AN INDEPENDENT EVALUATION OF A SOUTH-AFRICAN HYGROSCOPIC CLOUD SEEDING EXPERIMENT, 1991-1995

An independent statistical evaluation of a randomised hygroscopic clou d seeding experiment carried out in South Africa from 1991-1995 is pre sented, based on the original radar data for each storm. It is found t hat seeded storms clearly lasted longer than unseeded ones. It is sugg ested that the initiation of precipitation at a lower height in seeded than in unseeded clouds (demonstrated through microphysical measureme nts by the experimenters) led to a reduced dispersion by windshear of the precipitation falling from the updraft in the seeded clouds. A mor e concentrated downdraft region resulted, closer to the updraft. The s urface gust front was thereby intensified and its interaction with the storm inflow or other gust fronts enhanced convection. Some consequen ces of such a mechanism are explored. It is concluded that this experi ment offers a real hope of increasing precipitation from convective st orms and represents an important advance in weather modification.