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]