HAIL 



implies a basis for success in hail 

 prevention by cloud seeding through 

 the mechanism of drying out the re- 

 gion of the cloud in which hailstones 

 form. 



Although unresolved problems re- 

 main concerning the position of hail 

 growth with respect to the updraft 

 maximum and the liquid-water con- 

 centrations in hail-growth regions, 

 a picture is beginning to emerge of 

 a physically reasonable system for 

 hail growth and hail prevention that 

 is consistent with observations ob- 

 tained from field projects. 



Prevailing Scientific Controversy 



There is no general agreement on 

 the effectiveness of hail-prevention 

 techniques. Skepticism concerning the 

 claims of success in the Soviet Union 

 and concerning the reality of ap- 

 parent reductions in hail damage on 

 hail-suppression projects in this coun- 

 try loomed large in the development 

 of current plans for hailstorm re- 

 search in the United States. This is 

 illustrated by the following extract 

 from a planning document for the 

 National Hail Research Experiment 

 (NHRE): 



. . . This document i 

 tirely concerned with a discussion 

 of the need to complete success- 

 fully a Hail Suppression Test Pro- 

 gram, since it appears to us that 

 a National Hail Modification Pro- 

 gram is now premature. We must 

 first determine if hailstorms can 

 indeed be modified, and then learn 

 if it is worth the effort. 



This point of view (that so little 

 is known about hailstorms that the 

 primary hail research effort should be 

 so directed) is in conflict with the 

 point of view that current knowledge 



Figure V-13 — A MIDWEST THUNDERSTORM 



Temp Height 

 (°C) (kml 



-55 



-50- 



•10 



-40 



-30--8 



-20 



12 



10 



20-. 



-10- 



0--40 

 -37 



The figure shows a single, mature convective storm of the midwestern U.S. which 

 is apt to produce hailstones. A temperature and height scale are along the lefthand 

 margin. Note the base of the cloud at 3.7 kilometers. The vertical wind speed 

 profile is plotted over the cloud and indicates a maximum wind speed of 19 meters 

 per second near the middle level of the cloud. If the maximum speed of the updraft 

 exceeds the terminal velocity of the largest stable droplet, an accumulation zone 

 of supercooled water forms because of the chain-reaction mechanism triggered by 

 droplet breakup. The heavy line in the center section of the cloud is the 35-decibel 

 contour as seen by radar. The accumulation zone is within this area. It is this area 

 into which seeding material should be placed to be effective. 



155 



