Dr. James Carter is a professor in the Department of Geography-Geology at the University of Illinois. One of his many interesting pages collects photos and other reports (dating back to 1884) of so-called “hair ice,” “haareis,” or (my fav) “silk frost.” The fibrous ice crystals seem to be caused by the pore structure of certain woods, and only forms where the bark has been removed. Reportedly, the phenomenon is reproducible: if you find a piece of wood growing hair ice, you can warm it up, then re-freeze it, and it will grow hair ice again. [via Neatorama]