PART VI — PRECIPITATION AND REGIONAL WEATHER PHENOMENA 



Figure VI-5 — OPTIMUM SEEDING CONDITIONS 





The table presents stratified data from three sets of experiments in an effort to show 

 what factors are important in seeding in Colorado during the winter. The optimum 

 conditions are summarized as follows: (1) the 500 mb temperature should be 

 between -11° and -20°C; (2) the computed vertical gradient of potential con- 

 densate in the 700-500 mb layer should be 1.3 to 2.0 g/kg/100 mb; and (3) the 500 

 mb windspeed should be between 22 and 27 mps. The probability of each of these 

 events has been computed, but is not presented here. 



making them suitable for operational 

 use. 



Closely connected with new seed- 

 ing materials are advances in delivery 

 systems. Increased understanding of 

 diffusion processes now puts posi- 



tioning of generators, either airborne 

 or ground, on a more objective basis. 

 New devices for producing nuclei 

 permit more efficient use of nuclei 

 material. Advances in radar tech- 

 niques, coupled with improved under- 

 standing of cloud characteristics and 



dispersion properties, permit safer 

 and more effective use of aircraft in 

 seeding operations. The use of rocket- 

 launched, pyrotechnic seeding de- 

 vices is receiving considerable atten- 

 tion. 



Modeling — Mathematical models 

 play an increasingly important role in 

 both research and operational precipi- 

 tation-augmentation programs. They 

 are used operationally in recognizing 

 treatable situations, in choosing par- 

 ticular clouds to seed, in specifying 

 the position of mobile generators so 

 that the effect will be felt in the target 

 area, and in specifying the area of 

 effect from fixed generators. These 

 models, developed from the basic 

 laws of physics, are usually relatively 

 simple, and can be run on moderate- 

 size computers in near real-time. 



More sophisticated models have 

 been used only for research pro- 

 grams, in part because present- 

 generation computers are not capable 

 of handling them in the time-scale 

 needed for operational use. The value 

 of these models lies in suggesting 

 effects to look for in the field and 

 in suggesting factors to be studied 

 in more detail. Three types (scales) 

 of models are currently available: 

 (a) microphysics models, which con- 

 sider the formation and growth of 

 water droplets and ice crystals; (b) dy- 

 namic models, which consider motions 

 and processes within the cloud (see 

 Figure VI-6); and (c) airflow models, 

 which consider cloud-forming proc- 

 esses. None of these models alone is 

 adequate to describe the complexities 

 of precipitation augmentation; several 

 attempts are being made, therefore, 

 at combining or chaining them. 



Implications for Society 



Precipitation augmentation is be- 

 coming an active partner with the 

 other components of the water-re- 

 sources system. In many parts of the 

 nation, it may prove to be the most 

 economical and socially acceptable 

 method to increase usable water 

 supplies. 



176 



