130 



NATURE 



[June ii, 1903 



The results arrived at by a study of the data secured 

 during the four winters of 1898-99 to 1901-2, inclusive, in 

 regard to the relative frequency of occurrences of the 

 various types and the apparent connection between size and 

 form and the air temperatures, agree in general with the 

 results arrived at by many other meteorologists and ob- 

 servers, both in Europe and America, as set forth in the 

 work " Schneekrystalle, " by Dr. G. Hellmann, Berlin, 

 1893. 



Doubtless the actual connection between forms and sizes 

 of snow crystals and the temperature and density of the 

 air is much more intimate than our present knowledge would 

 indicate, because our studies are based on air temperatures 

 at the earth's surface, instead of in the cloud strata where 

 the snow crystals form. The temperature may often be 

 mild at the earth's surface when the crystals are develop- 

 ing at high altitudes where the cold is intense, and such 

 crystals should be classed with those deposited during ex- 

 treme cold. 



Structure of Snow Crystals. — The beautiful details, the 

 lines, rods, flowery geometrical tracings and delicate sym- 

 metrically arranged shadings to be found within the interior 

 portions of most of the more compact tabular crystals, and 



in less degree within the more open ones, have attracted 

 the attention of nearly all observers who have studied snow 

 crystals. That these interior details more or less perfectly 

 outline preexisting forms must have been early recognised, 

 yet the knowledge as to what they actually were reniained 

 long in obscurity, and a complete explanation of all of 

 them is yet to be found. The investigations of Drs. Norden- 

 skiold and G. Hellmann enable us to form a general con- 

 ception as to their true character. These observers dis- 

 covered that many of the lines, rods, and other configur- 

 ations within the crystals, that add so much to the beauty 

 of the forms, and which are so plainly revealed in the 

 photomicrographs, are due to minute inclusions of air. 

 This included air prevents a complete joining of the water 

 molecules ; the walls of the. resultant air tubes cause the 

 absorption and refraction of a part of the rays of light 

 entering the crystal ; hence, those portions appear darker 

 by transmitted light than do the other portions. The 

 softer and broader interior shadings may perhaps also be 

 due, in whole or part, to the same cause, but if so, the 

 corresponding inclusions of air must necessarily be much 

 more attenuated and more widely diffused than in the former 

 cases. We can only conjecture as to the manner in which 



NO. 1754, VOL. 681 



these minute air tubes and blisters are formed. It may 

 well be that some of them are the result of a sudden and 

 simultaneous rushing together of water molecules around 

 the crystal from all sides. This might result in the form- 

 ation of closely contiguous parallel ledges, or laterally pro- 

 jecting outgrowths that are separated from each other 

 during the initial impact by a narrow groove, or air space, 

 but are soon bridged over by subsequent growth. Similar 

 contiguous parallel growths occur frequently around the 

 angles of very short columnar forms, and lend plausibility 

 to this theory. Air spaces also exist within columnar 

 forms, as noted by Hellmann and Nordenskiold. They 

 seem to occur within such forms as hollow cup-like ex- 

 tensions, projecting perpendicularly within them from each 

 of the ends of the crystals. 



Modifications of Forms of Snow Crystals. — By close study 

 of the photomicrographs, we find that the most common 

 form outlined within the nuclear portions of the crystals 

 is a simple star of six rays, a solid hexagon, and a circle. 

 The subsequent additions assume a bewildering variety of 

 shapes, each of which usually differs widely from the one 

 that preceded it, and from the primitive nuclear form at its 

 centre. Bearing in mind, however, the tendency of the 

 crystals evolved within the upper clouds toward solidity, 

 and the tendency of those from the lower clouds to form 

 more branching open crystals, our task of deciphering the 



Fig. 2. — Nos. 730-7^8. 



hieroglyphics, and of tracing thereby the probable flights 

 of each individual crystal within the clouds, becomes much 

 easier than might be anticipated. 



Taking photomicrograph No. 821 as an example, we can 

 picture with some certainty its various flights within the 

 clouds during each stage of its growth. Star-shaped at 

 birth, it was probably carried upward by ascending air 

 currents, and at some upper level assumed the solid 

 hexagonal form that we see outlined around the star-shaped 

 nucleus. Having now become heavier, it probably de- 

 scended, and acquired further growth at some lower level, 

 such as that wherein it had its birth. 



Modifications of Forms due to other Causes. — As it is 

 generally conceded that winds play an important part in 

 modifying the forms of snow crystals, let us consider the 

 probable manner in which they operate to accomplish this. 



Aside from causing modifications by wafting the crystals 

 upward and downward within the clouds to regions varying 

 in temperature, humidity, density, &c., as previously noted, 

 the winds probably cause modifications in other ways. 

 Violent winds may prevent a perfect and orderly joining of 

 the aqueous molecules, causing imperfections in the forms, 

 or perhaps amorphous, granular aggregations. 



Again, they may waft greater quantities of water mole- 

 cules to one or more portions of a growing crystal, causing 

 abnormal growth to take place around such portions. 



More important still, violent vvinds often cause fractures 



