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January-February 2009

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Unusual Ice Formations: Studying the Natural Growths of Ice

In the August 1980 issue of Weatherwise a reader asked about a photo “showing plant-like shoots of ice that grew up from the soil in thin, parallel fragile bundles.” Dr. Charles Knight of the National Center for Atmospheric Research (NCAR) responded that this “is usually called ‘needle ice’” and gave a brief explanation for how it is thought to occur. The existence of this and similar forms of growing ice has been documented in articles and letters dating from the 1820s, but with digital cameras, the Internet, and search engines, many people are now able to share their findings with the world. As a result, a number of attractive formations of ice growing from different media have recently gained public exposure. The most commonly observed types of such ice are needle ice, ribbons of ice, hair ice, and ice on rocks. I believe this article represents the first time that these four types of growing ice have been discussed in one publication.

All of these forms of ice occur when water in the medium is still liquid while the air temperature falls below freezing. According to Knight, “Ice crystals have trouble penetrating wet soil because the ice surface must become very highly curved to penetrate between the soil particles. The small radii of curvature locally decreases the freezing point. The liquid water in the soil can therefore become supercooled with respect to bulk ice by conduction of heat through the ice itself, and, if the rate of cooling and the soil particle size distributions are just right, the supercooled water is drawn up through the soil to the bases of the ice columns, where it then freezes.”
The size, shape, and form of the growths of ice differ, but they seem to have a common base of formation. Of course, such ice formations are ephemeral, and with warmer temperatures and a little sunlight they are usually gone by midday. So our knowledge of them is based largely on photographs.

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