Hair ice, also known as ice wool or frost beard, is a type of ice that forms on dead wood and takes the shape of fine, silky hair. [1] It is somewhat uncommon, and has been reported mostly at latitudes between 45 and 55 °N in broadleaf forests. [1] [2] The meteorologist (and discoverer of continental drift) Alfred Wegener described hair ice on wet dead wood in 1918, [3] assuming some specific fungi as the catalyst, a theory mostly confirmed by Gerhart Wagner and Christian Mätzler in 2005. [4] [5] [6] In 2015, the fungus Exidiopsis effusa was identified as key to the formation of hair ice. [1]
Hair ice forms on moist, rotting wood from broadleaf trees when temperatures are slightly under 0 °C (32 °F) and the air is humid. [1] The hairs appear to root at the mouth of wood rays (never on the bark), and their thickness is similar to the diameter of the wood ray channels. [1] A piece of wood that produces hair ice once may continue to produce it over several years. [1]
Each of the smooth, silky hairs has a diameter of about 0.02 mm (0.0008 in) and a length of up to 20 cm (8 in). [1] The hairs are brittle, but take the shape of curls and waves. [1] They can maintain their shape for hours and sometimes days. [1] This long lifetime indicates that something is preventing the small ice crystals from recrystallizing into larger ones, since recrystallization normally occurs very quickly at temperatures near 0 °C (32 °F). [1]
In 2015, German and Swiss scientists identified the fungus Exidiopsis effusa as key to the formation of hair ice. [1] The fungus was found on every hair ice sample examined by the researchers, and disabling the fungus with fungicide or hot water prevented hair ice formation. [1] The fungus shapes the ice into fine hairs through an uncertain mechanism and likely stabilizes it by providing a recrystallization inhibitor similar to antifreeze proteins. [1] [2]