356 



SCIENCE 



[N. S. Vol. XLIV. No. 1132 



show the favorable reaction of suoh conditions 

 upon the prices of cereals and also of railroad 

 and other stocks. Thundershowers break our 

 s umm er droughts, cleanse our dusty air, re- 

 fresh our parched earth, replenish our failing 

 streams and brooks, bring us cool evenings 

 and nights after sultry and oppressive days." 



Hail consists of particles of ice from the 

 size of peas to that of oranges or larger which 

 fall from the clouds. True hail, which is 

 usually a summer phenomenon, and is charac- 

 terized generally by a central core of cloudy 

 ice surrounded by one or more layers of clear 

 ice, should not be confused with the small ice 

 pellets of winter. Hail rarely occurs without 

 a thunderstorm, of which hail may be said to 

 be a violent manifestation. Thus the distri- 

 bution of hail is limited, and patchy, falling 

 sometimes on parallel strips of land in the same 

 thunderstorm. As hail is an accompaniment 

 of thunderstorms, it occurs in the warm south- 

 east quadrant of a cyclone, or associated with 

 the over- and under-running winds on a wind- 

 shift line. For example, the passage of a 

 wind-shift line over the region from Illinois 

 to Maryland, June 20 to 22, 1915, was ac- 

 companied locally by very large hail : " tea- 

 cups " in Illinois and " baseballs " in Mary- 

 land. 7 The annual and diurnal periods of hail 

 occurrence are much the same as those of 

 thunderstorms, although more marked. Thus, 

 in the United States, the month with most 

 hail is May, and the time of day, mid-after- 

 noon ; while least hail falls in winter and in the 

 early morning. In distribution over the earth, 

 there is least hail in the polar regions where 

 the air seldom has sufficient moisture or is 

 sufficiently unstable to satisfy hail require- 

 ments. On the other hand, near the equator 

 at sea-level hail rarely occurs because the 

 freezing level is too high and the lower air too 

 warm to permit hailstones to reach the earth's 



7 See O. L. Fassig, Monthly Weather Bev., Sep- 

 tember, 1915, pp. 446-448; and " Climatological 

 Data for the United States by Sections," Vol. 2, 

 June, 1915, Illinois, Indiana, West Virginia and 

 Maryland sections. 



surface. Hail may fall on the ocean with 

 the passage of a wind-shift Hue. Its greatest 

 development comes in the subtropical deserts: 

 there the most frightful hailstorms occur — 

 storms in which men and beasts not killed out- 

 right may be frozen to death under the hail. 



The annual and diurnal periods and the lo- 

 cal distribution just mentioned are easily ex- 

 plained as follows : the moister and the warmer 

 the lower air, and the colder the upper air, the 

 faster and the farther will the warm air rise; 

 and the greater is the opportunity for hail 

 formation. The moisture comes from the 

 lower air, the cold from the expansion of this 

 air on rising. For instance, a mass of air at 

 30° C. and with a relative humidity of 50 per 

 cent, will reach 0° at 4.8 kilometers, and — 20° 

 at 7.9 kilometers' altitude, mixing being disre- 

 garded. Hail clouds frequently tower 8 or 10 

 kilometers above the earth's surface. Appar- 

 ently, hail originates when snow crystals be- 

 gin to form among raindrops, which are 

 usually carried up into the level where the 

 temperature is below freezing. A snow-flake 

 and an undercooled drop freeze together into 

 the opaque ice that forms the core of a hail- 

 stone. As this ice particle falls through the 

 layer of undercooled raindrops, which may be 

 3 to 4 kilometers thick, a layer of ice is added. 

 Then it may be caught in one of the tornadic 

 whirls, which evidently occur within thunder- 

 storms, and carried aloft. On being released, 

 perhaps near the top of the cloud, it may ac- 

 cumulate another layer of ice on the way 

 down. This cycle may be repeated many 

 times. Finally, when too heavy to be held in 

 the uprushing currents, or when the whirl col- 

 lapses, the hail, congealing moisture on its 

 cold surface as it falls, may descend to earth. 

 For example, some of the larger hailstones, 3 

 to 4 inches in diameter, falling in Annapolis, 

 June 22, 1915, had 20 to 25 layers of ice. The 

 hail was of diverse shapes. 8 That hail must 

 return to the upper part of the cloud after 

 having grown to a considerable size is evident 

 from the temperatures of — 5 to — 15° C. ob- 

 served in hailstones. Hail does not occur in 

 spite of hot weather, but because of the heat. 9 



s See Fassig, ibid. 



