Warm sunshine peeked under the brim of my hat, hovering low and to the south on its way to the winter solstice. The wind was brisk, though, stealing hard-earned heat through my thin running clothes. Just an hour ago, the breeze had whisked tiny snowflakes into a frenzy. In this transitional season, it feels like the thermometer is bouncing up and down like a toddler at Christmas. More than once, lately, I’ve stepped outside and been surprised.
The gravel road ended at a gate, but I followed mountain bike trail signs around it to the two-track beyond. Clubmosses trailed among the pine needles, and chickadees scolded overhead. Here, out of the wind, I had to take off my hat and gloves.
Then: Crunch! Crunch! Crunch! Here was another surprise, this time under my feet. The old roadbed was filled with fragile clusters of ice, pushing up through the soil. My clumsy feet had pulverized some, but in several places the ice remained beautifully sculpted into ribboned clusters a few inches high. Squatting down for a better look, I noticed soil particles, moss fragments, and grass blades frozen into the ice.
“Needle ice” seems to be the most scientific term for this phenomenon, but I’ve also heard it called frost pillars, frost castles, and ice filaments. The Swedes, Germans, and Japanese have their own words for this circumboreal art form, too.
While not confined to one region or habitat, needle ice does require a certain set of conditions in order to form. First, the soil must not yet be frozen, at least beyond the first thin crust. In contrast, the air temperature needs to be below freezing. Finally, the soil needs to have plenty of moisture, and just the right sized pores between the grains so that water can flow toward the growing ice.
What draws the liquid water toward the ice is a process known as ice segregation. Supercooled water – held in a liquid state below 32 degrees F – moves toward ice and adds on to it. When the two meet, ice grows away from the ice/water interface.
As the ice crystals expand upward, growing perpendicular to the surface, they may also push soil up or away, lift small pebbles into the air, and incorporate whatever debris is nearby. This fragile structure of ice and dirt is what crumbled under my running shoes. I’m not the only source of destruction, though. Once these frosted soils melt, they are loose and susceptible to erosion. If the needle ice forms on a slope, even just the action of lifting soil particles up and letting them down again will cause them to descend in the process of soil creep.
Only certain soils are frost-susceptible, though. Very dense, clay soils do not promote water flow, and clean sand is too well-drained. The ratio between pore size (the space between the grains) and particle surface area must be within a range that promotes capillary action. This is the ability of the water to flow through narrow spaces without the help of gravity, and often against it. When the nurse pricks your finger and captures your blood in the tiny tube, it is capillary action that causes the flow. Silty and loamy soils–with particle sizes in between clay and sand–tend to be the most frost-susceptible.
Knowing if a soil will promote ice needles is important if you’re going to build a road on top of it. As the freezing front descends deeper into the soil, the ice growth spreads horizontally into a lens shape, and can lift the soil above it up a foot or more in a frost heave. If just one place in the road is pushed up, the surface cracks, and a pothole forms.
While we may curse that deep and destructive side of frost, the aerial side is more enchanting. Similar processes of additive crystal formation can create beautiful ribbons of “ice flowers” on the stems of plants, or flowing “hair ice” growing on soggy wood. When ice attracts moisture out of thin air, we wake up in a magical, white world covered in hoar frost.
These interesting phenomena are most common in the transitional seasons, in weather that can surprise you from moment to moment. The surprise I’m waiting for now is waking up to a foot of skiable snow!
|photo by Jared Stanley|