Posted at 7:00 AM on December 12, 2007
by Paul Huttner
It’s a winter wonderland outside the Weather Lab this morning. The trees are frocked in white, looking like an early Christmas present today.
It's called "hoarfrost." It happens when ice crystals are deposited directly on surfaces like trees and fences that are exposed to the humid sub-freezing air.
Conditions were perfect overnight with ice fog around the metro, and as you wake this morning take a look out and enjoy the beautiful Currier and Ives display around the metro.
Sublimation is the state change from solid to gas.
The opposite of this is deposition or desublimation.
That term is even in your hoarfrost definition. All frost is deposition.
You are correct, per my post above:
"It's called "hoarfrost." It happens when ice crystals are deposited directly on surfaces like trees and fences that are exposed to the humid sub-freezing air."
And from the AMS definition of sublimation below:
sublimation—The process of phase transition from solid directly to vapor in the absence of melting.
Thus an ice crystal or icicle sublimes under low relative humidity at temperatures below 0°C. The process is analogous to evaporation of a liquid. Colloquially the terms are used interchangeably for the solid–vapor transition (evaporation). For growth, the term sublimation has been replaced by deposition since the 1970s. There is evidence that deposition nucleation does occur, although there may be an adsorbed layer prior to nucleation. It appears that most nuclei in the atmosphere require near–water saturation before they initiate ice.
I caught the info on Hoarfrost on ALC this evening; I didn't know if had an actual term (although I should have guessed). I was out enjoying my hobby this morning and happened to grab a photo just after sunrise. The frost was on everything and quite striking to be around. Thanks for this info!
Along with the hoarfrost, there was also a nice (or an ice:-) light pillar display along 494 in Bloomington that morning before sunrise.