Frost beard on my walk in the woods
Frost beard, Västerlanda, Sweden, 28th December 2025
Frost beard is also known as hair ice, and it's one of my favourite natural phenomena.
It only occurs on decaying broadleaf wood, and typically forms on snow-free ground, in conditions just below freezing and with high air humidity. Hair ice is created when carbon dioxide from the decomposition process forces water out of the wood, causing it to freeze into hair-like rays.
Frost beard, Västerlanda, Sweden, 28th December 2025
Normally, ice crystals this fine would merge into larger crystals and lose their shape, so something must be preventing this from happening, or hair ice wouldn't form.
Scientists have believed since the early 20th century that this something was probably antifreeze compounds formed by a winter-active fungus. But it would take almost a century before the fungus was identified in 2015 as Exidopsis effusa (which sadly doesn't have a common name), by Swiss and German scientists Hofmann, Preuss, and Mätzler.
Abstract. An unusual ice type, called hair ice, grows on the surface of dead wood of broad-leaf trees at temperatures slightly below 0 °C. W
