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<br />seedability (Bureau of Reclamation, 1983). Water and ice concentration <br />measurements from aircraft were also used in assessing the seeding poten- <br />tial of various cloud types at the PEP (Precipitation Enhancement Project) <br />site in Spain (WMO, 1982). General guidance on susceptibility to interven- <br />tion was derived from studies based on a MAY-B index which represented the <br />proportion of flight time the water and ice concentrations jointly <br />satisfied specific threshhold values. To identify cloud regions which had <br />characteristics that were more consistent with the concept of seedability, <br />PEP scientists developed a parameter called ROP (regions of potential) <br />which required that the averaged liquid water content exceed particular <br />values and that these regions persist for more than 10 min. <br /> <br />It is interesting to note that the decision to undertake area-wide experi- <br />ments to establish the effects of seeding was prompted by cloud studies <br />(Braham, 1960) that showed only a fraction of otherwise promising clouds <br />lasted long enough to be seedable and did not develop precipitation <br />naturally. With no means available to preselect only this fraction of <br />clouds for an experiment, it was concluded (Braham, 1981) that a very large <br />sample of clouds would be needed to test the seeding concept. By con- <br />ducting an area-wide experiment involving numerous clouds, it was hoped <br />that the cumulative effects of seeding the favorable fraction of clouds <br />could be detected in the precipitation at the ground. The occurrence of <br />suitable conditions, numerous convective clouds that would reach heights of <br />at least -5 oC, was predicted on the basis of thermodynamic and synoptic <br />parameters such as precipitable water, stability and wind direction <br />(Braham, 1966). Similar criteria were used in the Arizona Project (Battan <br />and Kassander, 1960) area-wide experiment. <br /> <br />13 <br />