Coating trees, powerlines, and even fences, hoar frost creates a spectacular winter scene admired by many. So, what exactly is rime ice and how does it form?
This unique kind of ice formation is caused by frozen water vapor formed when fog develops while temperatures are below freezing. In order for the fog to form, we need both the temperature and dew point temperature to be equal. Whenever these two temperatures meet, fog is likely to form as water vapor is condensed. There is a unique occurrence though when the temperatures are below freezing, which creates a type of ice known as rime ice.
The water vapor that becomes condensed is held in a state that keeps it in liquid form. These supercooled water droplets remain liquid while suspended in the air until they come into a contact with a solid object that is below freezing. Once they make contact with trees, power lines, etc. the water droplet instantly freezes and crystalizes, becoming a solid while attaching itself to the surface of that object. While roads are not usually impacted by this phenomenon, unless freezing drizzle is also present, most objects located above-ground will be coated in a prickly layer of rime ice.
The ice doesn't tend to stick around long, especially if sunshine is present during the day. It will usually melt within a matter of a few hours after sunrise. It certainly creates quite the view, though!
Meteorologist Andrew Stutzke
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