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<br />
<br />.
<br />
<br />quent transport and dispersion of the, seeding materi-
<br />als are needed. Current research using gaseous
<br />tracers such as sulfur hexafluoride is addressing
<br />these problems.
<br />There are indications that precipitation changes,
<br />either increases or decreases, can also occur at some
<br />distance beyond intended target areas. Improved
<br />quantification ofthese extended (extra-area) effects is
<br />needed to satisfy public concerns and assess hydro-
<br />logic impacts.
<br />Precipitation augmentation programs are unlikely
<br />to achieve higher scientific credibility until more com-
<br />plete understanding of the physical processes re-
<br />sponsible for any modification effect is established
<br />and linked by direct observation to the specific meth-
<br />odologyemployed. Continued research emphasizing
<br />in situ measurements, atmospheric tracers, a variety
<br />of remote sensing techniques, and multidimensional
<br />numerical cloud models that employ sophisticated
<br />microphysics offer improved prospects that this can
<br />be accomplished.
<br />
<br />c. Hail suppression
<br />The efficacy of projects intended to mitigate the
<br />severity of hailstorms remains indeterminate. Statis-
<br />tical assessments of certain operational projects indi-
<br />cate successful reduction of crop hail damage, but
<br />scientific establishment of cause and effect are incom-
<br />plete. Results of various operational and experimen-
<br />tal projects provide a range of ou.tcomes. Some
<br />suggest decreases in hailfall, but others have pro-
<br />duced inconclusive results, and some suggest in-
<br />creases. Given the diversity of conceptual models,
<br />cloud selection criteria, seeding agents, delivery tech-
<br />niques, assessment methods, and the storms them-
<br />selves, this is not unexpected. It is a direct reflection
<br />of storm complexity as well as the spatial and temporal
<br />variability of hail.
<br />Statistical evaluations using hail characteristics
<br />(i.e., kinetic energy, hailstone size, and area of hailfall)
<br />have often yielded inconclusive or inconsistent re-
<br />sults. Historic trends in crop hail damage have been
<br />used to evaluate many operational programs, but
<br />these data can be unreliable and so must be used
<br />cautiously.
<br />Our understanding of hailstorms is not yet sufficient
<br />to allow confident prediction of the effects of seeding
<br />individual storms, and the most appropriate seeding
<br />methodology has not been determined. The possibil-
<br />ity of increasing or decreasing both hail and rain in
<br />some circumstances is recognized, but numerical
<br />cloud models have recently affirmed thatthe desirable
<br />outcome, that is, a decrease in hail and an increase in
<br />rain, is possible. '
<br />Hail results in significant economic losses world-
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<br />wide; thus, research on hail suppression continues.
<br />As with precipitation augmentation efforts, increased
<br />in situ observations, remote sensing (e.g., multipa-
<br />rameter radar), and numerical cloud modeling capa-
<br />bilities continue to improve our understanding of hail-
<br />storms as a foundation for more effective scientific
<br />endeavors to suppress hail.
<br />
<br />d. Severe storms mitigation
<br />There is no generally accepted conceptual model
<br />for modifying tropical disturbances. Hurricane modi-
<br />fication experiments of the 1950s and 1960s were
<br />inconclusive. Although strong interest continued into
<br />the 1970s, no organized research effort was under-
<br />taken, and few studies have been devoted to this
<br />subject for the past 20 years.
<br />No sound physical hypotheses exist for the modifi-
<br />cation of tornadoes, or of damaging winds in general,
<br />and no scientific experimentation has been conducted.
<br />Experiments have been carried out to suppress light-
<br />ning but have not yet yielded methods sufficiently
<br />developed for application.
<br />
<br />3. Status of inadvertent weather
<br />modification
<br />
<br />There is ample evidence that agricultural and in-'
<br />dustrial activities modify local and sometimes regional
<br />weather conditions. Urbanization also results in local-
<br />ized and regional weather modification. Air quality,
<br />visibility, surface and low-level winds, humidities and
<br />temperatures, and cloud and precipitation processes
<br />are all affected by large urban areas.
<br />
<br />a. Impacts of agricultural practices
<br />Variations in cropping and irrigation practices are
<br />known to alter local albedos, humidity, surface tem-
<br />peratures, and roughness. Large-scale irrigation
<br />projects alter boundary-layer conditions, increasing
<br />the potential for' precipitation. Overgrazing of large
<br />grasslands and large-scale deforestation reduce
<br />evapotranspiration and change the surface rough-
<br />ness, altering the moisture, temperature, and winds in
<br />the boundary layer and causing a net loss of precipi-
<br />tation.
<br />
<br />b. Effects of urban areas
<br />Surface temperatures in urban areas are higher
<br />than those in adjacent rural surroundings. This heat
<br />island effect, coupled with increased roughness and
<br />urban structures, alters local winds and atmospheric
<br />circulation patterns leading to convergence zones
<br />which initiate clouds and precipitation under certain
<br />atmospheric conditions.
<br />
<br />Vol. 73, No.3, March 1992
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