316 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1960 



These cyclic zones may be produced by several different types of en- 

 vironments or processes. They could, for example, grow within and 

 at the edge of the main core of a very large convective column and 

 represent the fluctuating nature of the environment of such a com- 

 plex dynamic structure. In a mature storm such as produces the larger 

 hailstones, such columns must contain zones of subcooled cloud drop- 

 lets, mist and rain, small ice crystals, frozen raindrops, and wet snow- 

 flakes. 



A second situation, which might account for the variations observed, 

 would involve wet growth within the core of a convective column or 

 plume, followed by more rapid growth in a semiwet or dry condition as 

 the hailstone is carried outward but remains within the less turbulent 

 but colder zone of a horizontal ring vortex. The stone might be carried 

 completely around the vortex and then into the central core to become 

 again wet and denser as the porous structure becomes saturated with 

 water. Considerable field evidence is available to indicate that hail- 

 stones are carried outward in ring vortices, since they are often thrown 

 out of them and fall great distances through clear air. 



Another condition, which could also account for the observed growth 

 variations, would involve the successive development of convective 

 turrets pushing up into the area where the stones are growing, each 

 in turn having stronger vertical velocities and thus providing the in- 

 creased energy necessary to keep the growing stone suspended within 

 the cloud and at levels colder than 0° C. 



It is likely that all three of these conditions, as well as other com- 

 plex mechanisms, are underway simultaneously or going on in rapid 

 succession within the region where the stones are growing. The con- 

 sidera})le variation of growth patterns found in large hailstones from 

 a single storm, as shown in plates 7 and 8, is the strongest evidence 

 that this is probably the case. Larger stones do not fall from simple 

 convective cumulus. They invariably occur under conditions that in- 

 volve either an intense zone of large-scale convergence, as was the 

 case with the Grand Island storm, or the development of the giant, 

 isolated cumulonimbus that often occur east of the front range of the 

 Eockies downwind of some of the large mountain peaks initiating 

 them. 



4. FACTORS THAT CAUSE SUCH STORMS IN THE WESTERN GREAT PLAINS 



Easley [7] has studied and enumerated the various factors responsi- 

 ble for the intense hail incidence in the northeastern Colorado- 

 Nebraska-Kansas area of the Great Plains. These include proximity 

 of the mountains to the west, the ground elevation that is greater than 

 4,000 feet above sea level, the relatively low level of the freezing 

 isotherm, the quasi-stationary trough to the lee of the mountains, the 



