Hair ice was first studied in 1918. The weird phenomenon is a type of ice made up of fine, silky hair-like structures. It looks like someone just dropped a wig of white hair on the ground.
“When we saw hair ice for the first time on a forest walk, we were surprised by its beauty,” said 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.”
In 1918, Alfred Wegener took a closer look at a patch of hair ice. On the piece of wood, he noticed a white coating. This white layer was identified as fungus mycelium by his assistant. The presence of fungus mycelium suggested to Wegener there was some kind of relationship between the ice and the fungus in the wood.
About 90 years later, retired Swiss professor Gerhart Wagner found evidence of this. If you treated the wood with fungicide or dipped it in hot water, it stopped the growth of hair ice.
But, the particulars of hair ice were still unknown. Which fungus species is responsible? What process drives the growth of hair ice? That is what researchers strived to answer in a new study.
Gisela Preuß, a biologist, ventured into the woods near Brachbach, Germany several times between 2012 and 2014. She collected samples of wood with hair-ice on them and identified eleven different species of fungi.
One species was common in all of the samples: Exidiopsis effusa. “In more than half of the samples, it was the only species present,” according to Preuß.
Christian Mätzler, a physicist from the University of Bern in Switzerland, dived into the physics behind hair ice. He confirmed what was already suspected. Ice segregation is the key mechanism behind the production of ice filaments on the wood surface.
As water near a branch surface comes into contact with cold air, it freezes. This creates a sandwich effect with a thin film of liquid water between the ice and wood pores. Suction pushes the layer of thin water outward, causing it to freeze in the thin hair-like structures as it moves.
The hair ice picture below illustrates this mechanism.
The fungus plays an integral role in the structure of hair ice.
According to Mätzler:
“The same amount of ice is produced on wood with or without fungal activity, but without this activity the ice forms a crust-like structure. The action of the fungus is to enable the ice to form thin hairs – with a diameter of about 0.01 mm – and to keep this shape over many hours at temperatures close to 0°C. Our hypothesis includes that the hairs are stabilised by a recrystallisation inhibitor that is provided by the fungus.”
Hair ice looks beautiful, but it’s tricky to spot. It usually only forms under certain conditions. You need a humid winter night with the air temperature dropping slightly below freezing. Oh, and your best shot at seeing it is before the sun rises as it quickly melts.
The study was published in Biogeosciences.
Image credits: Christian Mätzler