The White in the Woods

The white patches have appeared in the woods again.  They only come on these sorts of nights – the clear, still, punishingly cold ones, when the stars glint.  Looming out of the dark, in the corner of your eye, they look like rubbish – like plastic bags strewn across the woodland floor among last year’s beech leaves.  It is an incongruous sight: the glaring whiteness punctuating the black before dawn.  But although it appears amongst the leaf litter, this phenomenon is anything but man-made trash.  A closer look reveals something that’s natural and almost unbelievably beautiful.  It’s feather frost.

Exquisite, iridescent filaments of ice, sparkling in the torchlight, each an individual strand.  They have sprouted in incredible profusion from twigs and branches that lie on the ground, splaying out like the finest hair, like Father Christmas’ beard, like the fake silky mane of a child’s toy horse.  On longer twigs these tendrils of ice often surge out laterally, then curve down from a central parting, an effect that increases the resemblance to human hair.  The soft, slow explosion of ice splits the dead bark, which peels back to make way for this remarkable winter bloom.  If you pick up one of these twigs without due care, or run a finger down it, the strands of ice coalesce and fall away, revealing clean yellow wood and surprisingly clear-cut ends that look like they have been trimmed with scissors.  Doing this feels like sacrilege, and I generally try not to dislodge the fronds of ice but leave them intact, to melt in their own time.  Sometimes I will pick up a frost-flowered twig and bring it indoors to show my children, who are used to that sort of thing.

From stubby twigs or chunks of wood, ones that were especially replete with water, come the most spectacular flowers of frost.  These are the ones that splay out in all directions, making a coiffured confection of delicate ice.  Sometimes gravity, and perhaps the gentlest breath of wind, swirls them into candyfloss curlicues that resemble nothing else in the natural world.  On the very best specimens, it’s impossible to see the wood whence all this unbelievable ice has emanated.  A ridiculous pompom of teased and twizzled whiteness nestles in the crunchy leaves like a formless creature from a vintage psychedelic children’s programme.

Feather frost is an attractive name, and the one I prefer for that reason, but the pedant in me has here overlooked the fact that it doesn’t resemble feathers and is not really frost.  Technically frost derives from water vapour that originates in the air, then freezes on contact with objects.  This phenomenon results from water inside sodden wood that freezes in a certain way, in the right conditions.  The phenomenon is also known as ‘frost flowers’, ‘hair ice’, ‘ice wool’ or ‘frost beard’ and doesn’t seem to be especially well known or well documented.  It’s fairly unusual, but probably not as rare as we might think.  Many people I have spoken to have never seen or even heard of it, which is a shame, as it’s a treasure to marvel at, a piece of pointless joy in an increasingly utilitarian world.  I want to spread the word.

The place where I live is the only one where I have ever seen feather frost, though it undoubtedly occurs elsewhere.  This spot is suitable because it receives high rainfall, is relatively sheltered and situated in a valley bottom close to a river, where the air is humid.  It also is one of the last parts of the valley to receive the morning sun.  For feather frost to form, conditions must be just right.  According to studies, the latitude has to be between 45 and 55 degrees North.  There needs to have been plenty of recent rain (not a problem on Dartmoor) and lying branches or twigs, not too rotten, but fallen in recent months and nicely waterlogged.  Occasionally frost flowers sprout on the branches of trees, where the wood is dead but not yet thoroughly rotten.  My theory is that well-rotted wood does not retain the clearly defined pores, known as medullary rays, underneath the bark that the strands of ice must sinter through for feather frost to properly form.  The wood must be from a deciduous tree, as softwoods don’t contain the necessary rays.  Here our flowers form on beechwood.  The night should be still, clear and cold, but not too cold: around -1 or -2 degrees Celsius.  Any wind would dislodge the filaments of ice as they grow.  I’ve never stayed out all night to watch the growth of frost flowers, but I picture them pushing through the pores like pasta through the holes of a very slow, very fine spaghetti machine.

How do the ‘hairs’ of ice stay separated and distinct, rather than forming into a single chunk of ice, which is what usually happens at these temperatures?  The reason is surprising.  Like many questions in nature, the answer is a fungus.  In this case it’s Exidiopsis effusa, a waxy, greyish, low-growing species, a fact that was only determined as recently as 2015 by a team of scientists in western Germany, which discovered that it was this species of fungus that enabled the filaments of ice to segregate distinctly as they emerged from the rays in the wood.  Water near the surface of the wood freezes on contact with the cold air; more water is drawn by suction outwards away from the wood, where it too becomes ice, adding a new layer, and so on, incrementally through the night.  The chemical properties of the fungus inhibit the crystallisation processes of the ice, maintaining the slender, distinct nature of each filament.  The scientists found that in more than half their sample, Exidiopsis effusa was the only fungus present.  They then analysed the ice itself and found that it contained the compounds lignin and tannin, by-products of the life processes of the humble fungus that enables the formation of this extraordinary phenomenon.