i4o WORK OF SNOW AND ICE 



water solidifies from the vapor of the atmosphere, it takes the form of separate 

 crystals (Fig. 142). The flakes are rarely perfect, but they are always crystals. 

 Snow crystals may continue to grow so long as they are in the atmosphere; or if 

 the air is warm or dry they shrink, from melting or evaporation. When they reach 

 the ground, the processes of growth and shrinking continue, and the crystals 

 increase or decrease according to circumstances. 



A glacier is a colossal aggregation of crystals grown from snowflakes to granules 

 of greater size and more compact form. The microscopic study of snowflakes shows 

 how they change from flakes to granules. The slender points and angles of new- 

 fallen flakes melt and evaporate more than the central portions. The water 

 (and doubtless the water vapor) thus formed gathers about the centers of the flakes 

 and, if the temperature is right, freezes there. 



These are first steps toward the pronounced granulation of snow which has 

 lain long on the ground. Measured from day to day, the larger granules beneath 

 the surface of coarse-grained snow are found to be growing. When the temper- 

 ature of the atmosphere is above the melting-point, the growth is faster than when 

 the air is colder, but there is an increase in the average size of the granules, and a 

 decrease In their number, under all conditions of temperature. Part of the increase 

 of the larger granules appears to be at the expense of the smaller ones; part doubt- 

 less comes from the moisture of the atmosphere which penetrates the snow and 

 condenses there, and part from the descent of water due to surface melting. 



Deep beneath the surface of a large body of snow, the larger part of the growth 

 of the large granules is probably at the expense of the small ones. To understand 

 how this takes place, it should be noted that the free surface of every granule is 

 constantly throwing off particles of water-vapor (i. e., evaporating); that the rate 

 of evaporation increases with the sharpness of the curve of the surface, and that 

 the smaller the particles, the sharper the curve; that the surface of a granule is 

 liable to receive and retain molecules evaporated from other granules, and that, 

 other things being equal, the retention of particles is most common on surfaces 

 of least curvature. It follows that the larger granules of less curvature will lose 

 less and gain more, on the average, than the smaller granules. The result is that 

 the larger granules grow at the expense of the smaller. 



Another factor affecting the growth of granules is pressure and tension. The 

 granules are compressed at their points of contact, and under tension elsewhere. 

 Tension increases the tendency to evaporation, and the capillary spaces adjacent 

 to the points of contact probably favor condensation. Pressure reduces the 

 melting-point, while tension raises it. Though the effect of this is slight, it is to 

 be correlated with the much more important fact that compression produces heat 

 which may bring the temperatlire of the ice to the melting temperature at some 

 points, while tension may reduce it to or below freezing temperature at adjacent 

 points. There is therefore a tendency for the ice to melt at points of contact and 

 compression, and for the water so produced to refreezc. at adjacent points -where the 

 surfaces of granules are under tension. This process becomes effective beneath a 

 considerable body of snow, and here the granules gradually lose the spheroidal 

 form assumed in the early stages of granulation, and become irregular polyhedrons, 

 interlocked into a mass of more or less solid ice. 



Whether these processes furnish an adequate explanation of the changes or 

 not, all gradations may be observed from snowflakes to granular neve, and thence 

 to the granules of glacier ice, ranging in size up to that of walnuts, and even 



