Sunday, January 31, 2010

Hoar Frost

After a night inside the cabin, the dogs are waiting impatiently by the door for their walk.  Their excitement is  "cooled" when they hit the 9 degree air but they plunge ahead into the snow, driven by physiologic needs that come from 10 hours without a tree.
Snow covers the ground except for where yesterday's tire track has exposed the gravel drive.  Their needs met, the dogs are tugging me back towards the warm cabin, but there on the drive lie delicate crystals of hoar frost.  The magical appearance of these small branching crystals is lost on them.
Whether this evaporated from a  lake or a distant ocean,  these water molecules traveled in the air unseen except as clouds until they suddenly found themselves transformed into delicate crystals hanging on small rocks in a deep valley.  The moist air, touching on the much colder frozen ground created these tiny icy fragments which never condensed into water.
According to Wikipedia"Frost is the solid deposition of  water vapor from saturated air. It is formed when solid surfaces are cooled to below the dew point of the adjacent air."  Not particularly poetic but scientifically accurate.  "If a solid surface is chilled below the dew point of the surrounding air and the surface itself is colder than freezing, frost will form on the surface. Frost consists of spicules of ice which grow out from the solid surface."
There are many kinds of frost.  If the freezer door isn't tightly closed, you get frost (and a few frostier words from your wife when she defrosts it.)  Hoar frost-scientifically called radiation frost-forms on the ground or other solid objects that are below the freezing point.  Cold but clear nights like last night, with its bright full moon, let heat escape from the ground.  Flood frost refers to this cold air which then moves downhill into valleys such as ours on Bull Creek, the perfect setting for this hoar frost.

A polar opposite (pun intended) condition can produce frost flowers that occur when there is a sudden freezing spell  but the ground is not already frozen. The freezing air forces out water contained in the plant stem into long frozen ice ribbons.  (See the December 5, 2009 blog.)
Photographing these delicate little crystals, I think back to Bill Bryson's words in his wonderful book, A Short History of Nearly Everything.
"Of the 3 percent of Earth's water that is fresh, most exists as ice sheets.  Only the tiniest amount- 0.036%- is found in lakes, rivers and reservoirs, and an even smaller part- just 0.001% exists in clouds or vapor."
How fortunate to have these crystals of frozen water vapor visit my drive.

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