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<br />II. THE PHYSICAL BASIS FOR CLOUD SEEDING <br /> <br />Simply put, for cloud development to oc~ur) t~ler~ is usually some <br />mechanism prOdlJcing vertical movement of water vapor. In rising, the <br />air cools to a temperature for which condensation forms the water <br />droplets of wt1ich clouds are comprised. <br /> <br />Some of the mechanisms causing upward air movement common in <br />Kansas are: (1) intense surface heating of the ground, (2) cold <br />frontal systems intruding and forcing air upward over the frontal <br />boundary} (3) relatively cold, sinkirlg air from the upper atmosphere <br />forcing air to rise a~ead of it, (4) the upslope f'lowing of air from <br />eastern Kansas into w~stern Kansas, and (5) regions of convergence <br />created by surface troughing or troughing aloft. <br /> <br />Water droplets themselves are formed by water vapor CQlldensing <br />upon microscopic particles called condensation l1uclei. Such <br />microscopic particles ~re present in the atmosphere worldwide and <br />irlclude dust, smoke, as well as salt crystals. <br /> <br />Also, there are a few kinds of special particles known as ice <br />nuclei---particles upon which condensed water may freeze) or upon <br />whict, ice crystals may form directly from water vapol'. Usually, there <br />is an abundance of condensation nuclei found in the atmosphere, but a <br />relative scarcity of ice nuclei. Clouds can be made up of small water <br />droplets or ice crystals, or a combination of both. <br /> <br />Sizes] concentrations, and chemical propel'ties of all nuclei <br />pl'esent in the atmosphere combine to play an import211t role in <br />det9rmining the efficiency with wt~ich a partic,~lar cloud system c~n <br />produce precipitation. At any given time there are massive amounts of <br />water in ttle atmosphere in the form of water vapor; oftentimes, no <br />precipitation occurs beCalJSe certain requi.red conditior15 for the <br />formation of precipitation are absent. <br /> <br />There are two natural cloud processes which are responsible for <br />cr'eating precipitation} the "warm cloud" process, and the "cold cloud'} <br />prQcess. <br /> <br />A "warm" cl6ud, i~ a cloud in which temperatures are never below <br />freezing and do not produce ice crystals. This cloud is generally <br />characterized by the relatively slow growth of cloud water droplets <br />which eventually attains the size and weigl,t to fall from the cloud. <br />In doing so, it sweeps out other clolJd droplets as it falls along its <br />path. While this type of cloud may appear occasionally in southwest <br />Kansas~ this is not the type of clol~d which plays any significant part <br />of the overall production of l'ainfall in this area. <br /> <br />Of primary importance to southwestern ~(ansas is the "cold" cloud. <br />"Cold" clouds have temperatlJres below freezillg in all of, or some part <br />of the cloud. Cold clouds are repoflsible for almost all of tt18 <br />significallt rainfall occurring in southwestern Kansas. <br /> <br />Withirl any cloud that has SOMe part of it colder than freezing, <br />~h2re.may be some water droplets ttlat have not yet turned from liquid <br />Into lee crystals; these are termed "supercooled" droplets. It is in <br />this supercooled part of thQ ~lol,.rd t~l~t c~n b~ fOlJr1d n mt~tlJre of <br />liquid water droplets al1d ice crystals susperlded together. Oftentimes, <br />there may be only unfrozen water droplets. It is this supercooled <br /> <br />5 <br />