Needle ice – This is a phenomenon that occurs when the soil temperature is above freezing (32 degrees F) and the air temperature is below freezing. Moisture in the soil is drawn up to the surface through capillary action where it freezes, building columns of needle-like ice crystals. If warmer daytime temperatures are not cold enough to freeze the ground, but then drop at night below freezing, it can create the right conditions for these exotic crystals to form. To form, the soil has to have a high water content and there are typically repeated freeze-thaw conditions for several days.
Needle ice is different than hoar frost, which is formed by water vapor from the air freezing and crystallizing on the surface of the soil or snow. I saw this yesterday on the trail at Curtis Farm Preserve. I'm sure the rain has long since washed it away, but more could form with the cold temps predicted over the next few days.
(Submitted by Lynn Knight, December 14, 2019)