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<br />I <br />] <br />I <br />I <br />I <br /> <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />\ <br />I <br />I <br />, I <br /> <br />II. THE PHYSICAL BASIS FOR CLOUD SEEDING <br /> <br />Much is still unknown about how some clouds grow, mature and <br />dissipate with the passage of time, especially very severe storms. <br />During the crop-growing period of the year, rapidly growing <br />convective clouds can quickly become severe, producing hail which <br />destroys both crops and property and produce highly destructive <br />surface winds as well as occasional tornadoes. The following is a <br />brief explanation of how such convective clouds form. <br /> <br />In order for a convective cloud to form, air containing water <br />vapor must first be cooled to condensation. Rising air can do this. <br />Rising air cools by expansion as atmospheric pressure decreases <br />causing air to cool adiabatically. Eventually, the air cools to a <br />temperature at which the atmospheric water vapor in it condenses <br />into water droplets. Condensation occurs first upon microscopic <br />particles called cloud condensation nuclei (CCN. CCN particles are <br />relatively abundant in the world atmosphere and includes dust, smoke <br />and salt particles. When a collection of these water droplets have <br />grown to sufficient size, they are seen as clouds. In Western Kansas <br />some of the mechanisms which causes air to lift and convective <br />clouds to form are: . <br /> <br />(1) surface heating - returning solar radiation to the <br />atmosphere - warm air rising <br /> <br />(2) intruding cold and warm frontal systems - forcing air to <br />rise over its advancing boundary <br /> <br />(3) relatively cold air in the upper atmosphere sinking into <br />warmer air ahead of it causing warm, moist air to rise <br /> <br />'\ <br /> <br />(4) upslope flow of air moving moisture from lower altitudes <br />in Eastern Kansas into the higher altitude areas of <br />Western Kansas and Eastern Colorado <br /> <br />(5) regions of horizontal convergence created by troughing at <br />the surface, or aloft, forcing air to rise <br /> <br />(6) upper level low pressure systems (usually associated with <br />lower convergence, sinking cold air or upslope effects) <br /> <br />(7) convective scale interaction resulting from thunderstorm <br />outflows digging under warm, humid air acting much like a <br />mini-cold front which forces air ahead of it to lift <br />rapidly <br /> <br />(8) gravity waves---very small, internal perturbations <br />traveling through the atmosphere, many of which are <br />created randomly and not always easily detectable in <br />real-time when they occur. <br /> <br />3 <br />