What is the name for this strange weather phenomenon? See Details Below.
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ETpro (
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August 20th, 2011
It was a frigid winter morning in St. Paul Minnesota. The temperature had been around 30° below zero F. (-34.44° C.) overnight. The sky was crystal clear and crisp. A beautiful winter sun had already climbed well into it at around 8 AM when I arrived at work, and it had warmed things to a relatively balmy -20° F. (-28.89° C.).
I had noted on the drive in that there had been a heavy frost overnight. Only after pulling into the parking lot and opening the car door did I realize that the white coating covering the blacktop lot was nothing like ordinary frost. Instead, it looked like a paper-thin covering of snow, with distinct crystals like snowflakes giving it depth and texture not seen in frost. More bizarre, the snow was still falling out of a clear blue sky with nary a cloud in sight. You could actually see the crystals seem to pop into existence around 12 to 15 feet up in the air and grow as they fell toward the ground. At around 8 feet up, they stopped growing and just fell to the parking lot as tiny snowflakes.
I’m guessing it happens when the ground gets extremely cold overnight then warmer, more moist air flows in raising the dew point. The relatively cold ground must super-chill air just above ground level below its dew point, and you get something like clear air frost forming.
What did I see? None of my colleagues at work knew what it was. Some were so baffled they had to go look for themselves before they would even believe it was happening. Many of these were native Minnesotans. What did I see? Was it such a rare meteorological phenomenon that it doesn’t even have a proper name?
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12 Answers
Nevermind, I misread the details.
I’ve never heard of it or seen it, but your explanation sounds spot on.
This is similar to how fog is formed. There’s a temp inversion where warmer air is above colder air, but this time, the cold air is colder than usual.
All I can find is this story that suggests it is snow from a long gone cloud that simply took its sweet time finally landing. I also think I remember that contrails from high flying planes in super cold can cause a bit of snow. Too lazy to look for a link though!
It’s August and humid and I’m sweaty. Thanks for the winter Q, I feel better!
@Blackberry Thanks. That’s my best guess. Maybe I can post it on some weather science site and find someone who can confirm it.
@Cruiser I appreciate the link, and it was interesting reading. I can see how that could happen, but that fails to explain what I saw for two reasons. I didn’t mention it in the details, but it was an absolutely calm day. The crystals were falling straight down toward the ground. It had been a quiet night before as well, with no foretasted precipitation and stars visible everywhere. And two, I could see them form above my head. So could the others who came out to the parking lot to view it. So I wasn’t hallucinating either. :-)
It sounds like diamond dust which is usually seen in polar regions.
Resident Fluther meteorologist here. It could be rime. This is almost exactly the same thing as hoar frost, so good work, @Hibernate !
@flutherother : Diamond dust is a bit different, and it doesn’t usually accumulate on anything. It is generally frozen ice crystals suspended in the air. It is useful in creating “sundogs”, though.
Anyway, have a look at the article on rime and the pics of hoar frost. Pretty cool stuff. I miss the cold!
@Les Hoarfrost, or rime, forms on the surface of objects or on the ground. The crystals in this case were forming in the air iself, which is different.
Well, then you were probably in an inversion, where the temperature at the surface was colder than aloft. The air above you (where you say you could see the crystals forming) was a bit warmer than where you were standing (and warmer air can hold more water vapor than cold), so when it mixed with the cold, dry air, crystals could form. This is fairly common (albeit, only in the cold places), and is similar to diamond dust (but diamond dust tends to be much smaller than your typical “snowflake” sized crystals).
Doesn’t matter who was right or wrong here. It’s all good and peachy we learned some new stuff. I did not know about diamond dust till today [I always considered it to be hoarfrost].
Now @ETpro knows about both of them and how they look like. If he remembers the exact image of what it was looking like he can compare [or google for more] pictures.
Thanks all. @Hibernate One of the guys that worked with me at the time thought it was called hoarfrost too, but I looked that up. It forms on the cold surfaces where it is seen, not in the air. It looks like @Les‘s suggestion of rime is out too. Again, as @flutherother noted both are strictly surface phenomena. I am sure that it was a temperature inversion at work.
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