Tamás Varga at Earthly Mission:
Hair ice is a type of ice that forms on dead wood and takes the shape of fine, silky hair. Also known as ice wool or frost beard, this natural phenomenon is pretty uncommon, having been reported mostly at latitudes between 45 and 55 °N, in broadleaf forests.
This unique type of ice forms on moist, rotting wood from broadleaf trees when temperatures are slightly under 0 °C (32 °F) and the air is humid. The smooth, silky hairs have a diameter of about 0.02 mm (0.0008 in) and a length of up to 20 cm (8 in). Although individual hairs are brittle, they usually take the shape of curls and waves.