The bare ground of the trail wound through dead leaves and patchy snow. At a short overhang in the trail, I noticed spiky threads of ice growing up from the soil in crunchy clusters. A careless boot revealed how fragile these formations are; the fine ice threads crumbled readily. This was needle ice, a common sight in the woods this winter.
Curious about the phenomenon, I got in touch with Dr. James R. Carter, a professor emeritus from Illinois State University. Dr. Carter has spent the last ten year observing these ice formations. His photographs and descriptions of different examples of needle ice are available here.
Carter explained that while needle ice somewhat resembles frost, it is a completely different phenomenon. Frost forms when water vapor is deposited onto a growing ice crystal; the water molecules move directly from the gas phase to the solid phase without ever becoming a liquid. Needle ice, on the other hand, forms from liquid water when a process called ice segregation occurs in the soil. » Continue Reading.