Hair Ice

A visit to the North Cardiff ridgeway during the brief cold snap last week (remember it?) revealed several rather beautiful patches of 'hair ice' on dead wood in a sheltered valley.

I was with a few others but none of us had seen this before, though we'd all recently read about it in connection with the cause of this strange phenomenon. It turns out that it was as recently as 2015 that the mystery was solved. A team of German scientists analysed samples of wood from beneath hair ice and found one fungus species common to all of them: the crust fungus Exidiopsis effusa. An online article (see here for link) provides this explanation for the mechanism involved:

"Liquid water near the branch surface freezes in contact with the cold air, creating an ice front and sandwiching a thin water film between this ice and the wood pores. Suction resulting from repelling intermolecular forces acting at this wood–water–ice sandwich then gets the water inside the wood pores to move towards the ice front, where it freezes and adds to the existing ice,” the scientists explained"

I think I can just about get my head around that!

I've since heard that there is some dispute about whether E. effusa is the sole causal organism, but that just complicates an otherwise neat story!