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<br />. .
<br />Combustion of fossil fuels increases atmospheric
<br />particulates and aerosols over and downwind of major
<br />cities. This reduces air quality and visibility and also
<br />influences the local development of clouds and pre-
<br />cipitation,
<br />Major cities with populations in excess of 1 to 2
<br />million, and located in continental climates, influence
<br />warm-season clouds and increase precipitation by
<br />10%-20%, with a lesser effect on precipitation in cold
<br />season. Recent studies of urban areas in tropical
<br />regions have confirmed that significant modification of
<br />weather conditions also occurs in this climatic zone
<br />leading to cloud and precipitation increases.
<br />
<br />c. Atmospheric influences from industrialization
<br />Atmospheric effluents from manufacturing and
<br />power generation facilities add significantly to aero-
<br />sols and trace gas constituents, increasing smog and
<br />degrading visibility. Ingestion by clouds of some
<br />emitted gases (e.g., sulfur dioxide, S02) results in the
<br />production of acidic precipitation, which can, in suffi-
<br />cient concentrations, adversely affect structures,
<br />vegetation, and water quality. Large cooling lakes and
<br />cooling towers cause localized fogs, low clouds, and
<br />icing under certain weather conditions.
<br />
<br />d. Effects of large-scale transportation corridors
<br />Condensation trails from jet aircraft often persist
<br />and in areas of frequent flights sometimes spread,
<br />creating cirrus and/or cirrostratus cloud decks. These
<br />reduce insolation and can lead to lower surface tem-
<br />peratures, The resulting cloudiness is most frequently
<br />found along major airport corridors of the United
<br />States and Europe. The jet-induced cloud decks can
<br />persist for many hours or even days. Traffic in major
<br />surface transportation corridors result in sizable re-
<br />leases of effluents (NO;s and other particulates)
<br />which affect regional visibility and degrade air quality.
<br />
<br />4. Environmental and societal impacts
<br />of weather modification
<br />
<br />The impacts of weather modification on society can
<br />be far reaching. Therefore, the ecological, hydrologi-
<br />. cal, socioeconomic, and legal ramifications of such
<br />activities must be considered and assessed. The
<br />complexity of the effects of altered weather have been
<br />found to lead, in most cases, to both benefits and
<br />problems in various societal sectors and environmen-
<br />tal areas. Wise use of planned weather modification
<br />should recognize this varying distribution of effects
<br />and plan to assess the impacts in the design, opera-
<br />tion, and evaluation of field projects. There may need
<br />to be compensation for those affected negaHvely and
<br />liabilities must be assessed and understood where
<br />
<br />336
<br />
<br />e
<br />possible, to inform the public and those who make
<br />decisions relating to the use of weather modification
<br />Many states have enacted laws that regulate the use
<br />of weather modification, and the federal government
<br />requires that all weather modification projects be
<br />reported annually,
<br />
<br />5. Recommendations
<br />
<br />The prospect of being able to predictably modify
<br />fogs, clouds, and precipitation in certain conditions
<br />requires continued assessment of planned weather
<br />modification techniques. Much is known about the
<br />physical processes involved in many aspects of inad-
<br />vertent weather modification, but important questions
<br />remain, including those relating to large-area irrigation
<br />projects and major transportation corridors. Improved
<br />observational facilities, computer capabilities, numeri-
<br />cal models, and understanding now permit more de-
<br />tailed examination of clouds and precipitation pro-
<br />cesses than ever before, and significant advances are
<br />consequently possible, However, many measure-
<br />ments within and near clouds are required to test and
<br />improve the models. More effort must be made to
<br />obtain these, not justfor sounder weather modification
<br />but for large-scale weather and climate prediction and
<br />other uses, such as the remote sensing of precipita-
<br />tion.
<br />As socioeconomic factors place increasing de-
<br />mands on finite water resources, the demand for
<br />viable weather modification methods will logically in-
<br />crease. The following tasks address the most press-
<br />ing questions:
<br />1 ) The physical processes and specific conditions
<br />under which it is possible to increase, decrease, or
<br />relocate precipitation should be fully defined. The
<br />degree of change possible must be quantified to
<br />establish whether economic benefits can be realized.
<br />The use of untested weather modification techniques
<br />during severe droughts, as a means of increasing
<br />precipitation, is not recommended. Opportunities to
<br />increase precipitation are typically minimal during
<br />droughts and only well-tested techniques should be
<br />considered, realizing that only limited precipitation
<br />augmentation will probably result. .
<br />2) Hail suppression concepts should be reexam-
<br />ined. refined, tested, and (if necessary) modified to
<br />determine whether conditions exist under which hail
<br />can be suppressed.
<br />3) The extent of impacts produced by inadvertent
<br />weather modification requires improved definition,
<br />Further atmospheric studies are needed of cities of
<br />varying types and in different physical settings to
<br />better understand and predict local and regional-scale
<br />
<br />Vol. 73, No.3, March 1992
<br />
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