Snow Crystals 



37 



Perfect crystals are frequently covered 

 over and lines of beauty obliterated by 

 such granular coatings as are shown 

 in 807. Such heavy granular-covered 

 crystals possess great interest for many 

 reasons : They show when the char- 

 acter of the snow is due to the aggre- 

 gation of relatively coarse cloud par- 

 ticles or minute rain drops, and not to 

 the aggregation of the much smaller 

 molecules of water presumably floating 

 freely about between them. They also 

 offer a complete explanation of the for- 

 mation and growth of the very large rain 

 drops that often fall from thunder clouds 

 and other rain storms, if we accept the 

 conclusion that such large drops result 

 from the melting or merging together 

 of one or more of the large granular 

 crystals. 



While most granular forms possess 

 true crystallic nuclei, there is reason to 

 suppose that they sometimes form di- 

 rectly from the particles of cloud or 

 mist. 



HOW THE DEvSIGNS ORIGINATE 



The beautiful details, the lines, rods, 

 flowery geometrical tracings, and deli- 

 cate symmetrically arranged shadings to 

 be found within the interior portions of 

 most of the more compact tabular crys- 

 tals, and in less degree within the more 

 open ones, 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 por- 

 tions. The softer and broader interior 

 shadings may perhaps also be due, in 

 whole or in part, to the same cause, but 

 if so, the corresponding inclusions of air 

 must necessarily be much more attenu- 

 ated and more widely diffused than in 

 the former cases. We can only conjec- 

 ture as to the .manner in which these 

 minute air tubes and blisters are formed. 



As no one can ever actually see the 

 extremely minute water particles rush 

 together and form themselves into snow 

 crystals, the material and the manner 

 in which the molecules of water are 

 joined to form snow crystals is largely 

 a matter of speculation. While it is 

 true that the snow crystals form within 

 the clouds, it does not therefore follow 

 that they are formed from the coarse par- 

 ticles of which the clouds are composed 

 in cold weather. 



We have good grounds for assuming 

 that the true snow crystals are formed 

 directly from the minute invisible atoms 

 or molecules of water in the air, and not 

 from the coarse particles in the clouds, 

 as it is unlikely that these coarse par- 

 ticles could unite into snow crystals in 

 so perfect a manner as to leave no trace 

 of their union even when examined 

 under powerful microscopes. 



THE U. S. WEATHER BUREAU* 



By Hon. James Wilson, Secretary of Agriculture 



THE year i902-'o3 marks a dis- 

 tinct advance by the Weather 

 Bureau in the science of me- 

 teorology, especially in two directions. 

 From the beginning of the weather fore- 



casts of the government, in 1871, the 

 necessary observations at the several 

 stations have always been reduced to the 

 sea-level plane. It was conceived some 

 years ago that the numerous defects in 



* From Hit- Report of the Secretary of Agriculture, Hon. James Wilson, for 1903. 



