Hair ice, also known as Frost Beard or Ice Wool, is very rare and only forms in very specific conditions, for many centuries its formation had been a complete mystery to science. However researchers found in 2015 that it is formed by a fungus, Exidiopsis effusa, it only forms between 45° and 55° North in humid air at temperatures just under OC.
The hair-like crystals are 0.01mm in diameter and contain an inhibitor made by the fungus which stops the ice from melting or forming into larger crystals, why and how is not known.
Sinuous shapes
The sinuous forms they take are very beautiful and can last for hours, sometimes even days. This long lifetime is due to the inhibitor preventing ice crystals from recrystallizing into larger ones, which would normally occur at temperatures near 0 °C, and the ‘hairs’ always root at the mouth of the wood rays and never on the bark.
It is also only found on the decaying wood of broadleaved species of tree, these were found on Beech wood near Croasdale farm in the Hodder valley and have re-appeared in the same location for years.
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Northwest Nature and History 