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(10 years or more). unless the effects due to seeding were very large (> 1 00 percent). More analysis of existing data sets should precede any renewed weather modification field efforts with convective complexes. In particular. the CCOPE data set from Montana and the Texas HIPLEX rawinsonde network data should be fully investigated. CCOPE included one of the largest mesoscale surface networks ever set up consisting of about 125 sta- tions. Multiple Doppler radar observations were made of the storms over this network. and several rawinsonde stations were also operated. It is hoped that a physical hypothesis applicable to larger clouds can be derived from this comprehensive set of obser- vations. Further. it is hoped that predictor variables will be found which permit testing of such a hypothe- sis through a physical and statistical randomized seeding experiment lasting no more than 5 years. The HIPLEX studies of the economic and environ- mental impacts of potential rainfall increases gener- ally confirmed and extended previously published results. Studies assessing the value to agriculture of potential operational projects were carried out in several states. including Montana. Kansas. and Texas. The results of these studies showed progres- sive increases in rangeland and crop yields with suc- cessive increments of growing season precipi- tation. with net benefits to the High Plains in the millions of dollars per year. Texas studies also showed that an estimated 23000 acre-feet of ground water could be replaced by a 10-percent increase in precipitation (Kenegla. 1979). The eco- nomic analysis assumed that the markets would expand so that commodity prices would not be reduced. The effects of the enhanced agricultural sector on other regional economic sectors were also recognized. The studies suggested a possible shift in land use or in species composition at given point toward those existing a few tens of kilometers to the east as a result of successful long-term weather modification operations. Much remains to be done to develop a reliable. scien- tifically sound convective cloud seeding technology capable of providing economically significant rainfall increases. That is not to say that current operational programs are not succeeding in increasing rainfa~1. However. natural variability of convective storms IS so large that demonstrating success beyond reasona- ble scientific doubt remains a formidable task. The importance of increased water resources to mankind cannot be overemphasized. Many areas of the world already experience economic hardship and even starvation due to limited natural rainfall. Therefore. it appears that society should continue efforts to develop a proven cloud seeding technology capable of increasing rainfall from convective clouds. WEATHER MODIFlCATION IN 'THE HIGH PLAINS Most seeding experiments in the High Plains have involved summertime cumulus clouds. These clouds range from the small. short-lived "puffy" ones having little or no potential to produce ram. to the large. violent "thunderheads:' whi~h may last for over an hour and produce heavy ram. The HIPLEX experiments were conducted in an effort to learn more about the physical properties of summer clouds and establish a sound scientific basis for increasing precipitation from them. The precipitation processes occurring in warm season convective clouds are summarized here as back- ground for the discussion of HIPLEX results. Localized vertical air motions known as convec- tive currents occur where the temperature decreases rapidly with increasing hE3ight. In the updrafts. air is cooled by expansion to its dew- point; that is. the temperature at which it becon:es saturated. With continued lifting, a cloud consIst- ing of tiny water droplets is formed from the water vapor that the air can no longer hold. In order for a cloud to rain. it must produce drops big enough to fall from cloud base to the ground without evaporating. The drops must be more than 1 mm (0.04 in) in diameter in a 1typical High Plains situation. Computations of droplet growth show that condensation alone would require many hours to produce a raindrop from a cloud droplet. Therefore. a precipitation particle must be the end product of one or more processes involving agglomeration of many cloud particles. Growth of liquid cloud droplets into raindrops is sometimes possible because of the differences in fall speed among cloud droplets of different sizes. which lead to collisions and coalescence. This sometimes is called the "warm rain" process. but a more accurate and descriptive term is the "coalescence" process. which is the merging of two drops into a single larger drop. One cloud seeding technique sometimes used is to introduce hygroscopic particles. such as powden3d common salt. to create some large drops and thereby accelerate the coalescence process (fig. 1.1). While the coalescence process is important in some High Plains cumulus. nearly all clouds big enough to produce significant rainfall tower to heights where the temperature is below 0 0 C (32 0 F). This development greatly increases the number of possible interactions among the cloud droplets and provides for an alternative precipi- tation process. known variously as the "ice 3