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<br />~ <br /> <br />II. THE PHYSICAL BASIS FOR CLOUD SEEDING <br /> <br />Much is yet unknown about how many kinds of clouds grow, mature <br />and dissipate with the passage of time, especially very severe storms. <br />The following is a simplified description of how clouds can form and <br />eventually develop into precipitating systems (and severe storms) <br />during the crop-growing period of the year that interests us: <br /> <br />I <br />I <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 the <br />lifting of air. As air rises, atmospheric pressure decreases causing <br />. the air to expand and cool, eventually cooling to a temperature at <br />which the water vapor within it condenses into water droplets. When a <br />collection of these water droplets have grown to sufficient size, they <br />are recognized as clouds. Some common mechanisms forcing the upward <br />movement of air are: <br /> <br />(1) surface heating of the ground <br /> <br />(2) intruding CDld frontal systems <br /> <br />(3) relatively cDld air in the upper atmosphere that sinks intD <br />warmer air ahead of it <br /> <br />(4) upslope flDw Df air moving frDm eastern tD western Kansas <br /> <br />(5) regions Df convergence created by troughing at the surface <br />or alDft <br /> <br />(6) upper level low pressure systems <br /> <br />I <br />I <br />J <br /> <br />(7) outflow boundaries, or gust fronts, created by the dDwndrafts <br />assDciated with precipitation falling from older, mDre mature <br />storms diggin~under warmer air---acting much like a mini- <br />cold front <br /> <br />(8) gravity waves---small, less well-discussed atmDspheric <br />perturbations often capable of 'causing convective instability <br /> <br />Water droplets are formed when rising, cDoling water vapDr <br />condenses upon microscDpic particles called cloud condensation nuclei <br />(CCN). CCN particles include dust, smDke and salt crystals---all <br />found in the atmDsphere, wDrldwide. There are other special kinds of <br />particles known as ice nuclei (IN)---particles upon which, if found in <br />the condensed water droplets, may cause them to freeze. Also, ice <br />crystals may form directly from water vapor upon ice nuclei. Usually, <br />there is an abundance of condensation nuclei found in the atmosphere, <br />but a relative scarcity of ice nuclei. CIDuds can be made up of <br />unfrDzen water droplets or ice crystals, Dr a combinatiDn of both. <br /> <br />Within a cloud that has some part colder than freezing, there may <br />be some water droplets still in the liquid state; these are termed <br />"supercooled" droplets. It is in the supercooled part of the cloud <br />that can be found suspended a mixture of both supercooled droplets and <br />ice crystals. Often there may be only supercooled water droplets. The <br />supercooled cloud volume is mDst important in creating rain and hail. <br />Supercooled water can remain unfrozen as low as -40C (-40 F) before <br />spontaneously changing to ice. <br />