HAILSTORMS AND HAILSTONES — SCHAEFER 345 



the Loomis School at Windsor, Conn. Subsequent detailed studies 

 have been made of these stones in the writer's laboratory. 



The stones show a variety of shapes and sizes although they have 

 certain features in common. All of them show banded structure, in- 

 dicating two or more changes in growth rate and/or environment. 

 These changes appear to the casual observer as concentric bands of 

 clear and cloudy ice, the so-called onionskin structure. Careful sec- 

 tioning and etching reveal structures quite similar to those described 

 and illustrated by List [6] and others. Typical stones are shown as 

 cross sections in plate 5. It will be noted that the general structure 

 of the stones is comprised of individual ice crystals of two sizes. The 

 large crystals range in size from 1 to 4 millimeters, the smaller ones 

 from 0.1 to 0.3 millimeter. This tenfold difference in size denotes 

 either large variations in growth rate, moisture availability, or a com- 

 bination of factors which caused the stones to grow in widely different 

 environments. 



3. COMMENTS ON POSSIBLE CAUSES OF OBSERVED FORMATIONS 



In examining the crystalline structure of many large hailstones, 

 it is noted that the smallest crystals are generally 10 to 30 times 

 larger than ordinary cloud droplets. This can mean several things. 

 The structures observed could be formed by the rapid freezing of 

 mistlike droplets as they contact the hail embryo to form a porous 

 graupel-like matrix. This porous, spongelike structure might then 

 soak up liquid water as the stone sweeps it up at a rate faster than 

 it can freeze upon contact. 



A somewhat similar mechanism may be postulated involving cirrus- 

 type ice crystals. In this case, however, water and ice crystals would 

 coexist, the size of the ice crystals and their packing determining the 

 final crystal sizes. Either of these processes could explain tlie uniform 

 bands of small crystals found in most large hailstones. 



The formation of the larger crystals may also be explained in at 

 least two ways. They could represent zones of slow, wet growth. The 

 crystals are often similar to those found in icicles or in hydromites, 

 both of which are formed by relatively slow development. Some hail- 

 stones are certainly formed in this manner, since the elongated crystals 

 observed as illustrated in plate 4, figure 2, could hardly be formed in 

 any other way. Where the large crystals tend to have a fairly uni- 

 form size, they might represent the accretion of large, frozen rain- 

 drops agglomerating on the surface as a slushlike mass that is freez- 

 ing at about the same rate as the water and ice mixture is being 

 deposited. 



One of the striking features of the crystalline pattern of large hail- 

 stones is the abrupt manner in which a change in crystal size occurs. 



