PART V — SEVERE STORMS 



stitute of the Hydrometeorological 

 Service in Nalchick, under the di- 

 rection of Sulakvelidze. This proj- 

 ect consists of hail-suppression ex- 

 peditions in the northern Caucasus, 

 Azerbaidjan, and Armenia. The 

 fourth is also in the Georgian S.S.R. 

 and is under the direction of Lomi- 

 nadze. Rockets are used in the first 

 two projects; guns are used exclu- 

 sively in the last two. The Ministry 

 of Agriculture furnishes the hardware 

 and crews for the field projects. 



Scientific Bases — All of these ef- 

 forts are based on the validity of the 

 relationship 



N ( N s ) 



R S = R 



where Rs is the mean-volume hail- 

 stone radius after seeding, 



Rn is the mean-volume hail- 

 stone radius without seed- 

 ing, 



Nn is the hailstone concentra- 

 tion without seeding, 



and Ns is the seeded hailstone con- 

 centration. 



A physical justification for the 

 validity of this relationship was given 

 by Iribarne and dePena and con- 

 firmed more recently by List and 

 Lozowski. The most important find- 

 ing of this theoretical work is that 

 the water content of a hail cloud 

 becomes effectively depleted by a 

 small number of hailstones, of the 

 order of 10 per cubic meter, so that 

 even modest artificial increases of 

 their concentration by two orders of 

 magnitude can be expected to de- 

 crease their size sufficiently to prevent 

 damage. It is this recognition that 

 brings hail-suppression experiments 

 into the realm of physical realization 

 and economic benefit. 



All experiments in the Soviet 

 Union seem to be designed in similar 

 fashion: hail forecast, radar analysis, 

 identification of the hail-spawning 



area in the cloud, and delivery of the 

 seeding agent into the hail cloud. 

 Forecasting skill has been developed 

 to the degree that special experiments 

 can be carried out to prevent the 

 development of impending hail, while 

 others are conducted to stop hail al- 

 ready falling. 



Reported Results — Soviet scien- 

 tists state that more than one million 

 hectares (3,900 square miles) were 

 protected in 1966. Hail damage in 

 the protected area was 3 to 5 times 

 smaller than in the unprotected area, 

 which means that the cost of pro- 

 tection amounts to barely 2 or 3 

 percent of the value of the crops 

 involved. For 1966, the total ex- 

 penditure for protection was 980,000 

 rubles, and the computed economic 

 effect was a saving of 24 million 

 rubles. 



Gaivoronskii and others have also 

 reported on hail-suppression experi- 

 ments in Moldavia, near the Bulgar- 

 ian eastern border. These experi- 

 ments utilize "Oblaka" rockets, a 

 type that has a caliber of 125 milli- 

 meters, weighs 33 kilograms, holds 

 3,200 grams of PbL- as a pyrotechnic 

 mixture, and delivers a total of 3 x 10 16 

 nuclei at -10° centigrade. Maximum 

 range and height are 12 and 9.5 

 kilometers, respectively. The authors 

 state that, in 1967, only 551 hectares 

 out of 100,000 hectares of crop were 

 damaged compared with 4,784 hec- 

 tares in the control area. A similar 

 effort with rockets is being carried 

 out by Kartsivadze. 



Evaluation — It appears from the 

 literature that the work in the Soviet 

 Union is already past the research 

 phase and well into the operational 

 stage. As tests in the research phase 

 were not randomized, however, a firm 

 statistical significance has not been 

 established. It is possible that the 

 discovery by Changnon of the oc- 

 currence of individual, short hail- 

 streaks rather than long hailswaths 

 may invalidate some of the conclu- 

 sions made by the experimenters. 

 Thus, a hailstreak may terminate by 

 itself, rather than as a result of the 



seeding action, before reaching the 

 boundary of the protected area, and 

 since there are no means of knowing 

 this beforehand such a case is counted 

 as a positive seeding result. These 

 conditions clearly point to the great 

 complexity of designing a randomized 

 experiment that would yield a unique 

 result in a relatively short time. 



There can be little doubt that the 

 basic approach of the Russian sci- 

 entists, to treat each hailstorm as an 

 individual case, is appealing; at the 

 least, it eliminates the great uncer- 

 tainty of the diffusional process from 

 surface generators to the storm. 



Hail Suppression: Switzerland 



The GROSSVERSUCH III hail-sup- 

 pression experiment was conducted in 

 Switzerland from 1957 to 1963 in the 

 Canton Ticino. The experimental 

 area appears to have been larger than 

 the canton, since generators and rain- 

 gauges were distributed over roughly 

 10,000 square kilometers, but the 

 size of the area instrumented with 

 24 surface Agl generators (type un- 

 specified) was only a minor part of 

 about 4,000 square kilometers, one- 

 half of which were in Italy. 



After many years of careful 

 freezing-nuclei measurements in 

 and downwind from Agl generator 

 sources it was concluded that, in 

 order to be effective, seeding from 

 the ground must be concentrated in 

 the regions and at the moment in 

 which storms form. It would appear, 

 however, that the analysis should 

 only be performed for the area coin- 

 ciding with the generator network. 

 Since this was not done, conclusions 

 reached in the experiment — to the 

 effect that "there is little doubt that 

 seeding has been very effective in in- 

 creasing the number of hail days" — 

 seem to be not entirely valid. 



Hail Suppression: France 



French efforts in operational hail 

 suppression are also continuing. Des- 



152 



