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A frost flower is a name commonly given to a condition in which thin layers of ice are extruded from long-stemmed plants in autumn or early winter. The thin layers of ice are often formed into exquisite patterns that curl into "petals" that resemble flowers.
Frost flower formations are also referred to as frost castles, ice castles, ice blossoms, or crystallofolia.
Types of frost flowers include needle ice, frost pillars or frost columns, extruded from pores in the soil, and ice ribbons, rabbit frost or rabbit ice, extruded from linear fissures in plant stems. While the term ice flower is also used as synonym to ice ribbons, it may be used to describe the unrelated phenomenon of window frost as well.
Hair ice, frost beard, ice wool, or feather frost describe a hairy, sometimes silky variant of frost flowers, extruded from openings of histological rays in the wood, which also requires the presence of fungus metabolism.
The formation of frost flowers is dependent on a freezing weather condition occurring when the ground is not already frozen. The sap in the stem of the plants will expand (water expands when frozen), causing long, thin cracks to form along the length of the stem. Water is then drawn through these cracks via capillary action and freezes upon contact with the air. As more water is drawn through the cracks it pushes the thin ice layers further from the stem, causing a thin "petal" to form. In the case of woody plants and (living or dead) tree branches the freezing water is squeezed through the pores of the plant forming long thin strings of ice that look uncannily like hair i.e. "hair ice" or "frost beard".
The petals of frost flowers are very delicate and will break when touched. They usually melt or sublimate when exposed to sunlight and are usually visible in the early morning or in shaded areas.
Examples of plants that often form frost flowers are white crownbeard (Verbesina virginica), commonly called frostweed, yellow ironweed (Verbesina alternifolia), and Helianthemum canadense. They have also been observed growing from fallen branches of conifers and contain enough hydraulic power to strip the bark off.
The meteorologist Alfred Wegener described hair ice on wet dead wood in 1918, assuming some specific fungi as catalysator, a theory mostly confirmed by Gerhart Wagner and Christian Mätzler in 2005.
On Sept. 2, 2009, a University of Washington biology team sailing back from the North Pole encountered these little flowery things growing on the frozen sea "like a meadow spreading off in all directions. Every available surface was covered with them." When allowed to melt, the one to two milliliters of water recovered was found to hold about a million bacteria. Professor Jody Deming believes that as the poles warm, there will be more and more of these meadows, because there will be more and more open sea that turns to thin ice in winter, and her team is eager to discover what the bacteria living in the frost flowers are doing.
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