234. 



KNOWLEDGE, 



[OCTOBEB, 1902. 



Now it is jirobablc that, as is tho case with a raindrop, or 

 with hail, in order to ^ive a siiowllakt> a stiirt in life tlicro 

 must he a tiny nucleus of dust, round whieli tiie condensinr; 

 vapour may pither. It is mainly a (juestiou of temperature 

 as to what form this condensinp; moisture will take, but 

 commonly when the temperature is above the freezinjj 

 )ioint rain is the outcome. When this process takes place 

 ill a body of air at or about the fr<>czing ])oiut, snow gets 

 its opportunity ; while when the condensed moisture does 

 not at once freeze solid, hail will bo more likely to occur. 

 At some times, indeed, l)oth snow and hail take the form 

 of little fluffy pellets of frozen moisture, and considerable 

 experience is necessary to distinguish between them. As 

 a general rule, the colder the weather the smaller the 

 snowflakes ; the large flakes, which children describe as 

 being due to the old woman plucking her geese, appearing 

 when the thermometer is not far away from the freezing 

 ])oint. Large flakes, indeed, are a conglomeration of 

 smaller flakes, and it is in the latter that the greatest 

 regularity and beauty of structure are to be seen. 



In order therefore that a snowflake may make a 

 successful journey through the atmosphere it should be 

 built up on a particle of dust, while if it should be 

 fortunate enough to commence its career at the top of a 

 cloud soaring many miles above the level of the earth, it 

 will thereby become still better equipped for adding to its 

 stores of frozen vapour. Between the growing snowflake 

 and the earth, it should be borne in mind, there are in 

 ordinary conditions strata of atmosphere that differ very 

 much as regards their temperature, and the amount of 

 moisture they contain. These different layers through 

 which the descending snowflake will pass favour its 

 development, for it often happens that in one layer of 

 atmosphere the flake gathers moisture which is promptly 

 frozen in the succeeding layer. In this connection it is 

 well to recall what happens when one holds a snowball, or 

 two pieces of melting ice in a warm hand for any length 

 of time, for either can be welded into a solid lump by a 

 little pressure, a process commonly called regelation, and 

 to be borne in mind when seeking for the causes that 

 favour the growth of a suowflake. 



Prom each layer of atmosphere through which it passes 

 the fluttering snowflake may therefore be thought of as 

 collecting a tribute of moisture, but unlike a hailstone it 

 makes these accretions in gentle fashion. There is a fuss 

 and a dash with the downward plunge of a hailstone, so 

 that the frozen moisture is welded around it with great 

 force and it quickly grows hard and solid. On the other 

 hand, with a snowflake the frozen moisture is not so much 

 welded as it is enmeshed, for on every snowflake, even in 

 its early moments, there are protuberances and spicules 

 that catch the floating moisture as in a tiny net. The 

 most common forms of snowflake have a solid nucleus 

 with rays ramified in different planes, others taking the 

 shape of six-sided needles or ])risms, or six-sided pyramids. 

 A complicated snowflake takes the form of a six-sided 

 ])rism from one or both ends of which six-sided plates are 

 projected. Another kind of snowflake is found to be 

 simply a thin lamina of frozen moisture, snowflakes of 

 this class Iteiug observed in great variety. Mauy inte- 

 resting sketches have been made of all these different 

 kinds of snowflakes, but this is work that requires further 

 elaboration by some observer willing to devote a little 

 time to this most interesting work of taking a picture of 

 the snowflakes as they reach the earth. It has been said 

 that the crystals in any given snowstorm have a family 

 likeness, each storm, as it were, having its own particular 

 type of snowflake. This is an interesting point to be 

 settled only by careful observation, and for the present, it 

 is enough to recognize the fact that although snowflakes 



seem all very much alike yet there is endless variety in 

 these " lovidy blossoms of the frost." 



It will be seen, then, that the conditions most favourable 

 for the production of large snowflakes are when the 

 atmosi)liere is freezing in some parts and thawing in 

 others. With those conditions the process of repelation of 

 moisture on the surface of the snowflake will proceed 

 apace. Under such favourable circumstances very large 

 flakes may be built up, although, as already mentioned, 

 these large structures are often but the result of flakes 

 that have collided in mid air and joined forces. These 

 large snowflakes are like very large hailstones, which are 

 often but a mass of ice formed by several hailstones 

 crushed together. Both as regards the snowflake and the 

 hailstones, these conglomerates are not properly to be 

 taken as showing to what size a single flake or stone may 

 grow. With this proviso it may be stated that one of these 

 conglomerate snowflakes was found to measure 3J inches in 

 length, 1 J inches in breadth, and H inches in thickness; the 

 flaki^ when melted yielded 2 J cubic inches of water. Such 

 large snowflakes as this cannot come to maturity when the 

 atmosphere is of a very low temperature all through. In 

 such circumstances there are no alternate layers of air of 

 varying conditions in temperature and moisture, and as a 

 result only small, dry flakes of snow are produced. This 

 is the kind of snow that falls in the polar regions, and it 

 is these cold weather snowflakes that are the most perfect 

 in form. Closely allied to the small and the large snow- 

 flakes is sleet. This commonly is objurgated as the most 

 unpleasant of all the children born of the atmosphere, but 

 it will perhaps be seen that rightly to understand the 

 whole story of a snowflake, something of the changes in 

 temperature that produce sleet need to be taken into 

 account. 



When lying on the grouad, snow, from a meteorological 

 point of view, is of much greater interest than when falling 

 through the air. In an ordinary way there is a constant 

 exchange of heat between the surface of the earth and the 

 atmosphere. Thus during the day the sun pours its 

 warmth down through the air to the earth, so that the 

 surface of the ground is raised in temperature. During 

 the night hours this acquired warmth is rapidly radiated 

 into space, and the temperature of the earth accordingly 

 falls. The atmosphere, moreover, that is everywhere in the 

 closest intimacy with the ground, is also affected by this 

 prodigal behaviour of the earth. Now, when the ground 

 is wrapped round in its mantle of snow, these imports and 

 exports of heat to and from the earth are interrupted. 

 In other words, the diurnal range of temperature is 

 greatly modified, so that all the time snow is on the 

 ground there is not that excessive expenditure of heat that 

 ordinarily takes place, and as a result the soil beneath the 

 snow IS maintained at an equable temperature. 



Anyone who has been on the snow a few thousand feet 

 above the level of the sea will have recognised the fact 

 that snow is a good radiator of heat. At such a height, 

 moreover, the atmosphere is dry and free from dust, so 

 that as the heat rays pass through the air, to and from 

 the surface of the snow, they have but little effect as 

 regards raising the temperature of the air. Air such as 

 this is said to be diathernianous, and heat rays passing 

 through such territory, so to si)eak, pay no toll. Similarly 

 snow, so long as it remains clean and free from impurities, 

 reflects the heat rays, but will not absorb them. Sup- 

 posing, however, that a little dirt or a plentiful sujiply of 

 coal-dust settles on such snow, heat is at once absorbed, 

 and the "frozen flowers" are destroyed. That the snow 

 is white is considered to be due to the fact that the ice 

 crystals of which each individual suowflake is built up, act 

 as so many miuiature prisms that blend the prismatic 



