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<br />'I <br /> <br />I <br /> <br />I <br /> <br />II. THE PHYSICAL BASIS FOR CLOUD SEEDING <br /> <br />I <br /> <br />Much is yet unknown ab~ut how some clouds grow, mature and <br />dissipate with the passage -of time, especially very severe storms. <br />The following 16 a simplified description of how convective clouds <br />can form and develop into precipitating systems (and severe star-IllS) <br />during the important crop-growing period of the year. <br /> <br />In order for our precipitating clouds to form. air containing <br />water vapor must first be cooled to condensation. This is done by <br />the lifting of air. As the air rises, atmospheric pressure <br />decreases causing the air to expand and cool, eventually cooling to <br />a temperature at which the water vapor wi thin 1 t condenses into <br />water droplets. When a collection of these water droplets have <br />grown to sufficient 'size, they are recognized as clouds. Some <br />common mechanisms causing the upward movement of air are: <br /> <br />(1) surface heating of the ground <br /> <br />(2) intruding cold frontal systems and warm frontal systems <br /> <br />(3) rela~ively cold air in the upper atmosphere which sinks <br />into warmer air ahead of it <br /> <br />(4) upslope flow of air moving from eastern to western Kansas <br /> <br />(5) regions at convergence created by troughing at the surface <br />or aloft <br /> <br />(6) upper level low pressure systems <br /> <br />(7) outflow boundaries <gust fronts) digging under warmer <br />air---acting much like a mini-cold front; created by the <br />downdrafts associated with precipiLation falling from <br />older, mature stor~ <br /> <br />(8) gravity waveB---s~ll, less well generally discussed <br />atmospheric perturbations often capable of causing <br />convective instability <br /> <br />Water droplets are formed when rising, cooling water vapor <br />condenses upon microscopic particles called cloud condensation <br />nuclei (CCN). CCN particles include dust, smoke and salt crystals-- <br />-all found in the atmosphere, worldwide. There are other special <br />kinds of particles known as ice nuclei (IN)---particles upon which, <br />it found in the condensed water droplets~ may cause them to freeze, <br />A.lso ice crystals may form directly from water vapor upon ice <br />nuclei. Usually there is an abundance af condensation nuclei found <br />in the atmosphere, but a relative scarcity of ice nuclei. Clouds <br />can be made up of unfrozen water droplets or ice crystals, or a <br />combination of both, <br /> <br />4 <br />