Page:Popular Science Monthly Volume 45.djvu/43

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31
FROST-FORMS ON ROAN MOUNTAIN.

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

PSM V45 D043 Frost aggregation in a windless spot.jpg

Fig. 3.

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