28 Feb 2022

Peculiar Hair Ice

Ardd Fotaneg · Botanic Garden

There are many reasons why I get excited about walking to work on cold frosty mornings. I like to take my time walking through the restored landscape, seeing the ice sparkle in the sun and the mist rising from the lakes… and I get to go on the hunt for hair ice! The conditions have to be perfect; below freezing and humid. Sometimes I’m lucky enough to spot a delicate bunch of shiny, icy strands nestled amongst the leaf litter.

Hair ice is a very unusual type of ice, and only forms under very specific conditions. It grows on the surface of dead wood from broad-leaf trees at temperatures just below 0°C, when conditions are calm and humid. The hairs are only 0.02 mm in diameter and can grow up to 20 cm long, looking like clumps of white candy floss protruding out of sticks and branches that have fallen to the ground. The silky smooth hairs can form beautiful waves or curls before they melt away in the sun. So, you have to be out early if you want to try and spot this icy spectacle for yourself!

Hair ice was described for the first time over a century ago, by Alfred Wegener, who also founded the theory of continental drifts. Wegener observed that the hair ice filaments only grew on branches that also had thin threads of mycelium belonging to a fungus. But, it wasn’t until 2015 that scientists identified the fungus responsible for the hair ice phenomenon: Exidiopsis effusa.

The presence of this fungus leads to a process called ‘ice segregation’:

  1. The liquid near the branch surface is in contact with the cold air, creating a thin film of ice, which sandwiches a thin layer of water between the ice and the wood pores.
  2. This creates a suction force, which pushes water from inside the wood pores towards the ice at the surface of the pore, where it freezes and adds to the existing ice.
  3. As this process repeats, and ice builds up, a thin ‘hair’ of ice is pushed out of the wood.

The fungus provides decomposed lignin and tannin as organic material, and it is thought that these act as ‘recrystallisation inhibitors,’ keeping the ice crystals from growing too large and stopping the strands from bunching up into more stable shapes. So when the fungus isn’t present, ice still forms, but in a crust-like structure instead of forming hairs.

This study has allowed us to understand the mystery of hair ice better, but more research will be needed to confirm this theory, and many more questions remain unanswered. The appearance of this peculiar hair ice still remains an enigma!

Next time there’s frost and high humidity, keep your eyes peeled as you walk through Fairy Woods and around the lakes of the newly-restored landscape – you might just be lucky enough to spot some of this icy fluff for yourself! But, you have to be early as it doesn’t take long for this beautiful display to melt away in the sun, and the presence of the fungus becomes hidden once again.



Hofmann, D., Preuss, G., Mätzler, C. (2015) Evidence for biological shaping of hair ice, Biogeosciences, Available: https://bg.copernicus.org/articles/12/4261/2015/

Met Office, Hair Ice, Available: https://www.metoffice.gov.uk/weather/learn-about/weather/types-of-weather/frost-and-ice/hair-ice