Ice Feathers?

[mtruman]

There was lots of rime ice on all the vertical surfaces on Franconia Ridge on Saturday as well as ice/frost on the trees, etc. Many of the formations looked like the picture below (taken on Little Haystack). People were using several different names for these. I had originally been thinking "frost feathers" but realize that this is wrong. Are they "ice feathers"? Is there a proper name or are they just rime?

[krummholz]

Those are quite amazing. They look exactly like feathers, but quite heavy and apparently formed out of water ice instead of the very delicate frost feathers that you see sometimes. I hope someone can give us a good theory about how they formed.

[Knapper]

What you captured is rime but it also goes by "frozen fog deposits", "frost feathers", "ice feathers" or "feathers of rime". All are correct but just stating they are rime is easier and correct as well. They form on the windward side of mountains that were in fog that was below freezing. The feathers grow into the wind and are a great way to see how wind is being deflected off various surfaces. The feathers you saw are just like the ones we get on the summit. Sometimes these feathers can be several feet long. They form when supercooled water droplets the size of nanometers that make up the fog hit another surface that is below freezing and freeze on contact. The difference between rime vs glaze is mostly color as both can create feathers although it is less common to see with glaze. The white color is from oxygen being trapped in pockets while the rime is forming. Because they freeze so rapidly, there is very little holding the feathers together which is why they break so easily when forming. It isn't until they start to partially melt that they become hardened.

How do they form? It is easier to draw then explain but I will try. When water freezes it does so in a linear fashion. Snowflakes typically have six sides based on how water molecules bond therfore the needles stick out further than the sides of the hexagon because they formed on the points of the hexagon typically in straight lines away from the center. Rime starts on a surface as a single point that starts to grow out as more molecules slam into that point. Then, through various alterations of airpressure, air flow, temperature, humidty and imperfections forming on the riming surface, other molecules start to cohere to the "sides" of the rime that has already stuck to the surface. So, if it were forming in a x, y, z plain where y is a pole lets say and x is the gound below it with z transecting the plain. The rime starts to grow more or less parallel with x as it grows out but the feathers begin to grow out the sides in the z direction as they bond with the rime forming out. This creates a "t" shape for the rime but then rime starts to form on the extending arms in the z plain creating several more x, y, z plains that rime are forming on of these plains. So the rime starts to take on a 3-d growth pattern. There comes a limit to how far out these structures can form based on location, wind speeds, temperature, and cohesion when forming. Because the bonds are relatively weak between the various branches, if another bit of rime shears them, they break. If winds shift, they break. If they start growing into each other limiting the amount of droplets slamming into them, they stop growing in that direction or slow their growth in that direction (hence why sometime one side is longer than the other). Hopefully this all makes sense, it is easier to draw then explain as a picture is worth 1000 words or more.

[mtruman]

I knew that this was the right place to ask the question. Thanks for the explanation Ryan. It all does make sense and I'm sure that you're right that a picture would be the easier way to explain it. I also know first hand about the "breaking easily when forming" part as they frequently bounced off the side of my head in the wind as we passed. In any event it was really fascinating seeing all the different types of rime forming on the various surfaces...

[krummholz]

Knapper, that was a great detailed explanation. All that geometry does actually kinda make sense. So, if I am understanding correctly, the "feathers" have a narrow vertical shape because it is easier for the rime to form straight into the wind direction rather than to form perpendicular to the wind.

Also, I had somehow thought rime formed on the leeward side rather than the windward side, but I gather it has to grow into the wind because the wind is what feeds it, so to speak, what keeps sending ice molecules that way.

[Bill O]

Rime forms when liquid cloud droplets freeze unto sub-freezing surfaces. The reason it can take on so many different looks depends on the temperature and wind speed.

[mtruman]

Knapper, that was a great detailed explanation. All that geometry does actually kinda make sense. So, if I am understanding correctly, the "feathers" have a narrow vertical shape because it is easier for the rime to form straight into the wind direction rather than to form perpendicular to the wind.

Also, I had somehow thought rime formed on the leeward side rather than the windward side, but I gather it has to grow into the wind because the wind is what feeds it, so to speak, what keeps sending ice molecules that way.

I can definitely attest to it growing on the windward side. I would have thought that it went the other way as well. Interesting.

[Bill O]

Also, I had somehow thought rime formed on the leeward side rather than the windward side, but I gather it has to grow into the wind because the wind is what feeds it, so to speak, what keeps sending ice molecules that way.

That's right. We're most familiar with seeing rime grow on the windy summit of Mount Washington. But, on sub-freezing, foggy mornings in the valleys (which usually have no wind) rime will grow in all directions.

[Knapper]

Bill O, to some degree, is correct that "rime" that forms in the valleys does grow on most outside surfaces regardless of direction. But to wake up and see white and then say "wow, look at all the rime that formed" is not always true. Rime is usually a "rapid" process usually confined to events of freezing fog mixed with winds found mostly on mountains, arctic regions, or on the sea(according to our mountain meteorology book from college). In valleys, it is more common to see advection frost which is similar to rime but on a smaller scale with lower densities. But, what is usually found in valley locations is a variation of Radiational frost (or hoar type frosts) with a subset of four types: air, surface, depth and crevase. But to be really specific, it all comes down to the densities between the two with rime being denser and harder than hoar frost. And even before it makes that jump, there is a three point scale of light, moderate, and heavy hoar frost based on amount, uniformity and density of the deposits. So, when my friends say their cars were covered in rime, I usually correct them and say it was covered by just frost, hoar frost or poconip (which is a word used by native american tribes out west to describe the the white deposits and the white death freezing fog could bring).

[Bill O]

Agreed, I did forget that disclaimer. Rime in the valleys is very rare. I've seen it only a handful of times outside the mountains, and everyone one of those might not have been rime.