Monday, December 28, 2020

When Jack Frost comes a’calling

hoarfrost

In folklore and literature, Jack Frost is often portrayed as a mischievous guy, sort of Old Man Winter’s younger self. He’s a personification of everything cold. In our region he’s a busy guy, at least for half of the year.

And an artistic one.

He gets credit for painting the trees orange and yellow and red in the fall. And we’re all familiar with ground frost, that harbinger of winter that looks like a dusting of snow. This phenomenon occurs when the temperature of objects near the ground falls below freezing. Water in the air freezes onto objects, sometimes as what looks like frozen dewdrops, sometimes as branched crystals.

Other times, Jack Frost picks up another brush to load everything with the lacy, feathery designs of hoarfrost.

Hoarfrost derives from the old English word “hoary,” meaning, getting on in age. It has the power to excite the poet in us. When you wake on a cold morning and look out to see the entire world — trees, bushes, your car — draped with lacy, feathery crystals glinting in the sunlight, it’s magical. The word “fairyland” comes to mind.

According to John Goff, the lead meteorologist at the National Weather Service Office in Burlington, hoarfrost is a “common occurrence” across the northern tier of the US, but almost nonexistent in areas with dryer, warmer climates. To form, hoarfrost requires a supersaturated column of cold air extending well above the surface of the ground.

Think a cold fog. Moisture in the air starts condensing around nuclei — particles of dust, for instance. Once that starts, the moisture in the air goes directly from a gas to a solid. It’s a process called sublimation, with ice crystals building up on everything, said Goff.

Hoarfrost appears as intricate, delicate and ephemeral crystal decorations. Jack Frost at his finest. These crystals reflect light from all their surfaces and thus look white. When the morning sunlight hits it … Wonderland.

Hoarfrost has one other requirement for formation: calm air conditions. It’s the calm air that allows those complex lacy depositions, said Goff.

That makes it different from rime ice, which also occurs on trees and structures.

Rime ice on Whiteface courtesy ASRC Whiteface Mountain Field StationRime, said Goff, forms when “you have super cold water vapor.” With rime the liquid water in the air essentially freezes into crystals on surfaces, building up and up and up. Unlike hoarfrost, rime forms when it’s windy, and its sideways formations often reflect that.

In the eastern US, rime is most common at elevations from 3,000 to 7,000 feet, said Goff, where you can get a fog bank (aka a cloud), very cold temperatures, and high winds. The fog droplets freeze quickly, producing much smaller crystals than, say, freezing raindrops.

The crystals build up on the windward side of objects, building a somewhat spongy, porous layer of heavy ice.

You can find impressive rime sculptures on the summits of the Appalachians from Mt. Mitchell in North Carolina through the Great Smokies, in the Adirondack High Peaks, at northern New England’s Mt. Mansfield and on the Presidential Range in New Hampshire.

Rime gets credit, along with fierce winds, for pruning subalpine and alpine trees on New England’s high peaks into krummholz, a German word meaning crooked wood. It can also form on aircraft. In this scenario, instead of super cold, moisture-laden winds depositing rime onto stationary objects, a jet flies at hundreds of miles an hour into a supercooled, moisture-laden cloud. Planes can pick up a lot of ice that way and it can affect their lift.

Rime ice is also common “on sea-faring vessels in northern oceans around the world,” said Goff. The ice can weigh down ships and prove hazardous to navigation. It’s not “water vapor freezing out of the air though,” he said. “It’s freezing ocean spray, of course. Nonetheless it’s still rime.”

But hoarfrost? Well, even if you’re not a fan of winter, it’s hard not to be impressed. People like posting hoarfrost photos to social media.

Meteorologists, however, don’t give it as much thought. “In the world of weather it’s a non-impactful event,” said Goff. “It’s more of a gee-whiz type of thing.”

Joe Rankin writes about forestry and nature. He lives in Maine. The Outside Story is assigned and edited by Northern Woodlands magazine, and sponsored by the Wellborn Ecology Fund of New Hampshire Charitable Foundation.

Photos from above: Hoarfrost courtesy Tig Tillinghast, and rime ice on Whiteface courtesy ASRC Whiteface Mountain Field Station.

Editor’s note: This first ran Feb. 23, 2018 on the Almanack.

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One Response

  1. Jim Fox says:

    Just today, a Canadian friend on Facebook had a photo of white fluffy filaments on rotted wood that looked like insect cocoons. Her post: “The other day the temperature dropped to near zero. In the forest while walking the dogs we came across these amazing things. They are forms of ice clinging to a certain type of wood that was lying broken on the ground.” A friend of hers identified it as ice-hair. and included this article. by European Geosciences Union

    Hair ice mystery solved
    Hair ice was found in a forest near Moosseedorf, Switzerland. Credit: Christian Mätzler
    You may have never seen or heard of it, but hair ice – a type of ice that has the shape of fine, silky hairs and resembles white candy floss – is remarkable. It grows on the rotten branches of certain trees when the weather conditions are just right, usually during humid winter nights when the air temperature drops slightly below 0°C. Now, a team of scientists in Germany and Switzerland have identified the missing ingredient that gives hair ice its peculiar shape: the fungus Exidiopsis effusa. The research is published today (22 July) in Biogeosciences, an open access journal of the European Geosciences Union (EGU).

    “When we saw hair ice for the first time on a forest walk, we were surprised by its beauty,” says Christian Mätzler from the Institute of Applied Physics at the University of Bern in Switzerland. “Sparked by curiosity, we started investigating this phenomenon, at first using simple tests, such as letting hair ice melt in our hands until it melted completely.”

    Then Mätzler, a physicist, joined forces with a chemist (Diana Hofmann) and a biologist (Gisela Preuß) in Germany. Inspired by earlier work, and by photographs of hair ice sent in from various countries, the team performed a set of experiments to figure out just what conditions are needed to grow this type of ice and what its properties are. In the process, they confirmed a 100-year-old theory for the origin of hair ice.

    Alfred Wegener, of tectonic-plate fame, was the first to study hair ice. In 1918, he noticed a whitish cobwebby coating on the surface of hair-ice-bearing wood, which his assistant identified as fungus mycelium – the mass of thin threads from where mushrooms grow. He suggested there was a relation between the ice and the fungus in the wood. Some 90 years later, Gerhart Wagner, a retired Swiss professor who has been researching hair ice for decades, found evidence of this relation: treating the wood with fungicide or dunking it in hot water suppressed the growth of hair ice. But the fungus species and the mechanism that drives the growth of hair ice was yet to be identified.

    Hair ice mystery solved
    Hair ice is growing on a branch in a forest near Brachbach, Germany. Credit: Gisela Preuß
    That was the aim of the researchers who have now published their work in Biogeosciences. Preuß studied samples of hair-ice-bearing wood collected in the winters of 2012, 2013 and 2014 in forests near Brachbach in western Germany. She analysed the wood samples using microscopic techniques and identified eleven different species of fungi. “One of them, Exidiopsis effusa, colonised all of our hair-ice-producing wood, and in more than half of the samples, it was the only species present,” she says.

    Mätzler, on the other hand, performed experiments designed to better understand the physics of hair ice on samples he collected in a forest at Moosseedorf, Switzerland. He found, confirming guesses by other researchers, that the driving mechanism responsible for producing ice filaments at the wood surface is ice segregation. Liquid water near the branch surface freezes in contact with the cold air, creating an ice front and ‘sandwiching’ a thin water film between this ice and the wood pores. Suction resulting from repelling intermolecular forces acting at this ‘wood-water-ice sandwich’ then gets the water inside the wood pores to move towards the ice front, where it freezes and adds to the existing ice. “Since the freezing front is situated at the mouth of the wood rays, the shape of the growing ice is determined by the wood rays at their mouth,” says Mätzler.

    “The same amount of ice is produced on wood with or without fungal activity, but without this activity the ice forms a crust-like structure. The action of the fungus is to enable the ice to form thin hairs – with a diameter of about 0.01 mm – and to keep this shape over many hours at temperatures close to 0°C. Our hypothesis includes that the hairs are stabilised by a recrystallisation inhibitor that is provided by the fungus.”

    Play
    This is a time-lapse of hair ice growth taken at the Gletschergarten in Luzern, Switzerland. Credit: Erich Albisser/Gletschergarten Luzern (http://www.gletschergarten.ch/)
    Hofmann then studied the hair ice itself. Her chemical analyses of the melted ice showed the water to contain fragments of the complex organic compounds lignin and tannin. Since these are metabolic products of the fungal activity, this finding further confirms the biological influence on hair ice. “These components may be the ones preventing the formation of large ice crystals at the wood surface,” says Hofmann.

    The researchers say a reason why it took almost 100 years to confirm Wegener’s hypothesis is that hair ice is a somewhat rare and fleeting phenomenon, spotted mainly in broadleaf forests at latitudes between 45 and 55°N. “Hair ice grows mostly during the night and melts again when the sun rises. It’s invisible in the snow and inconspicuous in hoarfrost,” says Preuß.

    Keep an eye out next time you go for an early morning walk in the woods. If you can find hair ice, you are in for a treat.

    Hair ice mystery solved
    Close-up image of melting hair ice. Credit: Christian Mätzler
    Explore further
    Researchers devise new way to monitor ‘health’ of ice shelves
    More information: Hofmann, D., Preuss, G., and Mätzler, C.: Evidence for biological shaping of hair ice, Biogeosciences, 12, 4261-4273, DOI: 10.5194/bg-12-4261-2015, 2015
    Journal information: Biogeosciences
    Provided by European Geosciences Union
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    https://phys.org/news/2015-07-fungus-responsible-peculiar-ice-filaments.html?fbclid=IwAR3yokVKeuuWpI5-tCSP-0h2z9DCgBwqnOWv20UaFwYf775s6JPolKzvVGo

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