You may have never seen or heard of it, but hair ice - a type of ice that has the shape of fine, silky hairs and resembles white candy floss - is remarkable. It grows on the rotten branches of certain trees when the weather conditions are just right, usually during humid winter nights when the air temperature drops slightly below 0°C. Now, a team of scientists in Germany and Switzerland have identified the missing ingredient that gives hair ice its peculiar shape: the fungus Exidiopsis effusa. The research is published today (22 July) in Biogeosciences, an open access journal of the European Geosciences Union (EGU). "When we saw hair ice for the first time on a forest walk, we were surprised by its beauty," says Christian Mätzler from the Institute of Applied Physics at the University of Bern in Switzerland. "Sparked by curiosity, we started investigating this phenomenon, at first using simple tests, such as letting hair ice melt in our hands until it melted completely."

Then Mätzler, a physicist, joined forces with a chemist (Diana Hofmann) and a biologist (Gisela Preuß) in Germany. Inspired by earlier work, and by photographs of hair ice sent in from various countries, the team performed a set of experiments to figure out just what conditions are needed to grow this type of ice and what its properties are. In the process, they confirmed a 100-year-old theory for the origin of hair ice.

Alfred Wegener, of tectonic-plate fame, was the first to study hair ice. In 1918, he noticed a whitish cobwebby coating on the surface of hair-ice-bearing wood, which his assistant identified as fungus mycelium - the mass of thin threads from where mushrooms grow. He suggested there was a relation between the ice and the fungus in the wood. Some 90 years later, Gerhart Wagner, a retired Swiss professor who has been researching hair ice for decades, found evidence of this relation: treating the wood with fungicide or dunking it in hot water suppressed the growth of hair ice. But the fungus species and the mechanism that drives the growth of hair ice was yet to be identified.

That was the aim of the researchers who have now published their work in Biogeosciences. Preuß studied samples of hair-ice-bearing wood collected in the winters of 2012, 2013 and 2014 in forests near Brachbach in western Germany. She analysed the wood samples using microscopic techniques and identified eleven different species of fungi. "One of them, Exidiopsis effusa, colonised all of our hair-ice-producing wood, and in more than half of the samples, it was the only species present," she says.