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]

• Frost flower
• Hoarfrost
• Needle ice