by Skip Via
skip@westvalleynaturalists.org
Over the past few years, we’ve presented several articles here on our web site dealing with different forms of ice–or more properly, frozen water or water vapor. Here are some:
- Hoar Frost or Rime Ice, or Something Else?
- Hoarfrost or Rime Ice?
- Graupeling with Winter
- Winter Wonders
In this article, we’ll take a look at another icy phenomenon–Needle Ice.
Needle ice occurs when the ground temperature is above freezing and the air temperature is below freezing. it most typically forms in loose soil high in silt and organic matter such as you might find along stream beds, in agricultural fields around loose rocks, and other places in which soil is deep and can absorb and retain lots of water. Water rises to the surface and then freezes into the needle-like columns that we see. The colder the air, the more needle ice you’ll see. Typical needle ice in these parts is an inch or two tall, but needle clusters as long as 8 inches have been found in the porous volcanic soil of Iceland. Needle ice here in the valley tends to be a fall phenomenon when cold air moves in and the ground surfaces are still above freezing.
Ice needles form perpendicular to the ground. Needle ice formations lift overlying materials (frozen soil, leaf cover, etc.) away from their original location. As the ice melts, the displaced matter typically doesn’t settle back into its original position. This combination can cause significant problems over time. In a field, it can lead to uneven surfaces sometimes called nubbins. If the needles form on a slope, the overlying material is deposited lower than its original location, enhancing erosion and soil creep or slope creep–a common phenomenon in Alaska. Needle ice can be a significant contributor to soil erosion along stream beds. Needle ice can also affect vegetation by lifting seeds and plants in the soil and redepositing them in such a way that they die or do not germinate.
Soil erosion from needle ice is not a significant problem in the West Valley area, although you can sometimes see its effects in the soil creep along some road cuts.
