The various types of snow crystals do not occur with equal frequency, or form an equal bulk of the total snowfall. The tabular crys tals, and especially those of an open, branchy structure, appear the most frequently, and the others in the following order: tabular crystals possessing solid nuclei; granular snow, minute irregular sub-crystalline varieties, solid tabular hexagons, columnar crystals, compound crys tals.
The columnar crystals vary much less than do the tabular ones; such variations as occur being chiefly those relating to their dimensions, rather than their structure or aspect. The com pound crystals exhibit a much greater variety of form and structure. The connecting bar varies greatly in both length and thickness in the same and in different snowfalls. The tab ular crystals attached to one or both ends of the columnar bar also vary greatly at different times, both as regards size, outline and struc ture, and these variations often apply to the tabular crystals attached to the same bar. Com monly, whenever the connecting bar is ex tremely short, one of the tabular crystals at tached to its ends will greatly exceed the other in size, and as a result the compound crystal (resembling a cuff-button in shape) is practi cally converted, for purposes of descent, into a tiny parachute, the minor tabular crystal as suming the role of parachutist; and in this order the crystals fall through the air and alight upon the earth.
The most important division into which snow-crystals are divided is the tabular class, and those grouped under this head occur the most frequently exhibit the greatest beauty and diversity of form, and form by far the larger part of the total snowfall. The varia tions of outline, structure, internal ornamenta tion, etc., of the individual crystals of this class, both in the same and in different snow falls, are all but infinite. No written descrip tion can adequately portray the wondrous beauty, complexity and perfect symmetry of many of them; it far transcends that of the crystals of any other mineral species. They occur in three principal subdivisions, and the frequency of the occurrence of each is in the following order: of a wholly open structure anchy); (2) those possessing nuclei of a close structure surrounded by growths of an open nature; (3) those whose structure is wholly close (solid). Owing to various un
favorable conditions, as violent winds and the collisions and fractures due to these, develop ment proceeding while they are in motion (as in falling) or while they are in close juxta position, and to other unknown causes, the majority of the crystals of snow fail to attain perfect symmetry in all their parts. Commonly the nuclei outline a minute star, a hexagon, or a circle. The number of snow-storms deposit ing large numbers of beautiful and symmetrical crystals at a given locality, in the northern part of the United States, during any one winter, does not usually exceed 16, and may not exceed eight. The great majority of the more perfect and beautiful crystals usually emanate from the clouds of the western, southwestern or north western segments of great storms. In general, similar types of crystals occur in most storms, within corresponding segments of each, due to some law of general distribution. Each cloud stratum, the high, the low and the interme diate, and the temperatures, air-pressures, etc., corresponding with each, seem to favor the for mation of a certain type of crystal; the low clouds give large branching crystals; the high clouds, small, compact ones; the intermediate clouds, medium-sized crystals, in form a com posite of the low and high types.
The feature of chief interest and importance concerning the internal structure of the crystals is the occurrence therein of minute inclusions of air and air-tubes. These appear by trans mitted light as dark lines or shadings, etc., be cause of the absorption, interference, or refrac tion of the rays of light that strike or pass through them. The beautiful and usually sym metrical arrangement of these air-tubes and compartments impart to this type of crystal its principal charm. These air-inclusions are of great interest, because of the great number of distinct and varied inclusions that often appear within the same and within different individual crystals, and also because it may be assumed that they were not formed in regular progress ive order, but in intermittent order from the nuclei outward, while each new and distinct outgrowth was being added to and arranged upon and around the growing crystals; and hence they outline the boundaries of each of the many pre-existing shapes the crystals suc cessively assumed in cloudland.