PART VI — PRECIPITATION AND REGIONAL WEATHER PHENOMENA 



One important factor in determin- 

 ing whether or not a given weather 

 situation is treatable is the number of 

 natural nuclei. Nuclei are needed to 

 convert vapor into liquid; other nuclei 

 are needed to convert liquid into ice. 

 The presence of ice crystals is con- 

 sidered critical to precipitation for- 

 mation in most clouds that occur in 

 the middle latitudes. If liquid drop- 

 lets are present at temperatures below 

 freezing, a nuclei deficit is implied. 

 Such a deficit in an otherwise suitable 

 cloud can be overcome by the addi- 

 tion of artificial nuclei. The addi- 

 tional nuclei will convert some of the 

 droplets into ice crystals, which will 

 grow at the expense of the liquid 

 droplets until they are large enough 

 to fall out, thereby initiating or in- 

 creasing precipitation. There are few 

 routine observations of natural nuclei 

 numbers, and most counts are made 

 at the surface, not aloft where the 

 clouds are. We have only rather 

 crude notions of how many nuclei 

 are needed in any given situation. 



Some of the other factors of im- 

 portance in the treatability of a 

 weather system are temperature 

 structure, wind, liquid water content 

 of the cloud, and cloud-droplet size 

 spectra. Again, we have fairly good 

 qualitative understanding of the role 

 of each factor, but we do not com- 

 pletely understand all the links in the 

 physical chain of events leading to 

 the desired result of the modification 

 attempt. In addition, some of the 

 pertinent factors are difficult to meas- 

 ure. Still other factors may be im- 

 portant in cloud treatability, but our 

 knowledge of them in real cloud 

 situations is too meager even for 

 qualitative statements. 



In some situations, theory and em- 

 pirical evidence have been united in 

 mathematical models. These models 

 simulate the atmosphere and can 

 predict the response of the cloud to 

 a given treatment. While the models 

 available today are comparatively 

 crude, they play a valuable role in 

 enabling scientists to recognize treat- 

 able situations. 



Treatment — After a situation is 

 identified as treatable, the appropriate 

 materials and techniques must be 

 chosen. The most frequently used 

 materials for weather-modification ac- 

 tivities are Agl and dry ice, but many 

 other substances have been used ex- 

 perimentally (salt, lead iodide, cal- 

 cium chloride, and a host of organics 

 including metaldehyde, phlorogluci- 

 nol, urea, and 1,5-dihydroxynaph- 

 thalene). The temperature at which 

 each of these agents becomes effective 

 is fairly well known (see Figure VI-4), 

 as is the particle-size requirement (for 

 Agl, on the order of 0.1 micron). 



Clouds can be classified into two 

 categories, cold and warm. Cold 

 clouds are those with temperatures 

 wholly or partly at or below 0° cen- 

 tigrade. Warm clouds are those ev- 

 erywhere warmer than U centigrade. 

 Materials that affect cold clouds 

 rarely have any effect on warm clouds. 

 Thus, the treatment material must 

 be matched to the situation. The 

 object is to change the size and/or 



state of the cloud particles. Precipi- 

 tation from warm clouds can be in- 

 creased if the small droplets can be 

 turned into big droplets. 



Hygroscopic materials should be 

 effective in warm clouds. They are, 

 in fact, being used experimentally, 

 though it has proved difficult both to 

 get the material ground to a small 

 enough size to stay in the cloud long 

 enough to be effective and to keep the 

 particles dry until they are released 

 to the atmosphere. Once a few drop- 

 lets large enough to begin to fall are 

 formed, coalescence should keep the 

 process going until precipitation falls 

 out of the cloud. 



Hygroscopic materials should also 

 be effective in cold clouds, but mate- 

 rials that initiate a phase change are 

 more efficient. Some cold-cloud 

 agents, such as dry ice, simply cool 

 the air and the vapor and liquid in 

 it to a temperature at which tiny ice 

 crystals form spontaneously. This 

 process is effective at air tempera- 

 tures a few degrees below freezing 



Figure VI-4 — TEMPERATURE DEPENDENCE OF NUCLEATING AGENTS 



The table lists some of the more prominent substances that are used as nucleating 

 agents and the temperature at which they become effective as nuclei. 



174 



