We woke up to a beautiful winter scene on Monday morning thanks to widespread hoar frost, which coated everything from trees to powerlines. So, what exactly is hoar frost and how does it form?
Hoar frost is caused by frozen water vapor formed during clear nights when fog develops while temperatures are below freezing. This was the case Monday morning when temperatures were in the teens, and the dew point was also in the teens. 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 frost known as hoar frost.
The water vapor that becomes condensed it 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 thick layer of frost.
Fog can form anytime the air temperature reaches the dew point temperature, even when below freezing like we saw Monday morning.
As for the term "hoar" it comes from the Old English word "har", meaning gray, venerable, old. The term was chosen as it is meant to express the resemblance of the white feathers of frost to an old man's beard, per dictionary.com. The frost 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