Category: "Film frost"

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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More frost patterns on black cars

January 7th, 2017

Black cars remain my favorite place to observe frost patterns. Here are a few I saw on one car today. 

More frost patterns on black cars

The straight lines of frost are more common when it is drier. And it has been quite dry here due to the very low temperatures. 

More frost patterns on black cars

 These pictures show an interesting mixture of straight lines and curved boundaries. 

More frost patterns on black cars

 And finally, the windshield had a pattern that resembled a hilly landscape. 

More frost patterns on black cars

But the pattern is actually quite flat. The frost is playing mind games on us, presenting an optical illusion of 3D topography. 

As with all cases of frost on surfaces, the ice initially got started when a thin layer of liquid (melt) froze in various spots. The ice that grew, first grew in the melt layer, then grew on top, essentially "sucking"** the vapor out of the surrounding air, thus drying out surrounding regions. This is why we see bare surfaces near the larger frost crystals. Those frost crystals grew from the vapor, just like snow, but are anchored to the surface because that's where the film froze. So, two types of crystallization are important: freezing of the melt (melt --> ice) then vapor deposition (vapor --> ice). 


- JN 

** The actual process is diffusion (the way perfume molecules reach our noses), but this term is a little more vivid.  


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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Choppy waves

January 11th, 2012

I thought this hoar frost pattern looked like rough seas. I see choppy, cusp-like waves down there.

Not having any pictures of rough seas, I hopped in our bathtub and kicked my legs around to make waves. Perhaps you can see a little resemblance.

However, the processes that caused the cusp-like, choppy wave pattern in the frost are completely different from those in the bathtub. Look more closely at the hoar-frost pattern:

It just goes to show you that hoar frost is never as simple as you’d think. Though from a distance it might look like white whiskers, up close it shows unexpected patterns. These patterns reveal something about how the crystals strained, twisted, and competed with each other.

Though it is true that the hoar crystals we see are built of deposited vapor molecules (invisible water molecules once floating in the air that happened to strike and freeze onto a cold surface), the story of hoar has an earlier beginning. In the beginning, the surface already had a thin film of liquid water. See the top panel in the sketch below.

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What We Sometimes Miss

March 8th, 2011

For the first few years in which I would excitedly go out on frosty mornings to photograph ice formations, I never paid any attention to frost on car bodies. Sometimes I would notice something on our car window, but that was basically it – I was essentially blind to ice in places where I didn’t expect to see anything interesting. Then one day, while returning from an area that often had fascinating puddles and ground ice, I walked next to a black car with the most stunning display of frost that I had ever seen. The car was completely covered roof, hood, and trunk with a thick, large, curvy white pattern of ice made distinct by the background of black underneath.

I spent the next hour or so taking pictures, returning home once to get another camera when my roll of film ran out. Though I understood roughly the processes involved, the initial freezing of a thin layer of water, making curvy ice patterns, followed by vapor depositing onto the frozen parts as hoar, making the ice white, there were other puzzling things that kept me entertained. However, the most puzzling thing of all was the fact that people would walk right by without even slowing down. Here was a strange and rare sight: strange because of the hastily dressed man (myself) leaning over a parked car with a tripod snapping pictures, and a rare yet striking display of curving frost in full view, and yet they paid me nor my prize no mind. It was as if I was the only person who could see the pattern.

The reverse thing happened to me just a few days ago. We had wet weather one day followed by a cold, clear night – perfect conditions for good hoary film frost. And indeed, many cars in our parking lot had beautiful curvy film-frost. I walked around, looking specifically for black cars, which show the most contrast to the white hoar, and photographed some on one car, but somehow overlooked the most amazing one of all: a speckled-seaweed-like pattern that I've seen only once before (see the Dec. 1 posting).

Even though the above was on a black car in a region I checked, I still missed it. But luckily, my neighbor caught it and emailed a few photos. In the image, some of the trails seem to cross over each other, but closer inspection instead suggests a coincidental merging of two trails on one side with a forking off on the other side.

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More Tales of Mystery and Observation

December 1st, 2010

When I stepped out early Saturday morning, the air seemed relatively warm, particularly compared to the cold snap we had last week. Indeed, it was much warmer, and yet the parking lot in our apartment complex had a glaze that was much more dangerous than that during the cold snap.

But it was the frost that I noticed. Of course, the air was relatively warm, but the clear sky cooled the surface. Recently, snow melted, leaving plenty of open water to evaporate – perfect conditions for film frost and hoar.

I saw “striped-tail” film frost(1) on two car roofs, both times on the sunroof glass. Why only on the glass? Perhaps the glass, being a poorer conductor of heat, had a lower surface temperature. This lower temperature would have produced a thicker film of water, and the thickness of the film seems to influence the pattern. The bigger mystery though is the cause of the pattern. In a short online article(2) last year, I suggested a cause for the stripes on each tail, though I can offer no explanation for either the nearly uniform width of the tails or the meandering. In the case below, the pattern is dense with lazily wandering striped tails. When I look at it, I think of seaweed.
The metal car surfaces often had nice curving film-frost with dense hoar, while others had or condensed droplets with more isolated, spiky hoar. The latter appeared on my own car.

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The Window of Many Cacti

February 14th, 2010
It’s been two weeks since our last frost, and judging from my first dozen shots, it seemed like my photography skills dropped from mediocre to downright pathetic. But then I got a few good shots of frost on windows. The fact that I shoot windows on cars means that I see some special forms that would not normally appear on house windows. The most common type I call ‘cactus frost’ because of its resemblance to saguaro cacti.

The resemblance may be a bit more obvious in the following shot, which I took two years ago on seeing this form for the first time.

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February 5th, 2010
Ripples in still water
When there is no pebble tossed
No wind to blow

--Grateful Dead “Ripple”

I don’t know where to start on this one. For some time I’ve been seeing concentric circular patterns on car windshields and car bodies – bands of white spreading out from a central point like ripples in a pond from a tossed pebble. Typically, they spread outward 3-10 inches or so before meeting up with ripples originating from another spot.

When I try to zoom in on the individual crystals, I usually see only vague outlines with the occasional recognizable form. What could cause this pattern? Here's a clue.

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Mystery of Whirlpool Hollows

February 3rd, 2010
I’ve seen this whirlpool pattern on two mornings on the same plastic side-mirrors of the same car.

The hollow columnar crystals are oriented lengthwise along concentric circles, which strongly suggests that an underlying film froze with the same rotating crystal orientation. This is strange. To see why I think it strange, we need to specify crystal direction. Consider the ice-crystal optic axis, the length-wise direction of the columns (or the direction straight into a stellar-star crystal). If we draw the optic axis for each crystal as an arrow, then we would have something like the following picture.

When a film of water on a smooth surface like glass or a car roof freezes, the preferred crystal orientation is that with the arrow pointing straight up and out of the surface. So, I find the above pattern mysterious - why don't the arrows have any trend toward pointing upward? Why do all  the arrows  stay in the same plane? Another mystery is the fact that I’ve seen this same whirlpool pattern with about the same center spot on both side mirrors on more than one morning. Perhaps the whirlpool pattern arises somehow because the surface is curved. Or maybe films of water on plastic freeze differently than films on smooth metal or glass. For now, I’ll call this the mystery of whirlpool hollows.

- JN