HAI! 



growth center. This embryo is conical 

 or spheroidal. It can be opaque or 

 clear ice. It is usually well recogniz- 

 able against the shell structure of the 

 remaining stone. 



One may conclude that the life 

 history of a hailstone can be organi- 

 cally subdivided into two major pe- 

 riods: (a) growth in a hail embryo 

 during the development cloud stage 

 of the hail cell, and (b) growth in a 

 hail shell during the mature-hail-cell 

 cloud stage. It is conceivable that 

 the former occurs during the de- 

 velopment phase of the cumulonim- 

 bus or hail cell, the latter when the 

 penetrative convection has been es- 

 tablished and a strong supporting 

 updraft has formed. 



Environmental Growth Condi- 

 tions — On the basis of List's theory 

 it is possible to derive four environ- 

 mental growth conditions from typi- 

 cal hailstone properties: 



1. It is unlikely that hailstones 

 are usually grown in the high 

 water content of an accumula- 

 tion level; if that were true, one 

 should observe soft, spongy 

 hailstones much more fre- 

 quently. 



2. It can be shown that hailstones 

 with many alternating layers 

 of clear and opaque ice may 

 have grown at high levels in 

 the cloud; at these levels, small 

 altitude variations cause large 

 variations of the growth con- 

 ditions. 



3. Hailstone structures that are 

 homogeneous over a large part 

 of the shell indicate that they 

 have grown in an updraft with 

 continuously increasing updraft 

 speed. 



4. The natural hailstone concen- 

 tration is of the order of 1 to 

 10 per cubic meter. This con- 

 centration effectively depletes 

 the cloud water content, as was 

 shown in 1960 by Iribarne 

 and dePena, which gives hope 



that hailstones could be made 

 smaller and less damaging 

 through a slight artificial in- 

 crease in the concentration of 

 about two orders of magnitude. 

 Amounts of seeding material 

 needed to accomplish this are 

 moderate. 



Hail-Suppression Experiments 



The problem of hail suppression 

 is economic as well as scientific. One 

 of the questions to be answered is: 

 Does agriculture suffer sufficiently 

 from hailstorms that prevention is 

 necessary? Some people believe that, 

 as long as we have a farm surplus 

 and pay farmers for not planting 

 certain crops, we do not need hail 

 suppression. While this may be true 

 now, in coming years we may need 

 every bushel of farm crop for our 

 food supply. This appears to be a 

 good time, therefore, to begin a hail- 

 suppression research program. Re- 

 search must be emphasized, since 

 too little is known about the hail 

 mechanism to permit a realistic hail- 

 suppression program to be conceived. 

 Also, little is known about the rela- 

 tive damage that is done by hail, 

 water, and wind during a storm. 



The research phase need not be 

 completed, however, before modifica- 

 tion experiments can be thought of. 

 On the contrary, the problem should 

 be considered as a field program in 

 experimental meteorology, where a 

 well-conceived experiment with hail 

 clouds is carried out with the poten- 

 tial of observing a cause-and-effect 

 relationship. Some hail clouds are 

 more suited to such an experiment 

 than others; for example, hail clouds 

 growing from the rear edge should 

 have a basically simpler structure 

 than hail clouds that grow from the 

 leading edge. Such clouds are also 

 easier to observe, as they are not 

 usually obscured by an overhanging 

 anvil. 



The National Hail Research Ex- 

 periment (NHRE) attempts to ac- 



complish exactly this ba 

 research objectives and suppression 

 operations — namely, to use aircraft, 

 radar, and surface networks for a 

 thorough study of the hailstorm 

 simultaneously with a well-designed 

 aircraft seeding program to which the 

 storm's reaction is observable. The 

 latter program cannot be conducted 

 entirely without statistical control. 



Hail Suppression: Soviet Union 



Much information has been ob- 

 tained from the operational hail- 

 suppression experiments in the Soviet 

 Union, specifically in the Caucasus. 

 Several books have been published, 

 and exchange visits between Soviet, 

 American, and Canadian scientists 

 have taken place, with many fruitful 

 discussions, although it has not been 

 possible to obtain a clear appraisal 

 of the validity of the claims made 

 by Soviet scientists. 



It appears that two major efforts 

 are under way in the Soviet Union 

 which differ basically in the means of 

 delivering the seeding agent into the 

 cloud. In one, guns and shells are 

 used; in the other, rockets. While the 

 guns have greater range and altitude 

 and deliver 100 to 200 grams of the 

 seeding agent (Agl or Pbln) by ex- 

 plosion of the "warhead," the rockets 

 can carry a larger amount of the 

 agent and deliver by burning a pyro- 

 technic mixture (3.2 kg). The rockets 

 are somewhat more versatile in de- 

 livery either on a ballistic curve 

 through the storm or vertically inside 

 the cloud when descending by para- 

 chute. 



One of four current projects in 

 the Soviet Union is carried out 

 through the Academy of Sciences of 

 the Georgian S.S.R. in the Alazani 

 Valley of the Caucasus, with Kart- 

 sivadze as the chief scientist. Another 

 is conducted by the Hydromete- 

 orological Service in Moldavia by 

 Gaivoronskii and others. The third, 

 and largest, project seems to be 

 conducted by the High Altitude In- 



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