Popular Science Monthly/Volume 53/May 1898/A Study of Snow Crystals
By W. A. BENTLEY and G. H. PERKINS.
MANY have admired snowflakes as they observed their exquisite outlines and varied forms, but few have ever given them careful study or distinguished the crystals of which a Hake is usually composed.
Extended examination of snow crystals has hitherto been very difficult because, except in a very uncomfortable atmosphere, the delicate structures speedily disappeared, and their outlines could be preserved for study and comparison only by the aid of skillfully executed drawings. Even these must often be hastily made, and could show little of the internal structure which is so important a feature of most snow crystals.Now, however, by any of the usual combinations of microscope and camera, these crystals can be easily and quickly photographed, and far more satisfactory representations obtained than were possible formerly. The term snow crystal is used because a snowflake may be a crystal or it may be, and usually is, a cluster of crystals.
The illustrations which accompany this article are all of them photomicrographs taken directly from crystals which were collected in northern Vermont during the past fifteen years. They are selected from over five hundred different forms. A close and minute study of many of them will reveal beauty and complexity of structure not seen by a casual observer. The methods employed in obtaining the illustrations have been very simple. It has been found that any apparatus which can be used for taking photomicrographs will serve to photograph snow crystals, but the microscope should be fitted with half-inch or two-thirds-inch objectives, of wide aperture and short axis; the focusing arrangement must work (juickly and accurately; the diaphragm aperture be small, not more than one-sixteenth inch; the illumination ordinary, uncondensed daylight; the exposure, rapid plates being used, from forty seconds to three hundred, as the light is strong or weak, camera bellows closed or elongated, etc. A black card placed between two pins which project from the stage on each side of the objective serves to exclude unwelcome light when the slides of the plate-holder are changed. Great care is necessary to prevent the crystals from melting, as this is one of the chief difficulties which must be overcome. On this account the observer must not breathe upon his slides, nor liandle them except with gloved hands. The whole work must be done in a cold room, with but one unscreened window. Crystals may be collected as they fall, upon a black card, and transferred by a bit of broom splint to a glass slide upon which they may be pressed flat with a feather.
Careful examination of the illustrations will soon convince one that, great as is the charm of outline, the internal ornamentation of snow crystals is far more wonderful and varied. Many of the specimens, we might almost say all of them, exhibit in their interior most fascinating arrangements of loops, lines, dots, and other figures in endless variety. So far as is known to the writer, these illustrations are the first which have been published that show in any adequate manner these interior figures, and surely they add greatly to our interest and delight as we study snow crystals. So varied are these figures that, although it is not difficult to find two or more crystals which are nearly if not quite the same in outline, it is almost impossible to find two which correspond exactly in their interior figures.
It is asserted by some observers that many of the lines or rods seen in the interior of snow crystals are really tubes filled with air.Perfect crystals are by no means always common in snow storms, most of the forms produced being more or less unsymmetrical or otherwise imperfect. It rarely happens that during a single winter there are more than a dozen good opportunities for securing
complete crystals, and there may not be half so many. The greater number of perfect crystals is found in widespread storms, or blizzards, while the local storms produce most often granular or imperfect forms. So marked is this distinction that very often the character and extent of a storm may be in general determined by an examination of the crystalline forms obtained. Extensive storms produce smaller crystals, more uniform in size, less clustered in flakes, and in greater variety than local storms. Figs. 1 to 20, inclusive, are crystals from general storms, while Figs. 23 and 24 are those of local storms. When the temperature is very low while a local storm is raging, its crystals resemble those of the blizzard more closely.
Some forms are common to both classes of storms. Probably because identical conditions do not occur frequently, the crystalline forms of each storm during a winter may differ from each other, one type appearing abundantly in one storm, a different type in the next, and so on. Conversely, the types most common in a given storm may reappear after an interval of months or years—as, for example, those obtained during the great blizzard of March, 1888, were repeated in the storms of February 16, 1892, and March 3, 1896, and most of these were of forms such as Figs. 13, 14, and 15, while unusual types, such as Figs. 7, 9, and 17, occurred in the storms of February 24, 1893, and February 13, 1894.
Not only do different storms afford different types of crystals, but different parts of the same storm, if it be general, give different forms. In this region, the northern and western portions of the storm area produce more perfect crystals than the southern and eastern, and from this we infer a difference in the atmospheric conditions in these portions, the former being more quiet and otherwise favorable to crystallization.
What has been called granular snow is shown in Figs. 4, 6, and 23. In this very common form we find only loose, irregular, subcrystalline forms, which are larger and heavier than others. This is formed in the middle or lower cloud layers, and when these are disturbed by wind, or otherwise rendered unsuitable for crystallization. Sometimes, perhaps always, these granular masses have nuclei of true crystals. Granular snow may explain the origin of the great raindrops which often fall during a thundershower. It is probable that such drops have a snow origin. Most, if not all, hailstones also originate in granular snow, as their thin, opaque centers and concentric rings of opaque, snowlike ice show.The superiority of photography over drawing in securing details of structure may be readily seen if one compares any of the accompanying figures with the ordinary drawings of snowflakes, or even with the finest illustrations hitherto published. It is unfortunate
that the depth and solidity seen in some crystals when the photographs are mounted as stereoscopic views can not be in some adequate manner reproduced in engravings, for this adds not a little to an understanding of the manner in which the crystal has been formed. Yet something of this can be seen in the figures as here given. A careful study of this internal structure not only reveals new and far greater elegance of form than the simple outlines exhibit, but by means of these wonderfully delicate and exquisite figures much may be learned of the history of each crystal, and the changes through which it has passed in its journey through cloud-land. Was ever life history written in more dainty hieroglyphics! It is well known that crystals which form in a low temperature are smaller and more compact than those formed in a warmer atmosphere. As the higher cloud strata are colder than those nearer the earth, the snow crystals which originate there are smaller and less branched than those from lower clouds. Such are shown in Figs. 2, 20, and 22, while crystals from the warmer clouds are more often like Figs. 24 and 25. The small, compact crystals of the upper clouds do not always remain of their original form or size, for, as they fall through layer after layer of clouds, each layer subjecting them to its own special conditions, they may be greatly modified, and by the time they reach the earth they may closely resemble the crystals from lower clouds, though they can usually be distinguished from them by an examination of the internal structure, as well as by, in some cases, their general form. All crystals falling from high cloud strata, the cirrus or cirro-stratus, are not changed; especially is this true in a great storm, or when the temperature of the lower clouds is low, and in any case some are much more completely transformed than others. One crystal may pass through cloud layers not very unlike that from which it came, and of course will not be greatly changed. Another may encounter here a quiet cloud layer and there a tumultuous layer, here a lower, there a higher temperature, here a dense and there a thin cloud mass, and by all of these conditions may be affected.
Examples of crystals which have been little changed are shown in Figs. 3, 7, and 8, while Figs. 12, 13, 14, and 16 show more completely modified crystals. Total transformation, such as the change of one type into another, does not often occur. The nucleus retains its original form, to which various additions are made during the downward passage. Composite crystals may, however, be formed during the passage through diverse cloud layers, though they are not common, as shown in Figs. 11 and 19. Usually, however, the tabular, compact, small crystals of the high clouds continue their development at lower levels upon the original plan, though becoming larger and more complex by the addition of brandies at the angles. The triangular forms, such as Figs. 7 and 9, are less common than the others figured, and occur usually in the greater storms. Fig. 17 shows a very unique composite crystal, which, beginning in the higher clouds as a simple hexagon, seen in the center, received the peculiar additions which are well shown in the figure. Fig. 11 is exceedingly unusual. It appears to be a composite crystal formed from two, each of which has been in some way broken apart, and the portions shown were so brought in contact as to unite and form a single crystal of very nearly the original form of each of its parts.
The above are some of the more important of the many interesting results which have come from our study of snow crystals. They are given not merely as of value in themselves, but also in the hope that others may be stimulated to undertake similar investigations, and that thus our knowledge of these charming objects may be greatly increased. After what has already been said, it should not be necessary to add that any one who engages in the study of snow crystals will speedily find his task both absorbing and delightful. There is no surer road to fairyland than that which leads to the observation of snow forms. To such a student the winter storm is no longer a gloomy phenomenon to be dreaded. Even a blizzard becomes a source of keenest enjoyment and satisfaction, as it brings to him, from the dark, surging ocean of clouds, forms that thrill his eager soul with pleasure.
Prof. William Trelease relates, in his address on Botanical Opportunity, that when engaged with Prof. Gray in collecting and republishing the botanical writings of Dr. Engelmann, they were both surprised to see how voluminous these works were. A few years later, when Prof. Trelease had to arrange in form for preservation Dr. Engelmann's manuscript notes, he was far more surprised at the extent of them than he had been on collecting his printed works, for when mounted and bound they formed sixty large volumes. "In addition to their intrinsic value, these are of more than usual interest as showing the methodical manner in which Dr. Engelmann worked. On his table seems to have been always a bundle of plants awaiting study. As each specimen was examined its salient features were noted and sketched on the back of an ever-ready prescription blank. When interrupted, he laid his unfinished sketch away with the specimen, to resume his observation and complete his study on the first opportunity, without any doubt as to what had been seen in the first instance. And so from individual to variety, from variety to species, from species to genus, and from genus to family, his observations were preserved in memoranda that facilitated the resumption of interrupted work at any time and after any lapse of time." Thus the off moments in the work of a busy physician were made to contribute to botanical knowledge, and a seemingly small opportunity for investigation was converted into a great one.