PRECIPITATIO I ' ICATION 



energetics. The seeding of small 

 cumuli over Florida and over 

 nearby ocean areas aims at 

 complete glaciation of the 

 clouds to secure the maximum 

 release of latent heat of fusion, 

 which in turn might cause 

 greatly expanded cloud devel- 

 opment. 



4. The optimum number of ice 

 particles (hence the seeding re- 

 quirement, if any) depends in a 

 complex way on the detailed 

 nature of the cloud and the de- 

 sired end product. For example 

 the Bureau of Reclamation 

 project in Colorado aims at 

 regulating the number of snow 

 crystals in the clouds to be the 

 minimum required in order that 

 their combined growth rate just 

 uses up the liquid water of the 

 cloud by the time the cloud 

 reaches the crest of the moun- 

 tain divide. A lesser number 

 would permit cloud liquid water 

 to pass over the divide and be 

 evaporated. A larger number, 

 and slower growth, might result 

 in individual crystals being too 

 small to fall out before crossing 

 the divide. 



Requirements for Scientific 

 Cloud Seeding 



The modern approach to cloud 

 seeding is to couple the treatment 

 method to the end object through 

 specification of the target cloud and 

 a knowledge of the intermediate phys- 

 ical processes. To accomplish this re- 

 quires elaborate systems for real-time 

 measurement of deterministic meteor- 

 ological factors, and real-time com- 

 puter modeling of the physical proc- 

 esses of the clouds to permit objective 

 decisions as to when, where, and how 

 to seed. 



Data Base and Related Technology 

 — The data base on which to develop 

 a scientific approach to cloud seeding 

 is uneven. In some areas it is fairly 

 good, in others almost totally lacking. 



The physical properties of cloud and 

 precipitation particles, and the par- 

 ticle-interaction coefficients, though 

 incomplete, are sufficient for most 

 purposes. Given an initial specifica- 

 tion of cloud properties, one can make 

 usable estimates of the growth of a 

 limited number of precipitation par- 

 ticles contained therein. Once the pre- 

 cipitation particles become sufficiently 

 numerous to interact appreciably, or 

 in the ever present case of the inter- 

 action of cloud drops, the bottleneck 

 is not so much the lack of physical 

 data as one of computer capability and 

 mathematical devices to allow one to 

 keep track of the large number of pos- 

 sible interactions. 



A more serious difficulty is the gen- 

 eral lack of data on the internal micro- 

 structure of clouds as a function of 

 cloud type, season, geography, and 

 meteorological situation. Instruments 

 for measuring ice and cloud nuclei are 

 essentially laboratory devices and 

 really not suitable for routine field 

 use. Only recently have tools been 

 developed for routine measurement of 

 cloud-particle spectra. We have many 

 measurements of nuclei and cloud- 

 particle spectra from research proj- 

 ects, but we still lack appropriate con- 

 cepts for generalizing them in ways 

 to permit useful extension to the un- 

 measured cloud situation. 



Interactions and Downwind Effects 

 — The feedback loops between the 

 physics of particles inside clouds and 

 the energetics of those clouds is al- 

 most totally unexplored. One can per- 

 ceive a definite effort in this area in 

 cloud physics today. Important ad- 

 vances are likely to come quickly in 

 terms of the interactions inside single 

 clouds. But the equally important 

 problem of interaction between clouds 

 and cloud systems on the mesoscale 

 seems much more difficult. Such in- 

 teractions are well known for the case 

 of natural clouds. One should suspect 

 them — indeed, there are signs point- 

 ing to them — in the case of clouds 

 altered by seeding. For example, 

 measurements on Project WHITETOP 

 indicated strongly that changes in 



rainfall due to seeding were accom 

 panied by changes of opposite sign 50 

 to 100 miles downwind. 



Water and Energy Budgets of 

 Clouds — An area of general meteor- 

 ology of great importance to cloud 

 seeding, and still inadequately ex- 

 plored, concerns the water and energy 

 budgets of clouds and cloud systems. 

 Seeding to change precipitation pre- 

 sumes to alter the water budget of the 

 target cloud system, yet studies of 

 the water and energy budgets of 

 mesoscale weather systems are almost 

 totally lacking. Braham carried out 

 such a study for thunderstorms in 

 1952. A study of the water budget of 

 the winter storms involved in the Bu- 

 reau of Reclamation seeding project 

 in Colorado is presently under way. 

 Virtually no other mesoscale weather 

 system has been so studied. The rea- 

 sons for this are primarily the inade- 

 quacy, for this purpose, of data from 

 the National Weather Service and 

 the great cost of obtaining additional 

 data specifically for such studies. Yet 

 cloud seeding can never be soundly 

 based until we know in considerable 

 detail the water budgets of both the 

 natural and treated storms. 



Looking to the Future 



The preceding paragraphs are con- 

 cerned mainly with topics in physical 

 meteorology concerned with seeding 

 clouds to alter the amount of precipi- 

 tation at the ground. There are a 

 number of other matters that must be 

 resolved before such seeding can be 

 adequate for public purposes. Some 

 of these are scientific in nature, others 

 are issues of economics, sociology, 

 and public policy. 



Unanswered Questions — Among 

 the most important issues to be faced 

 are four unanswered scientific ques- 

 tions: 



1. Under what specific meteoro- 

 logical conditions (including mi- 

 crophysics and energetics of 

 clouds) will a particular treat- 



171 



