PRECIPITATION \ 



Figure VI-3 — LATTICE STRUCTURES OF Agl AND ICE 



The models show the crystal structures of ice and silver iodide (Agl). In the model 

 of Agl, the white spheres are iodide ions and the black spheres silver ions. Although 

 the crystal structures of both molecules are similar, the lattice constant of Agl is 

 1.5% larger than that of ice. Partial compensation for the difference can be made by 

 coprecipitating silver bromide (AgBr) with Agl and substituting Br for as many as 

 30% of the I atoms in the Agl crystal structure, which will produce a unit cell up to 

 0.5% smaller than that of pure Agl. 



been used. In retrospect, some of 

 them were inadequate. Nevertheless, 

 the early programs did show that 

 cloud seeding has a tremendous po- 

 tential. 



While the bulk of the activity in 

 precipitation augmentation involves 

 seeding clouds with artificial nuclei, 

 other procedures have been proposed 

 and are being studied. Modification 

 of radiation processes is an example. 

 If a large area (several acres or more) 

 is covered with asphalt, the increased 

 heating of the air immediately over 

 the area can lead to strong convective 

 currents, sufficient under some cir- 

 cumstances to stimulate the precipita- 

 tion process. Another possibility in- 

 volves increasing the humidity high 

 in the air so that more water would 

 be available for the natural precipita- 

 tion processes. Several ideas have 

 been offered for extracting water 

 from coastal stratus clouds. 



The obvious goal for weather- 

 modification research, considered as 

 a whole, is to find the best system 

 for any given situation. However, 

 the wide variety of conditions under 

 which clouds and storms occur, cou- 

 pled with the different types of to- 

 pography over which these clouds 

 develop, show that several, perhaps 



many, procedures must be available 

 to get the best results from every 

 situation. It is unlikely that the real 

 world will ever see a truly "best" 

 system for all conditions. A reason- 

 able procedure, short of finding the 

 absolute "best" way, is to put the 

 available techniques, equipment, and 

 instrumentation together in such a 

 way that, under the existing condi- 

 tions, the desired effect is maximized. 

 In other words, optimize the available 

 systems. 



What Constitutes a Precipitation 

 Management System? 



A true precipitation-management 

 system, even a crude and inefficient 

 one, will have four major compo- 

 nents: (a) a component to analyze 

 present and expected water needs 

 and water sources, as well as the 

 anticipated effects of precipitation 

 management on such factors as the 

 economy and ecology of the area in 

 question, and arrive at a decision 

 to employ precipitation-management 

 techniques; (b) a component to recog- 

 nize a weather situation where the 

 application of precipitation-manage- 

 ment techniques would result in the 

 desired effect and also, hopefully, 

 those situations in which the result 



would be deleterious; (c) j compo- 

 nent to select the proper treatment 

 material and delivery system for the 

 situation at hand; and (d) a com- 

 ponent to assess the actual results of 

 the treatment in terms of useful water 

 on the ground, economic benefits and 

 disbenefits, and environmental con- 

 sequences. 



Analyzer Function — The first ac- 

 tivity of an operational system is to 

 determine when the application of 

 precipitation-management techniques 

 could contribute to the resolution of 

 a water problem of a particular area. 

 After the specific need is defined, 

 the various potential sources of addi- 

 tional water (e.g., the atmosphere, 

 water mining, re-use) are examined 

 to find the best way to fill the need. 

 The effects of the application of 

 precipitation-management techniques 

 on the economics, ecology, and so- 

 ciology of the area are examined. 



Another important consideration 

 is whether or not the increased pre- 

 cipitation would fall where a sub- 

 stantial portion of it would eventually 

 be usable. There are also legal ques- 

 tions that must be looked at, such 

 as ownership of the land being af- 

 fected, ownership of the moisture 

 being withdrawn, licensing and in- 

 demnification procedures, and report- 

 ing procedures. 



When all the available informa- 

 tion has been considered, a decision 

 is made. Precipitation-management 

 techniques may be inappropriate for 

 a variety of reasons, or they may be 

 the only techniques available. Usu- 

 ally, however, precipitation manage- 

 ment will be used in addition to other 

 methods of acquiring additional 

 water. 



Recognition — Once a decision has 

 been made to use weather modifica- 

 tion in the solution of a problem, 

 treatable situations must be identi- 

 fied. Many of the necessary condi- 

 tions for successful weather modifica- 

 tion are known, at least qualitatively, 

 but we do not yet know if these are 

 sufficient conditions. 



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