HURRICANES 



available from previous storms to 

 provide a clear definition of the nat- 

 ural variability of the parameter. 

 These points can be illustrated by 

 discussing the various measurements 

 that should be made. 



The following are either assumed 

 by the modification hypothesis or are 

 implied by results from the modeling 

 experiments: 



1. In hurricane clouds, large quan- 

 tities of water substance exist 

 in the liquid state at tempera- 

 tures lower than —4° centi- 

 grade. 



2. Introduction of silver iodide 

 crystals into these supercooled 

 clouds will cause the water 

 droplets to freeze and release 

 the latent heat of fusion. 



3. If the heat is released in the 

 annulus radially outward from 

 the mass of relatively warm air 

 in the center of the storm, it 

 should cause a temperature 

 change that will cause a reduc- 

 tion in the maximum tempera- 

 ture gradients in the hurricane. 



4. A reduction in the mean tem- 

 perature gradients must result 

 hydrostatically in a reduction 

 of the maximum pressure gradi- 

 ent in the storm. 



5. A reduction in the pressure 

 gradients should cause a reduc- 

 tion in the maximum winds in 

 the storm. 



6. The belt of maximum winds 

 should migrate outward after 

 the seeding has had time to 

 affect the storm. This action 

 presumably would be accom- 

 panied by development of a 

 larger eye, with the eye wall at 

 a larger radius, or, possibly, a 

 change in structure of the wall 

 cloud. 



All of the above suggest certain 

 measurements that should be made 



in the storm. If the changes in these 

 parameters occur at the right time, in 

 the right sequence, and with proper 

 magnitudes, the cumulative evidence 

 that the experiment was a success 

 could be very convincing. Efforts 

 were made to collect all of these data 

 in Debbie. In some cases, however, 

 the efforts were unsuccessful or the 

 data do not permit conclusive deduc- 

 tions. 



An aircraft was equipped to make 

 measurements of the character and 

 amount of water substance in the 

 lower levels of the supercooled layer 

 in Hurricane Debbie. While attempt- 

 ing to make the first pass across the 

 storm at the 20,000-foot level, a 

 supercharger malfunctioned and the 

 aircraft was no longer able to main- 

 tain that high an altitude. There are, 

 however, some qualitative observa- 

 tions which suggest there was a 

 change in character of the water 

 substance from predominantly super- 

 cooled water to a mixture of ice and 

 water. These observations are not 

 at the right level or of sufficient 

 detail and quality to document incon- 

 trovertibly that the seeding accom- 

 plished a major transformation in the 

 liquid-ice budget of the clouds. This 

 should not be interpreted to mean 

 that the seeding failed to accomplish 

 the desired effect, however. There 

 are just insufficient data to convince 

 a skeptic that the effect was actually 

 achieved. 



Very detailed and frequent obser- 

 vations of the temperature, pressure, 

 and winds were made along diameters 

 across the hurricane at the 12,000- 

 foot level. From these data we can 

 compute changes with time in the 

 parameters of their gradients at any 

 point along the diameter. The changes 

 in the maximum wind speed have 

 already been mentioned. 



The changes observed in tempera- 

 tures and temperature gradients are 

 not conclusive enough to support the 

 above hypotheses. On the other 

 hand, if the release of latent heat 

 was in the layers above 18,000 feet, 



one should not expect dramatic 

 changes in the temperature and its 

 gradient at 12,000 feet. We have in- 

 adequate temperature measurements 

 in the layer between 18,000 feet and 

 30,000 feet, since lack of properly 

 instrumented aircraft precluded the 

 acquisition of the type and quantity 

 of data needed. Furthermore, results 

 from the seeding simulation experi- 

 ment conducted with the model sug- 

 gested that the added heat is rapidly 

 dispersed and dramatic changes in 

 the temperatures are not likely to 

 occur. 



The changes in the pressure and 

 pressure gradients measured at 12,000 

 feet do give some support to the 

 success of the seeding and some indi- 

 cation that results conformed to the 

 hypothesis. But the great amount of 

 noise in the variations of this param- 

 eter and lack of adequate knowledge 

 concerning natural variations in hur- 

 ricanes make it impossible to say the 

 case is proved. Once again, the in- 

 dications are positive but inconclu- 

 sive. 



Intensive efforts were made to get 

 continuous coverage of the structure 

 of the storm by airborne radar and 

 by the ATS-3 satellite. This was 

 done with the hope that these data 

 would reveal the nature and time 

 of changes in the cloud structure 

 that might be caused by the seeding. 

 The radar pictures suggest that the 

 eye size did become larger after the 

 seedings; the changes in size even 

 appeared to have a periodicity sim- 

 ilar to that of the seedings: about an 

 hour and a quarter after several of 

 the seedings there was a rapid in- 

 crease in the area encompassed by 

 the wall cloud. 



One must be cautious, however, in 

 placing too much emphasis on this 

 evidence. The eye wall was pulsating 

 during most of the time the STORM- 

 FURY crews were monitoring it, so 

 there were many changes in size, 

 shape, and character of the eye be- 

 fore, during, and after the seeding. 

 There were also many problems with 



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