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I I I I I I I I I I I I I I I I I I I I II. THE PHYSICAL BASIS FOR CLOUD SEEDING Much is still unknown about how some clouds grow, mature and dissipate with the passage oftime, especially very severe storms. During the crop-growing period ofthe year rapidly growing convective clouds can quickly become severe producing hail which destroys both crops and property as well as producing highly destructive surface winds, including occasional tornadoes. The following is a brief explanation of how such convective clouds can form and the theory behind why hail suppression and rain stimulation is technologically feasible. In order for a convective cloud to form air containing water vapor must first be cooled to condensation. Rising air can do this. Rising air cools by expansion as atmospheric pressure decreases causing it to cool adiabatically. Eventually, the air cools to a temperature at which the atmospheric water vapor in it condenses into water droplets. Condensation occurs first upon microscopic particles called cloud condensation nuclei CCN. CCN particles are relatively abundant in the world atmosphere and includes dust, smoke and salt particles. When a collection of these water droplets have grown to sufficient size, they are seen as clouds. In Western Kansas some of the mechanisms which causes air to lift and convective clouds to form are: (1) surface heating - returning solar radiation to the atmosphere - warm air rising (2) intruding cold and warm frontal systems - forcing air to lift over its advancing boundary (3) relatively cold air in the upper atmosphere sinking into warmer air ahead of it causing warm, moist air to be displaced, or forced, upward (4) upslope air flow which moves moisture from lower altitud.es in Eastern Kansas into the higher altitude areas of Western Kansas and Eastern Colorado (5) regions of horizontal convergence created by troughing at the earth's surface, or aloft, forcing air to rise as it is squeezed together (6) upper level low pressure systems (usually associated with low-level convergence, sinking cold air and/or upslope effects) (7) convective scale interaction resulting from thunderstorm outflows digging under warm, humid air acting much like a mini-cold front forcing air ahead of it to lift rapidly (8) gravity waves---very small, internal perturbations traveling through the atmosphere, many of which are created randomly and not always easily detectable in real-time Other atmospheric particles are known as ice nuclei(IN), particles upon which, if found in condensed water droplets, enhance droplet freezing. Ice crystals also may form directly from water vapor upon ice nuclei. Despite the atmospheric abundance ofCCN, there is a relative scarcity of IN particles. It is this natural condition which we wish to address with cloud seeding. 3