydrometeors in clouds, offer new prospects for remote sensing of seeding signatures. Such radars have been successful in differentiating regions of graupel and hail from rainfall. Complementing such physical measurements, dem- onstration seeding trials have confirmed seeding ef- fects from both aerial and ground-based releases. Observations of increased ice-particle concentrations in seeded air volumes also tagged with tracer gases such as sulfur hexafluoride have increased confi- dence that observed effects follow from efforts to stimulate precipitation. Furthermore, recent advances in computer hard- ware, software, and physical understanding have allowed improvements in two- and three-dimensional numerical models which simulate cloud processes. These models continue to provide an ever-improving understanding of the complicated interactions within both natural and seeded clouds. Numerical simula- tions are now able to replicate many of the details observed in actual clouds. The impacts on cloud and precipitation development resulting from slight varia- tions in cloud microphysical or dynamic characteris- tics can be produced for the same cloud, for both natural and seeded circumstances. This is the only manner in which identical clouds can be studied in both treated and untreated versions, which may lead to more confident prediction of seeding effects and thus improved selection criteria for candidate clouds. Similarly, our ability to assess inadvertent modifica- tions of our atmos'pheric environment has also im- proved. Atmospheric changes that might have passed /1'1o.r-c.h 1992 331