Popular Science Monthly/Volume 45/May 1894/Frost-Forms on Roan Mountain
|FROST-FORMS ON ROAN MOUNTAIN.|
THIS is the only habitable high mountain peak east of the Pacific ranges. Its altitude, six thousand three hundred and thirteen feet above the sea level, tempered by its latitude, thirty-six degrees, together with its isolation from other mountains of similar height, renders it one of the most favorable places for the observation of atmospheric conditions. The clouds here usually float about level with the summit, though they sometimes rise as much as five hundred feet above it, or sink two
|Fig. 1.||Fig. 2.|
thousand feet below; so that it may be said to lie in the track of the clouds.
I regret that I was not better equipped for a thorough study of frost-forms produced by the lateral deposit of the frozen vapor in the clouds during the severe winter of 1892-'93, which I spent upon the summit of Roan Mountain for the sake of an invalid daughter. There was not a hygrometer within reach, hence the amount of moisture in the atmosphere at any given time can not be stated. The anemometer was frequently clogged by accumulations of frost upon it. Incessant winds and flying snow dust prevented the taking of clear photographs out of doors, and many plates were spoiled by inexperienced handling.
The factors in the production of these frost-forms are the frozen vapor and the wind. Their size, shape, and location are
controlled by the amount of moisture, the temperature, the direction and velocity of the wind, the shape, size, and situation of the objects on which they are deposited, and the size and nearness of the surrounding objects. The lower the temperature, the denser the cloud, the swifter the wind, and the more perfect the exposure, the more rapid the growth and the more profuse and elaborate the results.
Fig. 1 shows a six-sided wooden pillar with a deposit made in two hours. Wind, about thirty miles an hour; temperature, fifteen degrees below zero. Frost in the form of fir-tips, projecting three quarters of an inch from the corners, and one fourth to one half inch from the spaces intervening. A space two inches square contained twenty-five.
Fig. 2 shows the same pillar a week later, after five days of storm and two of sunshine. Frost-forms now projecting fourteen inches and glazed on outside.
There is no fixed proportion between the size of the base of the deposit and the deposit itself. It is remarkable for cohesive strength, stiffness, and tenacious grip upon its base. In the case of round bodies, such as trees or wires, it clasps but half the circumference, the other half being not even glazed (unless some large object be directly to leeward), and stands out on the windward side of its support, following its curves and angles with precision. Sometimes a tree or a grove of trees may be seen entirely white on one side and green on the other.
Unless there are numerous changes in the direction of the wind during the progress of construction, the first aggregations of particles have the same general configuration as the finished ornaments hundreds of times as large—six to eight inches wide at the base and projecting twelve to sixteen inches. A slight variation in the direction or velocity of the wind makes them more complex and adds greatly to their beauty; but a change of as much as sixty degrees in the direction wrenches them from their supports. They come away entire, and lie in heaps under the trees like autumn leaves, and may be collected and preserved in a cold, sheltered place until they gradually evaporate.
The process of formation is an interesting study. It is impossible to follow the course of the fine particles of snow dust which make up the most beautiful forms; but at a temperature of twenty-five to thirty degrees above zero the frozen moisture
comes in minute pellets of ice which may be watched with a good microscope as they strike a chosen spot. The development of the ice-forms is much more rapid than that of the snow-forms; otherwise the processes seem to be identical.
On the edges of fiat surfaces, and along the diameters of round bodies, lines of particles are deposited as the wind rushes past the obstruction. Then begins a twofold growth, caused by the direct application of other particles on the windward side, and by the rebound to the lines already laid of those particles which are driven violently against the surfaces between the lines. On smooth, narrow bodies, as this process is continued, the deposits along the sides or edges soon become so thick and long as to meet in the middle. On rough surfaces new lines and centers of groups are begun on all projections, however slight, and the particles rebound to them from the surrounding surfaces.
Fig. 3, a section of rough board, illustrates this. The deviation from the perpendicular in the frost-forms on the edges is due to the fact that the board was not accurately facing the wind.
There is, of course, a great variety of forms produced in different storms, all wonderful for delicacy of design and perfection of finish such as could not be imitated in any
|Fig. 5.||Fig. 6.|
material. Among them may be shown a branch of balsam fir (Abies Fraseri) (Fig. 4) which bears the heavy fringe of the storm of December 28th, when the wind blew at the rate of fifteen to twenty-five miles an hour, and the temperature was fifteen degrees above zero.
Fig. 5, a pillar and standpipe, shows the perfect fir-tip pattern of January 3d. Wind, fifteen to thirty miles an hour; temperature, ten degrees below zero. The lower temperature and swifter wind account mainly for the difference between this form and the preceding one. The leeward sides of pillar and pipe are thinly coated by the rebound of particles from the house wall.
Fig. 6, the accumulation on the tip of a blade of grass, seven eighths of an inch long. This fragment was broken off and brought into the house to show how all the grass was decorated by the storm of January 6th, with wind at forty miles an hour and temperature twenty degrees below zero. It was two inches and three quarters tall and weighed three quarters of an ounce avoirdupois, or more than five thousand times as much as the bit of grass inclosed by it. It was composed of ten large feathers, with the spaces between them filled with smaller ones—no shapeless snow about it. The tips of twigs, ends of fence rails, etc., projecting toward the wind, were all similarly decorated, but on different scales, according to their size and exposure.
Many curious and apparently contradictory effects are produced by the rebound from one surface to another. A post which
stood twenty feet from the house, in a small court inclosed on three sides, had a deposit on the face toward the house equal to that on the windward side, while the other sides were bare and dry.
Fig. 7 shows a wreath of plumes averaging six inches in length, formed altogether upon the leeward side of a tub, by the rebound of the vapor-laden wind from a high wall about three feet distant. It will be seen that the rebound from the tub again has produced a second series of forms around it on the ground, pointing toward the tub.
The most conspicuous and noteworthy example of this resilient force was exhibited at the close of the storm of January 6th to 8th, in a recess where a north wing joins the main hotel building. The speed of the wind varied from forty to sixty miles an hour during those three days, and the temperature was from fifteen to thirty degrees below zero.
Fig. 8 presents a sketch of the outlines. A B is the northwest corner of the main building, three stories high. C is two stories Fig. 8. high, and E D one story. F shows the direction of the wind, which varied little. A, B, D, and E had heavy, deep cornices of long, narrow plumes like pampas grass, averaging sixteen inches in length, inclined outward from base to tip at an angle of thirty degrees to the plane of the wall, and lying in a horizontal position. The plumes on A and E pointed north by northwest; those on B, west by northwest; all as directly toward the wind as was allowed by the laws governing their application to the walls and by the angles at which the wind struck the walls. Those on D, being formed by the rebound from the high wall C and the angle C B, pointed east, or toward C, though in all other respects similar to the others. On the weatherboards of A, B, and E the frost-feathers were short and broad. They stood vertically, with their bases on the edges of the boards, each row overlapping the row above, and each row formed by the downward rebound of particles from the thick edge of the board above it. The forms on all the upright corner boards (or facing boards) seemed to have been made later than those on the weatherboards, since they lay horizontally, with their tips pointing toward those on the weatherboards, and at a right angle to them. They must have been made by the rebound from the forms on the weatherboards, as their direction in every instance was exactly opposite to that of the cornice decorations on the same wall. After the first few hours it was impossible to brave the fury of the storm to watch the process of development, which is inferred from the results and the proved rules by which the work is done.
The forms on the weatherboards of D hung downward, while those on the opposite wall, B, stood upright. This must have been due to the rotary motion of the wind after it struck the three-story wall B and the two-story wall C, and, whirling downward and upward again from the ground, struck the one-story wall D.
Fig. 9 shows a section of the wall D in the beginning of that storm. Unfortunately, the other negatives of that group were spoiled.
On C the deposits on cornice and weatherboards nearest D partook of the shape and direction of those on D; and the same
was true of those nearest B. In the space intervening, the frost was laid on obliquely, resembling the first course of a heavy lattice. All these walls, as well as all others on the mountain top which faced the west and north, were completely covered, and presented the appearance of exquisitely chiseled marble.
On all flat surfaces, whether curved or rectilinear in outline, when they are suspended vertically, faced to the wind, so that it may blow past all sides unobstructed, the frost-forms lie at an angle of thirty degrees to the surface, with their bases to its edges, and point accurately toward the center.
On a flat surface having a rim that projects as much as half an inch, they are built on the inner edge of the rim, and extend toward the center at a right angle to the rim and parallel to the surface. When the rim is more shallow, their bases are set in the angle where the rim joins the surface, and they stand out from the surface at an angle of more than thirty degrees.
Fig. 10 is the lid of a cream freezer, showing frost-forms pointing toward the center and extending parallel to the face of the disk.
On a tumbler, three inches and a quarter in depth and two inches and a quarter in diameter at the top, placed with its mouth to the wind, the result was the same. The frost-forms pointed toward the center and were parallel to the bottom of the tumbler. It might be worth while to find out by experiments how deep and how wide a vessel would be required to cause them to deviate from this rule.
Fig. 11 exhibits an iron pipe elbow, part of the deposit on which was affected by the rebound from the longer curved side as the wind passed through it. If a straight section of pipe be placed so that the wind may pass through it unobstructed, the deposit is made on the windward end, of the same thickness as the metal; and it appears as though that part of the pipe had been
cast in the pattern prevailing in that storm, and whitened. The outer and inner longitudinal surfaces of the pipe are left bare and dry.
Very pretty experiments may be made with apples, chairs, wheels, tin cans, feathers, and other objects too numerous to mention.
Fig. 12 is an apple with a faithful imitation of a chrysanthemum on one side. This was made at a low temperature and was white. The most beautiful blossoms were those made of sleet, at a temperature of twenty-five to thirty degrees above zero. They were sometimes as large as the apples, and always had from twelve to twenty perfectly shaped petals, from one inch to two inches and three quarters long.
It often happens that the clouds clear away and the temperature rises a few degrees while the direction of the wind is still unchanged. Then the outer surfaces of the frost-forms become glazed and the softer filling is blown out. They may be taken off entire, and need no greater care in handling than fine china. They are thin as eggshells and translucent, and under the microscope show long rows of minute cells, separated by delicate filiform partitions. A contrary wind unclasps their hold and the ground is strewn with the curious wreckage. They may be kept for many days in a cold place.
In sheltered places, a little way down on the leeward side of the mountain, the deposits of frozen vapor are similar to the hoarfrost seen in the lowlands, but greatly exaggerated in size and profusion, and are usually in the form of small rosettes, set as thickly as possible upon all surfaces of trees, rocks, or buildings. The frost on the windows of all unoccupied rooms varies in shape and amount according as the temperature is higher or lower.
At fifteen degrees above zero, small fern-shaped figures are made, about a quarter or a half an inch long. At lower temperatures they decrease in size and increase in numbers, until.
|Fig. 11.||Fig. 12.|
at thirty degrees below zero, the panes are quite covered with tiny frost ferns, twenty-five of which have been counted in a space an inch square, every one perfect in outline. Above fifteen degrees above zero the shape changes to something like the Hypnum moss.
Fig. 13 represents part of a pane. The temperature fell below fifteen degrees for a short time, allowing the accumulation of a few of the fern-forms, and then rose rapidly to twenty-five degrees, with the result here shown. The moisture condenses upon the windows of inhabited rooms just about as it does everywhere else.
I watched throughout the winter for the stellar and hexagonal snowflakes, but never found them while the clouds enveloped the mountain. The particles of frozen vapor in the clouds resemble finely ground meal. When a cloud rises from fifteen to
twenty feet above any given place, several of these particles (usually six or eight) come down joined together like beads on a pin; when it rises fifty or a hundred feet, the little sticks of globules cross and adhere to each other in falling, and reach the earth in all the complex shapes commonly called snow crystals.