Hoar Frost

    Monday morning brought us near zero temperatures, along with it some magnificence displays of hoar frost. Those of us who live on the banks of Lower Wisconsin River enjoyed a thicker coat of frost. It's created by the rising mist of the river water which is warmer than the air. After I dressed in an extra layer of warm clothing for a walk to the riverbank additional winter wonders were revealed. The dirty looking snowbanks were covered with a light dusting of snow. The river in my area doesn't fully freeze over but the ice along the shore reaches out to the strong current. Looking just beyond the ice in the mist were dark shadows floating down river. A moment later they swam into an opening, as I thought they were a mix of common mergansers and mallard ducks. Farther out in the mist was the familiar honking of Canada geese. Across the river high above in the icy trees was the black silhouette of a bald eagle. It had already left the roost area to begin the hunt for the much needed meal to help produce extra body heat. Today the cloud cover saved the hoar frost from the rays of the rising sun.

    The name hoar comes from Old English, meaning showing signs of old age such as white hair. Checking some facts about hoar (radiation) frost I found that my above statement is only partially correct. The last step in creating air (vertical) hoar frost involves nighttime temperatures causing trees, plants and other objects to lose heat. When their surface temperature is below the air temperature ice crystals begin to collect on the branches. There are several forms of hoar frost but another one that we can relate to is surface hoar. Anyone ice fishing or skating will at sometime come across clear ice with surface hoar frost on it. It will appear as fern like crystals spreading across the ice before being covered by snow. Most of us will have seen the surface type on window panes or windshields. A weather man will tell you this all has something to do with the dew point. We don't often hear the term dew point in the winter forecast. Hoar frost and summer dew are cousins. The beautiful intricate designs of dew covered spider webs or plants have gone though a similar process at warmer temperatures. A couple photos from my river bank.

  

Looking west toward
Ferry Bluff

 Looking northeast
toward Sauk City











 
    Dew covered objects can be admired from a distance alone but take 
the time to check out the frost crystals close-up.