THIS

FROST-FORMS ON ROAN MOUNTAIN.

BY MRS. HELEN R. EDSON.

HIS 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

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 flat 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

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.

VOI. XLV.-3

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 resili