ROANOKE, Va. – The cold has been nasty to deal with over the past few days, but it has brought a couple of different phenomena not typically seen in Virginia.
NEEDLE ICE:
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This photo of needle ice sent in by Kevin Schrecengost shows ice basicaly shooting out of the ground. Essentially, that is what is happening.
This phenonemon occurs when the ground is saturated and above freezing, but the air is below freezing. The water at or near the surface will then freeze as it interacts with the below freezing air. Through a process known as capillary action, the water in the ground is brought to the surface and freezes. Capillary action is the ability of a liquid to flow in narrow spaces without the assistance of, or even in opposition to, external forces like gravity.
As the water freezes and expands, the ice is forced up thrugh the dirt. The soil will need to be loose or porous to be pushed up throught the ice. The brown color on top of the ice is the dirt forced up by the ice.
HOAR FROST:
This looks similar to the rime ice that developed across the region a couple of weeks ago, but it is different. This picture sent in by Becki Crowder Wells shows the feathery creations on the blades of grass.
Hoar frost develops when the frost point, the temperature at which frost develops, is below freezing. There must some moisture in the atmosphere for this kind of frost to develop. For development, the water vapor in the air will go directly to the solid state. This is also known as deposition. The result is feathery, spikey ice creations on grass, trees or anything else the frost wants to develop on.