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 static-mode 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.