Needle ice

Needle ice is a phenomenon that occurs when the temperature of the soil is above 0 C and the surface temperature of the air is below 0 C. The subterranean liquid water is brought to the surface via capillary action, where it freezes and contributes to a growing needle-like ice column.

Needle ice requires a flowing form of water underneath the surface, from that point it comes into contact with air that is below freezing. This area of the process usually occurs at night when temperature peaks its low point. From then on, it produces the needle like structure known as "Needle Ice".

The ice needles are typically a few centimetres long. While growing, they may lift or push away small soil particles. On sloped surfaces, needle ice may be a factor contributing to soil creep.

Alternate names for needle ice are "frost pillars" ("Säuleneis" in German), "frost column", "Kammeis" (a German term meaning "comb ice"), "Stängeleis" (another German term referring to the stem-like structures), "shimobashira" (霜柱, a Japanese term meaning frost pillars), or "pipkrake" (from Swedish pipa (tube) and krake (weak, fine), coined in 1907 by Henrik Hesselman).

The similar phenomena of frost flowers and hair ice can occur on living or dead plants, especially on wood.

Formation
In order for needle ice to form there needs to be a process of Ice Segregation, which only occurs in a porous medium when supercooled water freezes into existing ice, growing away from the ice/water interface. As water permeates the ice, it becomes segregated into separate pieces of ice in the form of lenses, ribbons, needles, layers or strands of ice.

Needle ice is commonly found along stream banks or soil terraces. It is also found by gaps around stones and others areas of patterned ground. The variety of soil properties also affects where it is found. Places where the soil is much deeper and richer can affect the growth of the ice. Consequently, the deeper the soil, the larger the water content allows it to develop. It can be evidently formed anywhere where underground water is exposed to open (freezing) air.

Needle ice is most suitable in soils with a high silt and organic matter content. Needle ice consists of groups of narrow ice slivers that are up to several centimeters long. The largest recorded needle ice was at 10 cm in length.

Needle ice grows up slowly from the moist and water-penetrable soil, and melts gradually in the sun. It can vary in appearance but always shows the consistent growth of ice perpendicular to the land surface. Needle ice looks like a bunch of filamentous crystals, and is in straight or curve shape. It usually forms in the morning when temperature drops below freezing point (0 °C).

Erosion
The emergence of needle ice has been recognized as a geomorphic agent of soil disturbance. The growth of needle ice lifts a detached, frozen soil crust riding on top of the layer of ice. When the crust and the ice melt, the soil surface settles back irregularly. This process is what loosens the soil surface, destroying the cohesion of the soil. Once all of it dries, the wind can just remove the soil particles resulting in erosion. This erosion is most commonly seen on banks, which has a major effect on them. Needle ice tends to move rocks in the soil upwards toward the surface. Then, the soil becomes infertile in those areas.

Plant growth
Needle ice affects the growth of plants. Seedlings are often heaved to this surface by needle ice. When the ground hardens the stems and roots of the seedling, they are gripped by the soil and then the formation of needle ice is what pushes them up and out the ground. When the needle ice melts, the seedlings do not settle correctly back into the ground causing these seedling to die. Even if the seedlings are partially heaved by the needle ice, they can still die due to root desiccation.