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. It is somewhat uncommon, and has been reported mostly at latitudes between 45–55 °N in broad leaf forests. Hair ice forms on moist, rotting wood from broad leaf trees when temperatures are slightly under 0 °C (32 °F) and the air is humid. 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). The hairs are brittle, but take the shape of curls and waves. They can maintain their shape for hours and sometimes days. A piece of wood that produces hair ice once may continue to produce it over several years.
In the year 2015, German and Swiss scientists identified the fungus Exidiopsis effusa as key to the formation of hair ice. 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.
Dohduhdah, Public domain, via Wikimedia Commons
Kostian, Public domain, via Wikimedia Commons