frozen in their fall, but congeal as they strike. In this way, ground, sidewalks, trees and other surfaces become covered with a coating of ice. Weather Bureau practise and popular consent join in designating this form of precipitation as an ice storm. The ice storm is apt to be followed by the destruction of tree branches snapped off by the wind, and by an unusual number of accidents in city street traffic.
Several conditions of temperature and precipitation may result in an ice storm. If the rain-drops fall through a stratum of air below freezing temperature and strike an object whose surface is also below freezing temperature a varnish of ice will be formed, and it is likely to increase in thickness so long as precipitation continues. When the temperature is very slightly above the freezing point, and the surface air is dry, it is possible that rapid evaporation may chill the varnish of water below the freezing point and change it to ice. Rain-drops in the air may be chilled to a temperature several degrees below the freezing point; they change to ice instantly as they strike.[1]
Damp snow and snow falling on tree limbs, poles and wires whose temperature is slightly above freezing, is very apt to cling to them. The weight of the accumulated snow may be sufficient to break tree limbs and line wires. Not only is there a considerable material damage; there is also a troublesome and expensive interruption of communication. Popularly, the condition is known as a white storm.
A temperature materially below 32° F following a white storm is apt to result in much damage to shade trees and orchards. Branches will bend freely, as a rule, when the temperature is above freezing; but they become brittle under intense cold if coated with ice. The distinction between an ice storm and a white storm is chiefly one of appearance. The Weather Bureau makes no distinction between them.
Hail.—Hail is a product of thunder-storms. The hailstone consists of alternate concentric layers of snow and ice. The manner of the formation of the hailstone is conjectural. About the only thing of which one may be certain is that the hailstone is alternately in layers of moist air below the freezing point and
- ↑ The Blue Hill Observatory reports rain falling when the temperature of the air was about 15° F—a very unusual phenomenon. It is not likely, however, that the rain-drops had reached a temperature much below freezing.
