RELATED UPDATE 2/9/09: One of our Photo Gallery users uploaded a video of hoarfrost falling from trees like snow, beneath a blue sky. I hadn't even thought of that possibility because typically wind is calm on nights with hoarfrost.
RELATED UPDATE 2/3/09: I finally got some pictures of hoarfrost here in Central Pennsylvania, though they don't hold a candle to those shown in the article below:
RELATED UPDATE 1/22/09: Ironically, the morning after I published this article, King5.com reports that "freezing fog" (probably accurate -- looks like rime to me from their slide show) has knocked out power for a couple thousand folks in Washington State. A photo gallery user has uploaded a photo of hoar frost there too, so I think we may be seeing both phenomenons in the same state. Here's a video of the damage:
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The first is called Hoar Frost and the Associated Press has uploaded some excellent examples of this phenomena during the last week, when Europe was experiencing an unusual cold wave:
Of course, they got the caption wrong both times - hoar frost comes from neither ice nor snow precipitation - it is ice crystals (frost) deposited on an object, according to WIkiPedia, which says that the process by which frost forms is Deposition, the opposite of sublimation; a NOAA dictionary says that it is sublimation. Either way, it is the same process, and it's the same for regular frost too -- water vapor becomes solid without going through the liquid phase). I'm not a moisture expert, and what isn't clear to me is why sometimes you get the regular, common frost that we all see all winter, and why you sometimes get the rare displays shown above. I suspect it has to do with the temperature, with lower temperatures allowing the complex displays to form. The photo below is one of those available on our 2009 Photo Gallery Calendar:
I'm honestly not sure if this is Hoar Frost or its cousin "Rime" but I'm leaning towards Hoar Frost. There is a distinct difference between Hoar Frost and Rime, which truly only occurs in very cold, wet locations such as the tops of mountains, for example Mount Mitchell in North Carolina and Mount Washington in New Hampshire. Rime (which some people also call Rime Ice) is technically a type of frost as well. It is characterized by a more icy, hard texture and is the result of freezing fog sticking to objects as the wind blows it by an object.
For that reason, rime is generally associated with only one side of an object, while hoar frost is not - it forms in calm air and is generally found 360 degrees around things. That's why I'm assuming that the photo below, taken in Washington State earlier this week, is rime rather than hoar frost.
Below is an example of a formation that is definitely Rime, taken by at the top of Mount Washington, New Hampshire. This is an extreme example which breaks the rule above because it never gets warm enough to melt at Mt. Washington, and accumulates over time with winds from different directions.
Below is a time-lapse video from Mount Washington showing rime accumulating over time (perhaps it should be called a "rime-lapse video"!
Rime is different yet from ice accumulated via sea spray, which you see often at cold coastal locations (the most famous of course being the oft-emailed 'Swiss Freeze"'), in fact Photo Gallery user BIGSILVER just uploaded this good example of where NOT to mount your satellite dish (taken last month):
Here's another - the AP took a picture of this phenomenon late last week after record cold hit New England:
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