Observations on the Growth of Needle Ice

Observations of needle ice were made in the field and in the laboratory. In both situations multitiered needles formed during a single freeze‐thaw cycle. Four freeze‐thaw cycles were induced in the laboratory. Ice needles grew during each cycle but became less abundant as the soil surface dried out. During the first cycle two‐tiered needles grew; in later cycles a layer of frozen soil developed under a single layer of needles. Measurements of soil heat flow, changes of soil moisture, and loss of moisture by evaporation allowed calculations of the heat balance of the surface to be made. These measurements demonstrate the importance of heat released in the conversion of water into ice. Comparison of field and laboratory results shows a contrast in the mechanism of heat loss in the two situations (radiative as opposed to evaporative), but this contrast is unimportant in the formation of ice needles. Water availability appears to be a more important control of ice needle growth. Development of multitiered ice needles is not dependent on the number of freeze‐thaw cycles but may result from a critical balance between the soil cooling rate, the amount of available moisture, and the release of heat by conversion of water into ice.