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Publication Abstract

Evaluations of Illinois Weather Modification Projects of 1976 - 1980: a Summary Changnon, Stanley A., Jr., and Chin Fei Hsu, 1981  Illinois State Water Survey, Champaign, IL,  ISWS C-148    Full Text Available

This report summarizes the key results of statistical and meteorological assessments of five summer weather modification projects in Illinois. During the 1976-1980 period, there were eight 1-summer weather modification projects in Illinois, each attempting to increase summer rainfall through cloud seeding with silver iodide. All the projects were operated in the same format involving meteorological forecasting and direction of the cloud seeding, aircraft for cloud base and mid-cloud seeding, and a weather radar for operations and data collection. These projects all involved cumulus cloud seeding for selected periods within the mid-June through early September period. Meteorological conditions related to the rain production of cumuliform showers and thunderstorms are generally uniform during this period (Changnon and Huff, 1980). These eight projects, their locations and periods of operations are identified in table 1, and the areas appear in figure 1.

The Illinois State Water Survey has had an extensive research and services program dealing with planned weather modification for 20 years (Changnon, 1979). One of the primary goals of this effort is to ascertain whether summer cloud seeding can produce agriculturally beneficial increases in rainfall under varying summer conditions.

One of the potential ways to gather information for addressing this complex scientific issue of weather modification is examination of operational (non-experimental) projects. If quality operational records and project data are collected, these projects represent an opportunity for learning whether the techniques employed may have altered the rainfall. As will be noted in table 1, many of the projects did not extend for long periods of time, all being two months or less. The actual number of days during which clouds were seeded is also not extensive.

A major NSF-supported research project conducted by the Water Survey concerns the development of techniques to evaluate operational projects, and one phase of that work is considering whether it is physically and statistically proper to combine the results of several operational projects of identical nature in the same climate zone. In the Illinois situation, this could mean the eight projects of 1976-1980 need not be analyzed as eight, separate events (each with too little data for meaningful conclusions), but rather their data combined and considered to represent a single, 8-summer project, giving a sizable data sample. The eight projects were all conducted in central and southern Illinois, an area of uniform rainfall climate in Illinois during summer (Changnon and Huff, 1980), and they all basically utilized the same seeding approaches and facilities. However, this report does not deal with the potential for this possible future research.

Over the past six years there have been preliminary, statistically focused analyses of five of these projects (Changnon and Towery, 1977; Changnon, Hsu, and Towery, 1978; Changnon and Hsu, 1980; and Hsu and Changnon, 1981). The five projects (those asterisked in table 1) are in a variety of locales. The aim of this report is to bring together the major findings of these statistically focused and reasonably limited assessments in a single document.

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