PRECII I 



or lower. Other materials, such as 

 silver iodide, are known to be effec- 

 tive, but why they work is not clearly 

 understood. The crystal structure of 

 Agl is quite similar to that of ice, 

 and this was thought to be the rea- 

 son for its effectiveness. Recent stud- 

 ies suggest that pure Agl is a rather 

 poor nucleating material, and that it 

 must be contaminated with some 

 other material to be useful in weather 

 modification. 



Different methods are needed to 

 deliver the various materials to the 

 cloud. Dry ice is dropped into clouds, 

 usually from an airplane. The size of 

 the dry-ice pellets depends on the 

 vertical thickness of the cloud. Silver 

 iodide can be released from the air 

 or from the ground. Ground releases 

 rely on the horizontal and vertical 

 airflow to carry the material to the 

 cloud. 



One major problem is to confine 

 the effects of treatment to a desig- 

 nated target area. The point on the 

 ground where the effects will be felt 

 is determined by the point of release 

 of the material, the concentration of 

 the material at the release point, the 

 diffusion of the material (a function 

 of the three-dimensional wind field), 

 the time required for the material to 

 become effective once it is in the 

 cloud, and the time required for the 

 altered cloud characteristics to show 

 up on the ground. The usual pro- 

 cedure involves assumptions about 

 mean values and average times, with 

 reliance on the skill of the operator 

 to integrate the various factors sub- 

 jectively. Several mathematical mod- 

 els have been developed that predict 

 the area of effect; as these models, 

 and the data they use, improve, tar- 

 geting procedures should also im- 

 prove. 



Despite the uncertainties in how 

 the material works, how much is 

 needed, and where and how it should 

 be released, present capabilities are 

 sufficient to warrant a certain number 

 of operational precipitation-modifica- 

 tion programs. In these cases, the 



areas to be affected are relatively 

 small and the objectives sufficiently 

 narrow so that the uncertainties can 

 be taken into account in the program 

 designs. 



Evaluation — The final phase of 

 a functioning weather-modification 

 system is evaluation of the results. 

 Evaluation techniques include the 

 standard statistical approaches: target 

 vs. control; treat vs. no treat; ran- 

 domized crossover, and so on. Both 

 parametric and nonparametric statis- 

 tics are used. A few new variations 

 have been considered but are not 

 being used except experimentally. 

 Given a suitable experimental design, 

 existing statistical evaluation pro- 

 cedures are acceptable for programs 

 that go on for several years and in 

 which the evaluation can wait until 

 the end of the program. 



Full evaluation includes not only 

 the amount of precipitation produced 

 but also the economic consequences 

 of the activity and the effects on the 

 social and biological environment. 



Current Scientific Status 



Large quantities of data at or near 

 the earth's surface have been gath- 

 ered from experimental areas. Upper- 

 air data are generally insufficient in 

 terms and frequency and density. 

 Because most weather-modification 

 activities are rather small and inde- 

 pendent of one another, data gather- 

 ing is not standardized with respect 

 to time of observation, duration, 

 precision, or reliability. Some of the 

 data from commercial programs are 

 not readily available. Perhaps the 

 greatest limitation of the present data 

 base is the scarcity of measurements 

 of some of the important factors in 

 precipitation augmentation, such as 

 natural nuclei counts. Lack of suit- 

 able instruments is, in part, respon- 

 sible for this situation. 



Extra-Area Effects — While scien- 

 tists have not had the quality data 

 they would have liked, significant 

 advances have occurred in the past 



few years. One interesting phenom- 

 enon was recently recognized: In 

 major field programs for increasing 

 rain, changes in the precipitation pat- 

 tern well outside the designated tar- 

 get areas have been noted. The 

 changes were patterns of negative 

 and positive anomalies, but the in- 

 creases were more substantial than 

 the decreases. This suggests that 

 some sort of dynamic effect is caused 

 by cloud seeding, resulting in an 

 average precipitation increase over a 

 very large area. These effects are 

 sometimes felt upwind and laterally 

 as well as downwind of the target 

 area. In at least one experiment, the 

 precipitation of an entire area was 

 increased, with target-area precipita- 

 tion significantly greater even when 

 compared with the precipitation-in- 

 creased controls. How universal these 

 effects are and under what conditions 

 they occur are not clearly understood. 

 The importance of this phenomenon 

 in evaluation is obvious. 



The Significance of Cloud-Top 

 Temperature — One of the most im- 

 portant discoveries of the 1960's was 

 identification of the importance of 

 cloud-top temperature on the effec- 

 tiveness of cloud seeding. Stratifica- 

 tion of data by temperature indicates 

 large precipitation increases from 

 seeded winter orographic clouds when 

 the temperature at or near the cloud 

 top is between about — 15 and — 20° 

 centigrade. When the temperature is 

 — 25 or colder, precipitation de- 

 creases from the same kind of clouds 

 are observed. This suggests that suf- 

 ficient natural nuclei have a negative 

 influence on the precipitation process. 

 Figure VI-5 summarizes some of the 

 above data. 



Technological Improvements — Im- 

 portant advances have been made 

 in finding seeding materials other 

 than silver iodide and dry ice. Many 

 organic and inorganic materials have 

 been studied in the field and in the 

 laboratory. Several of the organics 

 have been found superior to silver 

 iodide in many respects, including 

 cost, and work is progressing on 



175 



