Hoarfrost - December 11, 2004
Dense freezing fog formed across the Sioux Falls area during the early morning hours on December 11, 2004. Temperatures were in the lower 20s, with visibilities in the freezing fog down to 1/4 to 1/2 mile at times. There was a relatively light south wind - less than 10 mph when visibilities were at their lowest - but it was enough to cause the frost to develop primarily on the windward side of objects (i.e. on the south facing side). This can be seen in some of the photos below.
(click the photos below to view higher resolution enlargements) |
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Hoarfrost is defined in the AMS Glossary of Meteorology, 2nd edition (
© 2000) as:
—A deposit of interlocking ice crystals (hoar crystals) formed by direct deposition on objects, usually those of small diameter freely exposed to the air, such as tree branches, plant stems and leaf edges, wires, poles, etc.
Also, frost may form on the skin of an aircraft when a cold aircraft flies into air that is warm and moist or when it passes through air that is supersaturated with water vapor. The deposition of hoarfrost is similar to the process by which dew is formed, except that the temperature of the befrosted object must be below freezing. It forms when air with a dewpoint below freezing is brought to saturation by cooling. In addition to its formation on freely exposed objects (air hoar), hoarfrost also forms inside unheated buildings and vehicles, in caves, in crevasses (crevasse hoar), on snow surfaces (surface hoar), and in air spaces within snow, especially below a snow crust (depth hoar). Hoarfrost is more fluffy and feathery than rime, which in turn is lighter than glaze. Observationally, hoarfrost is designated light or heavy (frost) depending upon the amount and uniformity of deposition.
