Originally Posted by
JamesNoBrakes
Regular frost also goes from the vapor state to the solid state. It's just the extent of the crystals and that hoarfrost only forms by deposition. You are trying to somehow claim that hoarfrost doesn't adhere because it uniquely goes from a gas to a solid? While the crystals can be broken quite easily, they are still attached at the base. See my comment at the end.
No, not all frost goes directly to the solid state:
Window frost
Window frost (also called
fern frost or
ice flowers) forms when a glass pane is exposed to very cold air on the outside and warmer, moderately moist air on the inside. If the pane is not a good
insulator (for example, if it is a single pane window), water vapour condenses on the glass forming frost patterns. With very low temperatures outside, frost can appear on the bottom of the window even with double pane energy efficient windows because the air convection between two panes of glass ensures that the bottom part of the glazing unit is colder than the top part. On unheated motor vehicles the frost will usually form on the outside surface of the glass first. The glass surface influences the shape of crystals, so imperfections, scratches, or dust can modify the way ice
nucleates. The patterns in window frost form a
fractal with a
fractal dimension greater than one but less than two. This is a consequence of the nucleation process being constrained to unfold in two dimensions, unlike a snowflake which is shaped by a similar process but forms in three dimensions and has a fractal dimension greater than two.
[8]
If the indoor air is very
humid, rather than moderately so,
water will first condense in small droplets and then freeze into
clear ice.
Similar patterns of freezing may occur on other smooth vertical surfaces, but they seldom are as obvious or spectacular as on clear glass.
No one is saying that hoarfrost is not a meteorological term or phenomenon, only that it's simply the extent to which the crystals grow based on higher humidity.
Yes, zerozero said hoarfrost is an urban myth that came out of the bush. I was responding to him.
It's still frost. You are attempting to make it out to be something it is not. It still adheres to surfaces just like any other frost...because it is frost.
Well then describe the adhering mechanism for two solids that come into contact. Also if all frost is the same then why use a different term for it?
The reference that we were asking for was the one that says it doesn't adhere to surfaces.
Without spending a lot of time on finding better texts (and I hate to use this reference because they misused the term sublimation):
"Hoar frost occurs when a sub-zero surface comes into contact with moist air. The water vapour in the air turns directly into ice by sublimation, forming a white crystalline ice coating which can normally be brushed off."
https://www.skybrary.aero/index.php/Hoar_Frost
If you want to use the cold wing scenario, a cold-soaked fuel tank can make frost deposit directly on the fuel tank area even when there's no frost in the environment otherwise and the air temperature is well above freezing. It's the same mechanism. It adheres and no, it doesn't just brush off.
WHAT?? Completely different mechanism. The cold fuel cools the warm air around the wing to the saturation point and water vapor condenses into water where it attaches to the surface which is below freezing so the condensed water then freezes into ice or frost adhering to the wing.
"there's no frost in the environment" this sentence just doesn't make any sense. There's always some water vapor in the air.
Due to the low sun and cold temps, hoarfrost often forms on our car windows in the winter time, even in the daytime, so we spend a few minutes scraping it off. Forms on the metal body as well. Sometimes it's damn near impossible to get off without heating the car from the inside to at least partially melt it. If it behaved as you claim, I could just blow it off with my breath.
That's not hoarfrost. See above. The windshield radiates to cold space on a clear night thereby cooling it below ambient temperatures. The air immediately above the window is cooled, saturates, and water condenses onto the window where it then freezes, adhering to the windshield. Ambient air temperature can be above freezing.
The only situations I've observed in Alaska where ice does not adhere is dry snow when it's below freezing
Exactly!
I've never seen a form of frost that can be brushed or polished off. I've seen plenty of people try...but it's still there after they attempt to brush/polish it.
I've seen it and it done many times. I've even watched it sublimate away once we started taxi'ing in a Piper. It is very common in drier climates.
You seem to be implying the "polish smooth" thing about it...
I have no idea what you mean by this. The whole point of this argument is if the hoarfrost in the pictures that FAASTER posted would prevent the wing from providing adequate lift. Read any aircraft icing source and they will describe that ice either roughens the surface or deforms the shape of the airfoil enough that it disrupts and detaches the airflow from the wing. I made the point, actually several points, that 1) the hoarfrost is not rough (enough) and those pictures clearly show surface features under the frost such that it's probably 1/32" thick or less and 2) other surface imperfections are much more significant than the thin, light, and conforming layer of hoarfrost yet the FAA hypocritically ignores these.
So just to be clear, you are saying that the wing in the photos posted is unsafe and will not generate adequate lift?
Look up turbulators.