An Independent Statistical Reevaluation of the South African Hygroscopic Flare Seeding Experiment

The original statistical evaluation of the South African hygroscopic flare seeding experiment described by Mather et al. focused on the amount of radar-estimated rain produced by randomly seeded and unseeded convective cloud complexes in 10-min time periods from −10 to +60 min with respect to their time of selection. Statistically significant differences in the quartile values in some of the 10-min time periods were reported, with indications that the response time to seeding increased with increasing cloud system size. It was found also that the rain mass of the seeded cloud complexes in the 10-min period prior to selection was greater than that of their unseeded counterparts; the statistical evaluation did not take this apparent “inadvertent bias” into account, however. The evaluation included all 5 yr of the experiment, although the design was changed after the third year to allow for seeding larger cloud systems with a larger number of flares per convective cloud system. An independent statistical reevaluation of the South African cloud seeding experiment using hygroscopic flares is reported on here. Several interesting aspects of the results are found that require a physical explanation and, perhaps, a revision to Mather et al.’s seeding hypothesis; nothing was found that contradicts the claim that there is statistical evidence that hygroscopic flare seeding increased the rain mass from the South African convective cloud systems in the experiment, however.