The scientific evidence for enhancing rainfall from convective clouds by static-mode and dynamic-mode seeding with glaciogenic agents is examined and critically assessed. The assessment uses, as a measure of proof of concept, the criteria for success of any cloud seeding activity that was recommended in the Scientific Background for the 1998 AMS Policy Statement on Planned and Inadvertent Weather Modification, criteria that require both statistical and physical evidence. Based on a rigorous examination of the accumulated results of the numerous experimental tests of the staticmode and dynamic-mode seeding concepts conducted over the past four decades, it has been found that they have not yet provided either the statistical or physical evidence required to establish their scientific validity. Thus, the conclusion of several high-level reviews of weather modification conducted by the Advisory Committee on Weather Control, the National Academy of Sciences, and the Weather Modification Advisory Board during the period from 1957 to 1978 that cloud seeding was promising, unproven, and worth pursuing is still valid today.
The research and experiments related to the static-mode and dynamic-mode seeding concepts, especially those conducted since 1978, provided physical insights about some important cold–cloud precipitation development mechanisms and the possible effect of glaciogenic seeding on them. Exploratory, post hoc analyses of some of the experiments have suggested positive effects of seeding under restricted meteorological conditions, at extended times after seeding and, in general, for reasons not contemplated in the guiding conceptual seeding models; however, these exploratory results have never been confirmed through subsequent experimentation. New experiments are needed to resolve the uncertainties, inconsistencies, and deficiencies in the statistical and physical evidence in support of static-mode and dynamic-mode seeding of convective clouds obtained thus far. Considering the statistically positive result of hygroscopic flare seeding of cold convective clouds in South Africa and its replication in Mexico, and of hygroscopic particle seeding of warm convective clouds in Thailand, efforts to obtain the physical evidence required to place the hygroscopic seeding concept on a secure scientific foundation is, perhaps, a more immediate and higher-priority investment.
Future experiments on glaciogenic seeding of convective clouds, indeed any cloud seeding technique, should feature well-defined physical–statistical tests of the seeding concepts, in accordance with the proof-of-concept criteria, in order to establish their scientific validity. People with water interests at stake who are investing in operational glaciogenic cloud seeding projects for precipitation enhancement should be aware of the inherent risks of applying an unproven cloud seeding technology and provide a means of evaluation that allows for an assessment of the scientific integrity and cost effectiveness of the operational seeding projects. Those who are contemplating investing in operational hygroscopic seeding projects for precipitation enhancement based on the statistically positive experimental results in South Africa, Thailand, and Mexico should be aware that, in the absence of physical evidence required by the proof-of-concept criteria, this cloud seeding technology is also unproven.