Hair ice fungus: what it is and how this rare magical winter sight forms

At first glance hair ice doesn’t look like much. But against the dark, mouldering litter of wind-fallen twigs, branches and leaves, a pearly luminescence catches the curious eye, drawing you in for a closer look. Only then do you notice the wispy weirdness. A fluff that looks more fabric than ice, a perfectly coiffured beard of fine threads radiating out from the dead wood. What could it be… fibre-optic candyfloss?

This is exactly how it happened for me one crisp winter day in the Scottish Highlands. It was a moment right up there with more obviously exciting encounters I’ve had with eagles and dolphins. But why is hair ice so rare, or indeed relevant to this column? After all, it’s just ice, right? Well, technically, no.

What is hair ice?

Hair ice is most often seen on a winter’s morning, having grown the previous night on rotting wood. It only occurs when several other factors are just right: moisture, atmospheric conditions and temperature. And there has to be a secret extra ingredient – a fungus called Exidiopsis effusa.

Rather brilliantly, this micro magic was first discovered by Alfred Wegener, the same man who noticed continental drift, one of Earth’s most macro phenomena. In 1918, he spotted that wood that hosted this strange ice had a delicate filigree of cottony fibres on its surface – the mycelial threads of a fungus – and postulated that this was relevant to the ice formation.

How is hair ice formed?

The hair ice itself is formed of a multitude of threads, each less than 0.02mm in diameter, which seem to grow from the bare wood (not bark) of dead deciduous wood of several species.

The air temperature needs to be just under 0°C, and colder than the wood, which cannot be frozen. It turns out that the diameter of each ice thread coincides with the diameter of the tubes in the dead wood and is formed by a process of exudation.

Until recently, how the mechanism worked was a mystery. But in 2015, a Swiss study led by Dr Christian Mätzler officially made the link to the Exidiopsis fungus.

His team learned that, under certain conditions, a process called ice segregation occurs. This is when cold air freezes water on the surface of the wood, sandwiching a thin layer of water between the ice and the rotten wood. At this ‘ice-front’, the water in the pores of the dead wood expands and draws more water out from the pores on the wood’s surface, where it in turn freezes and adds to the base of the growing ice filament.

Traces of the metabolic products of the fungal activity on the dead wood – lignin and tannin – were found in the melted hair ice. It is thought that these act a little like anti-freeze proteins, preventing big crystals forming on the surface of the wood and also acting as a stabilising agent.

The alchemy that produces this stunning curiosity is nearly as beautiful as the collective ice threads themselves. It’s a real winter wonder, seldom seen – not just because the conditions required are so specific, but also because the effect is almost invisible if there is snow or frost on the ground. If you find it once, it’s worth making a note of the location, as the phenomenon may repeat itself over several years.