Initial Results from the 1989 Cloud Seeding Experiment in Illinois

Authors

  • Robert R Czys Illinois State Water Survey Atmospheric Sciences Division Champaign, IL
  • Stanley A Changnon Illinois State Water Survey Atmospheric Sciences Division Champaign, IL
  • Mary Schoen Petersen Illinois State Water Survey Atmospheric Sciences Division Champaign, IL
  • Robert W Scott Illinois State Water Survey Atmospheric Sciences Division Champaign, IL
  • Nancy E Westcott Illinois State Water Survey Atmospheric Sciences Division Champaign, IL

Abstract

Some early results from the 1989 cloud seeding experiment conducted in Illinois are reported in this paper. This exploratory field project was designed to achieve four primary objectives: 1) to obtain data on the largest possible sample of clouds (treated and natural); 2) to test some of the early physical steps of the dynamic seeding hypothesis; 3) provide data for the development of analytical tools for discerning seeding effects; and 4) to improve basic knowledge about natural cloud and precipitation processes in the Midwest. The treatment randomization was based on "floating" experimental units, initially defined by a single cumulus congestus. Analysis of predictor variables revealed significant differences between the AgI and sand treated clouds at the time of treatment in many aspects that might govern future cloud growth. A Seedability Index composed of criteria physically consistent with the dynamic seeding hypothesis is described which was developed as an initial approach to addressing the problem of the bad draw revealed by the predictor variable analysis. The temporal series of the empirically-defined Seedability Index revealed that seedable conditions did not remain constant over the course of the field experiment and that even the seedable conditions for pairs of experimental units obtained on the same day were not always comparable. These findings illustrate the large inherent natural variability which has come into play in other cloud seeding experiments, and has frustrated efforts to randomly select two populations of clouds having sufficient similarity in individual characteristics to allow valid comparisons.

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Scientific Papers