FCBBUABY, 1902.] 



KNOWLEDGE. 



43 



regions it is so advantageous to reach. Probably at the 

 end of its long journey the incipient hailstone will be 

 far up in one of the cirrus clouds, surrounded by particles 

 of moisture frozen by the cold rarefied air into ice 

 crvstAls. so that in its "new situation the hailstone woiild 

 find ample supplies of the material so necessary for its 

 growth. 



In such company it is not long before the moisture 

 on tho atom of dust also freezes. The form which the 

 frozen moisture will take depends on circumstances, but 

 there are many possibilities before it. Thus it may 

 crystallize as a" tiny pellet of snow, or it may take the 

 shape of an ice crvstal. or It may commence as a snow- 

 tlake : while in certain circumstances it will simply take 

 the form of a frozen rain-drop. Any of these shapes 

 will sen-e as an excellent starting point from which to 

 commence the earthward journey. 



During all the time of its upward joiuncy the force 

 of gravitation has been steadily pulling at the rising 

 atom of dust and its load of "moisture. Few things 

 floating in the air can long resist this imperative call 

 to return to the earth. 



Falling slowly downwards, the motion being slow at 

 first because the bulk of frozeu moisture is small, the 

 hailstone at once commences to attract to itself other 

 particles of frozen moisture. These adhere to it much 

 in the same way that snow-flakes will adhere to anyone 

 travelling quickly through a snow stonn ; so that as 

 tho hailstone pushes its way downwards it grows in 

 bulk. Moreover, as its weight increases it may happen 

 that its centre of gravity shifts, and it becomes accord- 

 ingly of an irregular "shape. This accident indeed 

 accounts for many of the curious shapes assumed by 

 hailstones and gives them that peg-top shape which is 

 so often observed. It is to be remembered also that a 

 hailstone takes a long time to drop from the clouds to 

 tlie earth, it being calculated that the journey may often 

 occupy ten minutes. In this interval most of the trans- 

 formations occur that produce the full-grown hailstone. 



Imagining now the journey to be well started it will 

 at once be realised that the travelling hailstone will pass 

 through strata of air that differ vei-y much as regards 

 temperature and moisture. Some of the air will be 

 above the freezing point and other layers will be below 

 it : while it will be no uncommon episode for the 

 dropping hailstone to plunge sheer through a cloud that 

 may be many thousands of feet thick. The hailstone 

 itself, with its heart of ice, is always below the freezing 

 point, so that any moisture that settles on it is promptly 

 frozen and forms a girdle of ice around the central 

 nucleus. An examination indeed of any hailstone shows 

 that these icy girdles are its most characteristic feature. 

 It will also be observed that these girdles or zones are 

 of two kinds, and that they are alternately clear and 

 opaque. It is these zones that tell the most concerning 

 the incidents of a wonderful journey, for they are pro- 

 duced by the different strata of air through which the 

 hailstone passed, each country, as it were, over which 

 the journey was made impressing its characteristics on 

 the flying traveller. 



When the hailstone passed through air that was below 

 the freezing point the moisture that settled upon it was 

 frozen in the form of a clear zone of ice ; while, on the 

 other hand, when the air and its contained moisture 

 were above the freezing point the girdle of ice was 

 opaque. 



A further importa.nt consideration as regards the hail- 

 stone is that moisture may often be reduced in tem- 

 perature below the freezing point without actually con- 



gealing. It is a common experiment thus to treat 

 moistiu-c, but it is always found that the slightest 

 agitation of this cooled liquid at once causes it to 

 crystallize. When, therefore, the hailstones come pelting 

 through air in this condition it will readily be under- 

 stood that the commotion produces a plentiful supply 

 of ice crystals, many of which are quickly annexed by 

 the hailstones, which are thereby greatly increased 

 in size. 



The foregoing are the most common conditions that 

 f;,vour the growth of a hailstone, and it will he concluded 

 that the essential conditions required ai-e layers of air 

 of different temperatures. Now it frequently happens 

 that hail accompanies a thundei-storm or a tornado ; 

 these two phenomena being veiy neai-ly related. In both 

 there is an atmospheric whirl, which, in the tornado, 

 produces a strong wind that is commonly of a destructive 

 chai-acter. If, then, a hailstone should "be going through 

 its evolutions in the neighboiu-hood of one of these 

 storms it stands a good chance of being whirled round 

 and round in the air, a process that may continue for 

 a considerable time. This violent treatment, however, 

 has the same effect as if the hailstone were falling down- 

 wards through the air, and the result is that it may be 

 cai-ried again and again through fii-sb a cold stratum of 

 air and then through a wann one. As already seen 

 these ai-e the very conditions that favour the growth of 

 hailstones, and hence it is that hail so commonly 

 accompanies thiuidei-stonns, toniadoes, and such like 

 atmospheric disturbances. 



The whirling of hailstones through the air cannot, 

 however, continue indefinitely, for presently they grow 

 so heavy that they fall in a rattling stream from the 

 edge of the cloud. Obsen'ation shows that hail showers 

 often pass across the countiy in parallel lines; but it 

 will be gathered that this is"owing, as described above, 

 to the fact that the stones arc ejected from the sides 

 of the storm cloud and not so much from its centre. 

 Hailstorms, as a rule, ai-e not of a very large area, and 

 are much longer than they are wide. The width is 

 regulated by the dimensions of the cloud, the length 

 being governed by the distance to which the internal 

 energy of the stonn lu-ges the stonn-cloud forward. 



Hail occui-s more frequently during the day than in 

 the night, and in summer than in winter. It also falls 

 more copiously over the land than over the sea, where 

 it is but rai-ely obsei-ved. Hail, indeed, is a turbulent 

 child, and it does best in those localities and at those 

 seasons when the atmosphere is in a variable mood. At 

 such times the cross cuiTents in the air produce those 

 eddies which are the most favourable for the growth of 

 storm-clouds, out of which leap the tornado and the 

 thundei-stomi. Plains also ai-e more often visited by 

 hail than mountainous regions, for here again the 

 atmosphere is more likely to be in an unstable condition 

 because such exposed ground often varies greatly in its 

 temperature. Hail, moreover, is rarely met with in the 

 Arctic regions, thunderstoi-ms being equally rare in this 

 locality. It is this circumstance, among others, that has 

 caused some people to give atmospheric electricity a 

 prominent position in relation to hail formation, and 

 more especially so because lightning and hailstorms 

 frequently occur together. Caution, however, is always 

 necessary when putting electricity forward as a cause, 

 for to do so is often to explain one mystery by another. 

 From what has been said it will be gathered that thei-e 

 are simpler explanations of the flight of a hailstone, and 

 it is along these more obvious lines that the history of 

 this interesting phenomenon is nowadays being studied. 



