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<br />Case studies of a dry, an I'average," and a wet year were carried out to <br />compare actual conditions against the mean values. The studies illustrated <br />that the long-term potential for deficit could be examined for individual <br />years and actual deficits verified. <br /> <br />Surface precipitation data for 30 years from almost 300 observing sites in <br />Oklahoma were used to prepare a climatology of precipitation characteristics. <br />These data were analyzed for the State as a whole and also for the nine <br />climate divisions within the State. Both daily and weekly precipitation <br />characteristics were described. The conclusions of the study most signifi- <br />cant to weather modification are as follows: (1) over 95 percent of the time <br />there are rain-producing clouds each week in each of the western three <br />climate divisions in the spring and summer; (2) it is extremely rare to find <br />2 weeks in a row with no rain in each of these three areas; (3) the most <br />frequent value for 24-hour rainfall at a gage is somewhat less than 12 mm; <br />(4) although there are some heavy, flood-producing rains in every section of <br />Oklahoma, these are to be found mostly in the eastern half of the State; <br />(5) assuming that the existence of rain indicates the presence of seedable <br />clouds, this climatology suggests that western Oklahoma has an ample supply <br />of seeding opportunities for a successful operational weather modification <br />program; and (6) even in the driest 5 years out of the period from 1951 <br />through 1980, there were enough spring and summer rain-producing clouds in <br />the western third of the State to provide an adequate supply of seeding <br />opportunities for a successful operational program. <br /> <br />Because agricultural drought is more concerned with soil moisture than with <br />precipitation, the Oklahoma Climatological Survey prepared a soil moisture <br />climatology. Three indicators of soil moisture were analyzed: model- <br />determined soil moisture, the Crop Moisture Index (Sadowski, 1975), and the <br />Palmer Drought Index (Palmer, 1965). The soil moisture was calculated from a <br />simple model that uses precipitation, temperature, and plant available <br />soil-water capacity. It can be used as an indicator of antecedent moisture <br />conditions, and it reflects the amount of moisture available for plant <br />growth. <br /> <br />26 <br />