Hoar Frost vs Rime Ice
HARRISONBURG, Va. (WHSV) - Some of us may have woken up Tuesday morning and seen what looked like a thin blanket of snow on the trees and the ground. But, it was clear all night, so what was it?
In the winter, there are two weather phenomena that would cause the beautiful ice accretions to develop overnight, known as hoar frost and rime ice. They may look very similar, but they form under very different circumstances (well, besides the fact that it’s quite cold).
Hoar frost, which is what we saw across the region Tuesday morning, forms on clear nights with little-to-no wind and very cold temperatures. As the Earth radiates its heat away into space at night, the surfaces of many objects near the ground become super-cooled, where temperatures drop very rapidly. Once the water vapor in the air comes into contact with these objects, they cool so quickly that they totally bypass the liquid phase and freeze on contact. This allows ice crystals to form over these objects, and what we get are beautiful ice-ridden landscapes.
Rime ice, on the other hand, forms when fog develops in areas where the air temperatures are below freezing. You’ll often hear meteorologists use the term “freezing fog” to denote when rime ice may occur. Fog is made up of tiny water droplets, as it is essentially a cloud near the surface. So in this case, water is already in the liquid phase. When these water droplets come into contact with objects that are below freezing, the droplets freeze to form ice. One of the tell-tale signs of rime ice formation is the formation of ice along one side of an object, almost as if the wind blew the ice in one direction.
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