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Weather Almanac

Weather Almanac for March 2010

THE FROST BEARD: HOAR FROST

As the amazing spectacle of the Vancouver 2010 Olympic Winter Games comes to an end, we in BC know that we will continue to have spectacular performances of power and speed and beauty of our natural environment to amaze us. We will have golden sunsets and silver sheens and linings always.

One such spectacle visited last Sunday morning. Although the day dawned foggy here in the Robson Valley, shrouding the peaks and ridges around town, by noon, things were changing. The ceiling lifted upward along the flanks, although the ridgelines and peaks still hid from view. As I left the house, I saw the trees in the yard and those on the mountain flanks gleaming in a white beard that rivaled mine. (The word hoar likely originates from a German word meaning “white or grayish with age” like an old man’s beard and might be the origin of the German title of respect Herr.) Close inspection revealed each twig end and needle sported a long white crystal that reminded me of a frozen white inchworm: hoar frost.

This piece is peppered—perhaps I should say salted”—with the best of the sixty-plus photos I took that day.

Hoar Frost

The definition of hoar frost as found in the American Meteorological Society Glossary of Meteorology states hoar frost is a “deposit of interlocking ice crystals formed by the direct deposition on objects.” “By direct deposition,” means that the water vapor turned directly to ice, the opposite process of sublimation where the ice goes directly to vapor. Such deposition conditions happen when the relative humidity is slightly over saturation (supersaturation) above the depositing surfaces. While this often happens under foggy conditions, hoar frost can form without fog under clear skies when the air’s relative humidity exceeds 90 percent and the air temperature falls below -8oC (18 oF).

Under such conditions, when there is a “frost seed,” a spot where the building ice crystal can attach, the vapor moves out of the gaseous state and deposits as an ice crystal. When the formation is rapid, the frost is opaque, grainy and dense. This ice formation is termed rime frost and is harder than hoar frost.

When the deposition is slow growing in steps or layers, the vapor builds crystals that are more intricate, and we have hoar frost rather than rime. Hoar is delicate and can form as a fern, feather, flower, or spike formation. Hoar usually forms in the light wind conditions of a radiation fog or frost.

Hoar frost crystals often initially form on plant tips, but can form on any other surface. In mountainous areas, hoar can form on the surface snow when the wind slowly wafts humid air over a cold snow surface. If such hoar layers are buried by snows, they produce dangerous conditions for avalanches as the hoar layer is weakly bound to the underlying and overlaying snow. When forces overburden this hoar layer, they can give way and release an avalanche.

Hoar frost can also form on ice surfaces on bodies of water, a version of which is termed “frost flowers” found on sea ice. Like all hoar frost, these flowers are delicate and often disappear when morning sun breaks through to touch them. When this happens, they sublime back to vapor, disappearing magically in a blink. Hoar frost often develops along the banks of fast moving streams or rivers when the water remains unfrozen and the air layer next to the shoreline becomes highly humidified.

Often, the ideal, stereotypical vista of a winter wonderland are forested mountain scenes of heavy hoar frost formation, even when we cannot see the surfaces all around sporting ferns, feathers, needles, and spikes of ice. Such was the day seen in the accompanying photos.

However, my first breathtaking experience with hoar frost came not in the winter dawning of a mountain morning. It came on a mid-Autumn morning on Vancouver Island’s Saanich Peninsula north of Victoria. That morning as I left the house, each blade of grass and the rest of the low landscape bushes and stems sported a fuzzy beard that reminded me of our Siamese cat’s tail when she got spooked. It actually looked like a hand’s depth of snowfall had fallen overnight. But as the sun’s rays slowly swept across the yard, the retreat of the shadow front marked the death of the surface hoar, and a return to green and brown.

Some call these hoar formations air hoar. The hoar that forms on a snow or ice surface is called surface hoar. An interesting form crevasse hoar develops when humid air seeps into a cold cave or crevasse. Since caves tend to be protective environments, crevasse hoar crystals can grow quite large, inches in breadth. Another interesting location for hoar frost formation is within air pockets in a loose soil and this is termed depth hoar.

Hoar Frost in Verse and Song

The beauty of hoar frost has inspired poets through the ages. One of the oldest came from the pen of Chinese poet Li Bai (Li Po) written in the Eighth Century:

The floor before my bed is bright:
Moonlight - like hoarfrost - in my room.
I lift my head and watch the moon.
I drop my head and think of home.
Translation: Vikram Seth, 1992, from 3 Chinese Poets

Scottish bard Robbie Burns wrote in “To A Mouse” (1785)

To thole the Winter's sleety dribble,
An' cranreuch cauld!

(thole = to endure; dribble = drizzle; cranreuch = hoar frost; cauld = cold)

Percy Bysshe Shelley in “The Invitation” saw the beauty of hoar as

The brightest hour of unborn Spring
Through the Winter wandering,
Found, it seems, the halcyon morn
To hoar February born;
Bending from Heaven, in azure mirth,
It kiss’d the forehead of the earth,
And smiled upon the silent sea,
And bade the frozen streams be free,
And waked to music all their fountains
And breathed upon the frozen mountains,
Making the wintry world appear
Like one on whom thou smilest, dear.

From D.H. Lawrence’s pen we find the lines from “Almond Blossom”

In the distance like hoar-frost, like silvery ghosts communing on a green hill,
Hoar-frost-like and mysterious.

In song, Ewan McColl and Peggy Seeger wrote in the “Moving On Song”

Born at the back of a blackthorn hedge
When the white hoar frost lay all around
No eastern kings came bearing gifts
Instead the order came to shift
You'd better get born in someplace else.

Bal Sagoth, a symphonic black metal band from Yorkshire, England, sang in “Spellcraft and Moonfire Beyond The Citadel Of Frost”

The Lord of Wolves haunts the forest,
In brooding winter's icy rapture,
Hoarfrost glimmers 'neath the moon...



Learn More From These Relevant Books
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Written by
Keith C. Heidorn, PhD, THE WEATHER DOCTOR,
March 1, 2010


The Weather Doctor's Weather Almanac: "The Frost Beard: Hoar Frost" and all photographs
©2010, Keith C. Heidorn, PhD. All Rights Reserved.
Correspondence may be sent via email to: see@islandnet.com.

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