PART VI — PRECIPITATION AND REGIONAL WEATHER PHENOMENA 



4. There are several feedback loops 

 whereby a change in the micro- 

 physical character of a cloud 

 parcel, as a result of precipita- 

 tion development, feeds back 

 into the energetics of the cloud 

 and thereby alters the boundary 

 conditions in which the precipi- 

 tation processes operate. These 

 feedback loops are largely unex- 

 plored. They range in scale from 

 the release of heat of phase 

 change, causing a small cloud 

 parcel to accelerate upward, 

 thereby increasing its conden- 

 sate load, to large-scale, long- 

 range effects whereby a major 

 change in the cloud system at 

 one point induces adjustments 

 in the atmosphere tens or hun- 

 dreds of cloud-diameters away. 



Natural Nuclei and their Relation 

 to Weather Modification 



Almost all U.S. efforts to change 

 precipitation through cloud seeding 

 (whether to increase, decrease, or re- 

 distribute either rain or snow) rest on 

 the observation that the normal be- 

 havior of a cloud can be altered 

 through the introduction of large 

 numbers of suitable nuclei. 



There are two types of natural 

 nuclei, serving two different func- 

 tions, in natural clouds: 



1. Cloud nuclei (small soluble par- 

 ticles of the order of 0.1 to 3 

 microns in diameter), which 

 serve as condensation centers 

 for liquid cloud droplets. 



2. Ice nuclei (probably clay min- 

 erals about 1 micron in diam- 

 eter, although the exact nature 

 of these particles is still in ques- 

 tion), which serve as centers of 

 initiation of ice particles either 

 by freezing drops or directly 

 from the vapor. 



Ice nuclei are necessary for snow 

 production. Snow generated aloft may 

 melt inside a cloud on its way to the 



ground and land as rain. Rain may 

 also be initiated by a few specially 

 favorable cloud nuclei acting through 

 an all-liquid process. 



The relative importance of the two 

 known precipitation mechanisms is 

 not fully worked out. However, it 

 appears that the all-liquid process is 

 more important in warmer seasons 

 and in maritime air masses, whereas 

 the ice-crystal mechanism is probably 

 more important in colder seasons and 

 in continental weather events. 



The ice-crystal mechanism of pre- 

 cipitation development was the first 

 precipitation process proposed. It ap- 

 peared to explain most available ob- 

 servations until the late 1940's, when 

 meteorologists began to make meas- 

 urements inside clouds and to examine 

 them with radar. The all-liquid pre- 

 cipitation mechanism was essentially 

 unknown before about 1950; even to- 

 day its relative importance is not clear. 



The common occurrence of super- 

 cooled clouds was taken as evidence 

 to show that concentrations of nat- 

 ural ice nuclei were often insufficient 

 for effective precipitation production. 

 Proponents of seeding thus argued 

 that, through the addition of artificial 

 nuclei, one could enhance the effi- 

 ciency of the ice-crystal mechanism 

 and thereby increase rain at the 

 ground. 



Technology quickly provided effi- 

 cient tools for releasing large numbers 

 of artificial ice nuclei. Present-day 

 seeding generators, burning an ace- 

 tone solution of silver iodide (Agl), 

 yield effective ice nuclei concentra- 

 tions of about 10 13 to 10 14 crystals 

 per gram of Agl at —10° centigrade, 

 increasing to about 10 1 '' crystals per 

 gram of Agl at —20° centigrade. This 

 means that a single gram of Agl, if 

 completely and properly dispersed, 

 would be capable of seeding 100 cubic 

 kilometers. Technology has not yet, 

 however, produced adequate tools for 

 measuring the concentrations of nat- 

 ural ice nuclei. 



A more realistic, more scientific ap- 

 proach to cloud seeding for altering 

 precipitation is beginning to emerge. 

 This approach recognizes, and at- 

 tempts to relate, several interdepend- 

 ent factors: 



1. There are two known precipita- 

 tion mechanisms, only one of 

 which depends on ice nuclei and 

 only one of which is readily 

 accessible through present-day 

 seeding technology. 



2. The concentrations of natural 

 nuclei, both cloud and ice par- 

 ticles, and the internal structure 

 of clouds of any given type 

 differ importantly from time to 

 time and place to place. For 

 example, a substantial differ- 

 ence between cloud spectra in 

 maritime and continental cumuli 

 is recognized as due to differ- 

 ences in the cloud nuclei; ba- 

 sically, it is this difference in 

 drop spectra that gives mari- 

 time clouds their propensity for 

 warm rain. As a consequence 

 of such differences, natural 

 clouds differ markedly in their 

 response to seeding. 



Not all responses to seeding 

 are desirable. To give an ex- 

 ample, Project WHITETOP 

 found that Agl seeding of 

 summertime cumulus clouds in 

 Missouri may have decreased 

 the rainfall by as much as 40 

 to 50 percent on days with 

 south winds. 



3. The development of precipita- 

 tion takes considerable time, in 

 many cases about the same as 

 the lifetime of the cloud parcels 

 that nurture the precipitation 

 development. Thus, most seed- 

 ing efforts attempt to alter the 

 time required for precipitation 

 development relative to the life 

 of the cloud, or, alternatively, 

 attempt to extend the life of 

 the cloud by activating feed- 

 back loops between changes in 

 cloud microstructure and cloud 



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