Category: "Hoar frost"

Hoar Frost on Plastic

February 17th, 2020

The pattern of hoar frost depends on not just the current and prior conditions, but also on the surface. If the surface is less attractive to water, that is, more hydrophobic, then the hoar crystals tend to be more further spaced apart.

Hoar Frost on Plastic

Or even more widely spaced apart. (Click on an image to enlarge it.)

Hoar Frost on Plastic

These two images show hoar frost on plastic surfaces (garbage bin lids). If you look closely at the above picture, you will see that the hoar columns are growing off little mounds. These mounds are frozen droplets. You'll also notice that the directions of the ice columns are not random, but instead a given ice column tends to be pointing in nearly the same direction of its neighbors.

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The Curious World of Ice and Snow: Part 1 of 3

February 4th, 2020

In 2012, I gave a "science cafe" talk with a local series sponsored by the Pacific Science Center, KCTS public television, and Science on Tap. The title was "The curious world of ice and snow". The location was a bar in Kirkland, but open to all ages. When I showed up with my family, they tried to seat us in the backup room, the regular room having filled up, but I said "Oh, well I'm the speaker" and they kindly created a space for my family in the regular room. I was indeed surprised at the crowd. People are apparently more interested in ice than I thought. (Hmm, but where are they when I post here?)

The Curious World of Ice and Snow: Part 1 of 3

Click on any image to see an enlargement.

The basic structure of each talk was to give a lecture of about 30 minutes and then allow up to an hour (I think) for the Q&A. In my excitement, I had created 41 slides, in retrospect too many for the allotted time.

Given all the time spent preparing the slides, I hope that by posting them here that even more folks can enjoy the images and discussions. But, instead of unloading all of them on you at once, I will break the discussion into three sections. By adding the following table of contents, each section will have 14 new slides and the total will be 42, which according to Douglas Adams* is a really special number.

The Curious World of Ice and Snow: Part 1 of 3

The contents of this section is part "1", written in green font.

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Martini Hoar (raise a tiny glass?)

October 19th, 2019

The hoar-frost crystal shoots up like a thin, solid straw, then suddenly opens up into a cup-like shape. I have seen it often enough to give it a name: "martini hoar".

Martini Hoar (raise a tiny glass?)

The cup can be weirdly segmented and polyhedral, but it nevertheless widens suddenly. Here are a few more (Sorry for poor photos—someday, I hope, I'll get better about photography.)

Martini Hoar (raise a tiny glass?)

Here is a larger view of the region. Note the similar hoar coming down from the top, but without a clear view of the base.

Martini Hoar (raise a tiny glass?)

This sudden widening feature has bothered me for awhile, but I was delighted the other day to figure out a plausible reason. My delight was made even greater because the reason involved measurements I made in the lab two decades back. The measurements were to understand snow-crystal habit, but apply equally well to hoar frost because hoar grows just like snow except it is attached to the ground.

Now that I have viewed some of the older pictures I took, the actual growth phenomenon looks more complicated in terms of crystal shape, so I am not so sure my reasoning explains things so simply. Nevertheless, it should apply well to many cases, and at least is worth learning because it involves important growth principles that also apply to snow.

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Puddle gets its grooves (upon freezing), part I

February 16th, 2019

These grooves appear on the surface of frozen puddles.

Puddle gets its grooves (upon freezing), part I

Most grooves are straight lines, and most of these also appear to have relatively symmetric sides, such as those marked #s 1, 2, & 4 in the above image. But some, such as #3, have sides that are much wider. Grooves can intersect each other at a "T" intersection, such as #1 and #3, or they can cross, as a 4-way intersection as in #2. Some, such as #4 start and end on nothing apparent and intersect no other grooves.

Sometimes the sides show small steps or sub-grooves, particularly on the wider sides, such as on #3. On grooves that curve, such as in #1 and #2 in the image below, one side can have a fern or foliage type of texture.

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Film frost grains and radiative cooling of the ground

December 10th, 2017

December 8th brought the first frost to the Seattle area. This doesn't mean that this is the first time this season that the ground reached 0 degrees C or lower. True, we had gotten snow in late November, though this by itself doesn't mean the ground reached 0 C because snow deposition differs from frost formation. The snow was below zero when it formed in the air, but for the rest of its existence, it could have been melting and the ground itself likely sped up the process by remaining slightly above 0 C. But in contrast with the snow, for frost to form, the ground itself must cool below 0 C.

When I used to put out some metal plates with recording thermocouples, I didn't see visible frost until about -5 C, but that was based on just a handful of measurements. Anyway, what this means is that we may have had a few mornings with some patches of ground (including anything connected to the ground) dipping slightly below zero but with no obvious frost appearing. Also keep in mind that "ground temperatures" reported at weather stations are 1.5-2.0 meters above the ground, and thus may be 5-10 degrees C warmer at night than some ground patches. Why? At night, the ground cools by radiating, and if the atmosphere is not radiating much down, then considerable cooling happens. This is why clear nights are the ones with the most frost or dew.

Anyway, here is one shot of some of this "first frost" on my car window.

Film frost grains and radiative cooling of the ground

The image shows patches of different texture. These regions differ in texture because they are tiny hoar-frost (i.e., vapor-grown) crystals of differing size or orientation. That is, they stick up differently in different patches. They stick up differently because they sprouted off of a thin layer of film-frost that had a different crystal orientation. So, the patchy look comes from the different grains in the film-frost. See some of my previous posts with diagrams about this phenomena (category: "film frost"). Here is one with particularly helpful diagrams.

-- JN

Concentric Film Frost

October 25th, 2017

October 16, 2017, the first frost of the 2017-18 winter as far as I know. Frost in Eastern Washington anyway, those of us on the west side of the Cascade Mts have not gotten any yet. Marc Fairbanks, an avid nature observer from the mountains near Cle Elum, sent me the following shot (click on the image to fully appreciate the pattern).

Concentric Film Frost

This is primarily film-frost, that is, is based on ice that formed when liquid film froze. But as I discussed in a previous post, some important growth processes here also occur through the vapor phase:

I had even earlier noted some "ripple" patterns on a windshield and mused about their formation:

As I'll discuss next, there are some other illuminating observations here.

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Glaze-rime Ice Buildup Facing Creek

November 23rd, 2014

Here's a reed of some sort, with a glaze of clear rime on the side facing a creek. How did the clear ice get there, and what is the white ice on top of it?

The fact that the clear ice is just on the side facing the creek indicates that the ice came from droplets blown off the creek. To have the ice buildup just on the side means that the droplets must have frozen soon after landing, but slow enough to spread out and fill gaps.

So, the droplet must have been larger than typical cloud droplets, and the temperature must have been within a few degrees of freezing. Clearly, the flowing water must have been above freezing, so the droplets must have cooled to below zero while drifting through the air.

A closer shot shows that the outermost ice, the white ice, is hoarfrost. Only hoarfrost would show the flat crystalline facets. Definitive proof that that ice grew out of the vapor that was blowing by. You can also see some hoar on the other side of the reed.


A Change in Habit

November 23rd, 2014

The two main growth modes of snow and hoar (i.e., ice growth from the vapor) are columnar and tabular. In columnar, the shape is long and thin, like a pencil or cluster of pencils (bound snugly with a rubber band). In tabular, the shape is plate-like or fan-like. When a crystal form has an easily expressed shape, like a column or plate, we say it has a certain habit.

In hoar frost that had been growing for several nights, I saw some that had changed their habit.

On the first night, when the vapor was more plentiful, large columnar forms appeared. This habit grows between -3 and -8 C. On a subsequent night, the temperature got colder, below -8 C, leading to tabular growth. More vapor exists out near the ends, so this is where we see the tabular habit.

The arrows above show the fast growth directions. There is a transition that sometimes happens when both of the main growth directions are fast, making a flare, or trumpet-horn shape (other times, it may be more of a cup or vase). Such a transition is far more common in hoar than in snow, because, I believe, hoar on the ground can experience a much higher excess of vapor.

The dark region in the background is the water of a creek.


Smaller Hoar Growing on Larger Hoar

November 22nd, 2014

Cold air came down the interior, spilling through gaps in the Cascades, cooling the western side. In the Seattle area, we got our first frost on Nov. 11, but the days stayed cold. Areas in the shade never lost their hoar, so the hoar frost kept growing.

But does a hoar-frost crystal continue growing the same way as on the previous night? Does it keep extending uniformly, getting larger and larger, keeping the same general shape?

Or do smaller hoar crystals form on previously grown larger crystals?

A few days later, I found myself walking in Maple Falls near Mt. Baker. In a shady pocket near my feet, I saw some hoar. Subtly different -- it appeared like small, translucent leaves encrusted on their edges with hoarfrost. But it turned out that the "small leaves" were actually tabular hoar (plate-shaped). That is, hoar encrusted on hoar: smaller crystals on larger crystals.

When frost resumes growing on previously grown frost, the other possibility may also happen; that is, the hoar just resumes growing the way it did the previous night. But I think this is generally quite rare because it would require very slow and constant growth conditions, which is generally incompatible with the diurnal temperature cycle. See the sketch below for both possibilities.

Note that I write above 'smaller hoar crystals grow on the larger ones' even though technically, they are all the same crystal. You can see this is true by the fact that their faces all line up.

The view that got me to look closer:

- JN

Crystal-to-crystal “communication” through vapor and heat

January 5th, 2013

Two mornings ago, I saw this on the windshield of a parked car.

The bulls-eye pattern wasn’t centered on any particular feature on the windshield, and there were similar, though less developed, patterns nearby. See them on the photo below.

The dark parts are largely frost-free regions, and thus are regions that dried out during the crystallization event. (It was still dark when I took the shots.) Later, under a brighter sky, I saw a different pattern on the hood of another car, a pattern that I figure has a similar cause. In the hood case, shown below, the dry regions are bright due to reflection of the sky.

Now, about those concentric rings …

I puzzled over a larger such pattern in the Feb. 5, 2010 posting “Ripples”. The causative process that I proposed back then seems consistent with this newer observation, but I will clarify it here.

Before the first frost formed, the windshield, though seemingly dry, nevertheless had a thin layer of liquid water. This thin film had cooled to some temperature below freezing. (See the sketch “How hoar frost forms” in the Jan. 11, 2012 posting.) As the windshield and water film continued to cool, freezing was inevitable; the only issue was where. The first such spot to freeze must have had some feature, however minute, that was advantageous to freezing. It may have been a nucleant particle (e.g., a mineral dust grain, a type of bacteria, or even a tiny fleck of ice that wafted in from somewhere else), a slightly cooler spot, or a slightly thicker water film (e.g., from a scratch or indentation). But for whatever reason, the ice formed there first.

Now suppose the ice spreads outward from the nucleation spot in all directions with about the same rate. I suspect this happens when the film is extremely thin, as it would have been under the relatively dry conditions of this day. So, the frozen film grows outward, roughly as a disc. See the sketch below; the black dot in the center is the first place that froze.

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