Hoarfrost and Yukimarimo

Hoarfrost patterns are often described as being feathery, needle-like or spiny. But this morning in a double-glass window that is no longer perfectly sealed, there was hoarfrost with a fiddle-head pattern. ( If we have to insist on comparing one aspect of nature to another!) Here’s what I saw in a time span of about 10 minutes, with and without a sunrise in the background.

 

 

Hoarfrost forms when a surface’s temperature has lost enough heat to be below freezing, and at the same time an envelope of surrounding air is supersaturated with water vapour. Supersaturation  is a term used to describe greater than 100% relative humidity, a condition that arises when there is a lack of condensation nuclei. Similar to the way dew forms, the initial stages of hoarfrost formation at times feature liquid droplets. But the drops are supercooled somewhere between -1 and -2o C and eventually freeze as the temperature dips into the -3 to -5 o C range. Other parts of the hoarfrost, however, may go through a direct gas-to-solid transition.

In Antarctica, hoarfrost can form under similar conditions on the surface of snow. But crystal formation at low temperatures(< -59o C ) can cause electrostatic attractions between crystals. A gentle wind can then free hoarfrost and due to the static forces, the crystals clump together into intriguing iceballs. These have been dubbed yukimarimo, —again from our human tendency to compare one unfamiliar observation to something witnessed before, as marimo is a ball-like growth of water algae in Japanese.

Here is some yukimarimo filling a footstep.

beauty_footprint
A boot-print filled with yukimarimo. From antarcticsun.usap.gov

On the same day that I learned about yukimarimo, we were walking in the park, a week after an ice storm. It was difficult to take a step without cracking the snow’s icy crust. In the “pothole-footprints” left behind by previous visitors were tiny little balls of snow. Was this  yukimarimo on a smaller scale? Not at all actually. Before I even had a chance to check Environment Canada’s weather site for the absence of precipitation since the ice storm, I noticed the the pellets were scattered in various spots, regardless of the type of surface. Recently there had been a brief, localized episode of sleet. But in the same way that we pin similar labels on different observations, we have a strong tendency to apply a recently learned concept to unrelated phenomena. And if we do not catch ourselves in the act, then as Holmes warned:

‘It is a capital mistake to theorize before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts.’

-A Scandal in Bohemia

 

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Sources:

American Meteorological Society http://glossary.ametsoc.org/wiki/Supersaturation

Weather online www.weatheronline.co.uk/reports/wxfacts/Hoar-Frost.htm

T. Kameda , H. Yoshimi, N. Azuma and H. Motoyama (1999). “Observation of “yukimarimo” on the snow surface of the inland plateau, Antarctic ice sheet“. Journal of Glaciology45 (150): 394–396.

The Antarctic Sun antarcticsun.usap.gov/features/contentHandler.cfm?id=2068

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At the Mercy of Diffusion

Both the mechanism and appearance of hoarfrost are sublime. Hoarfrost results when water vapor encounters a very cold surface and undergoes reverse sublimation. Such ice crystals normally grow on the ground, but it can also form on twigs or on a poorly insulated pane of glass.The latter is usually called  “window frost” because of its flatter appearance. But it’s no less interesting.

103hoarfrostEnergy considerations maximize hydrogen bonding and determine the hexagonal nature of snow crystals, yet it will not predict the pattern of window frost. In this picture above, there are parts that resemble radial spider webs; other sections look like the chaotic streets of downtown Boston. And there are artsy stickmen and elongated patterns resembling those of airport runways.

In order to grow, ice crystals need a “seed”, and each imperfection along the surface of the glass provides the nucleation site. The distribution of dust or soap residue is rarely uniform. Scratches are random. And who’s to say that every point on the surface is exactly at the same temperature? All of these differences are make each hoarfrost event unique.

If we go back in time, we can ask: why was the water vapor that crystallized evenly distributed in the air when it evaporated from concentrated sources? As water molecules evaporate from a living body or from a body of water, they are initially concentrated among themselves. As a result, they are more likely to collide with molecules of their own type. But the collisions send them in random directions. Slowly, air molecules which are themselves bouncing all over the place find themselves mixed in that thinning concentration of water molecules. As long as temperature is high enough, there’s not much of a force discriminating among water vapor and other air molecules. Everything eventually becomes evenly mixed, similar to but faster than the way a dye can spread evenly in water without any human stirring.

Of course, a cold surface slows all molecules that collide with it, but now the fact that attractions between water molecules can be much stronger than those between air molecules becomes significant and leads to the loss of that freedom of random motion that they previously “enjoyed”.

It’s similar to the blogosphere. So much of what is typed, even though it may be advertised through a hashtag or by word of mouth, comes across billions of other thoughts and messages, and the ideas of a specific blog spread thinly among the vast volume of what’s in real or virtual libraries. The content must find the right surface under the right conditions to crystallize in some other mind. And even then, like the frost itself, there’s usually no permanence. Most often, it does not even get to that stage. Our words are like molecules at the mercy of diffusion.

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