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Needle ice is a needle-shaped column of ice formed by groundwater. Needle ice forms when the temperature of the soil is above and the surface temperature of the air is below . Liquid water underground rises to the surface by capillary action, and then freezes and contributes to a growing needle-like ice column. The process usually occurs at night when the air temperature reaches its minimum.
The ice needles are typically a few centimetres long. While growing, they may lift or push away small soil particles. On sloped surfaces, needle ice may be a factor contributing to soil creep.Isbell, D.: Needle Ice on Mt. Osceola , EPOD of July 10, 2005. URL last accessed 2007-12-07.Pidwirny, M.: Fundamentals of Physical Geography, 2nd ed., section 10(ag), Periglacial Processes and Landforms. URL last accessed 2007-12-07.
Alternate names for needle ice are "frost pillars" ("Säuleneis" in German language), "frost column", "Spew Ice", "Kammeis" (a German term meaning "comb ice"), "Stängeleis" (another German term referring to the stem-like structures), "shimobashira" (霜柱, a Japanese term meaning frost pillars), or "pipkrake" (from Swedish language pipa (tube) and krake (weak, fine), coined in 1907 by Henrik Hesselman).Lawler, D. M.: "Some observations on needle ice", Weather, vol. 44, pp. 406–409; 1989.
The similar phenomena of and hair ice can occur on living or dead plants, especially on wood.
Needle ice is commonly found along stream banks or soil terraces. It is also found by gaps around stones and others areas of patterned ground. The variety of soil properties also affects where it is found. Places where the soil is much deeper and richer can affect the growth of the ice. Consequently, the deeper the soil, the larger the water content allows it to develop. It can be evidently formed anywhere where underground water is exposed to open (freezing) air.
Needle ice is most suitable in soils with a high silt and organic matter content. Needle ice consists of groups of narrow ice slivers that are up to several centimeters long. Although the literature states that the largest recorded needle ice was at 10 cm in length, specimens 15-20 cm in length have been observed at Gerðuberg, for example.
Needle ice grows up slowly from the moist and water-penetrable soil, and melts gradually in the sun. It can vary in appearance but always shows the consistent growth of ice perpendicular to the surface of the ground. Needle ice looks like a series of filamentous crystals, and is straight or curved in shape. It usually forms in the morning when the temperature drops below freezing point (0 °C).
The growth of needle ice lifts a detached, frozen soil crust riding on top of the layer of ice. When the crust and the ice melt, the soil surface settles back irregularly. This phenomenon is linked to erosion, particularly on streambanks.
Needle ice tends to move rocks in the soil up toward the surface and to shift rocks on the surface into nearby depressions. Depressions caused by needle ice activity are known as needle-ice pans, and lumps caused by needle ice are known as "nubbins".
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