The structural formation of snow crystals is generally found to be of hexagonal shape, usually six-cornered or pointed, although rare types have been discovered and photographed where such was not the case, as the trigonal crystals shown. Snow crystals have been classified, as to structural formation, into two types; the tubular and columnar. The columnar types are formed of long, slender, needle-like crystals or columns, usually tapering at one end, while the tubular crystals are developed upon an extremely thin tubular plane. Frequently we find that two types have united, thus forming the "compound" crystal, which is rare, and frequently a very beautiful, showy snow jewel.
The tubular crystals are of more common occurrence and exhibit greater beauty and diversity of outline than the plain columnar types.
The internal formation and design of the snow crystal is of great importance and interest, and the delicately etched markings which occur upon their surfaces, and are so well brought out in the photo-micrographic illustrations, are due to certain minute air inclusions or small air tubes. When the light falls upon the crystal, these air tubes appear as dark tracings or lines and shadings and go to form and carry out the design of each individual crystal.
[Illustration: 63. Solid, big storm type]
[Illustration: 64. A very symmetrical crystal]
[Illustration: 65. High altitude crystal]
[Illustration: 66. Freak crystal formed by broken sections uniting]
During great snowstorms the winds within such storms blowing spirally inward toward the storm centre near the earth, and at the same time upward and outward, above, exert the vast powers of bringing together the material, the water vapours, which in conjunction with the icy breath of the raging blizzard, perfects the formation of the snow crystal.
Far above the clouds, in the vast silences of space, in thinnest air, supported solely by up-rushing winds, the little snow crystals form and multiply, embellished and enlarged by their continual warring contact with the elements, until at last they descend earthward.
Many of these beautiful crystals are doubtless great travellers, for they are frequently, when first generated in space, exceedingly light in formation, so much so that not until they have been buffeted about repeatedly by the Storm King, do they gain sufficiently in structure and in weight to descend. They are gradually built up and become heavier by the varying conditions of air pressure, degrees of humidity, aided also by electric currents.