Jack Frost’s Latest Artwork

From Central Park in Winter by Thomas Nast, 1864.

Earlier this week a visitor from the north -- an arctic airmass -- brought bitter cold from the North Pole and Siberia to the Laramie Basin.  Jack Frost showed up too, creating his distinctive artwork wherever conditions were right.  Like Jokul Frosti (Icicle Frost) who lived during the time of the Norse Vikings, Jack wandered through the Basin just before dawn decorating window panes, door knobs, windshields, fenceposts and twigs.  Conditions along the Laramie River were perfect and Jack left a trail of hoarfrost sculptures, including the frozen floral offerings I found floating on river ice.

I shot photo after photo as I walked along the riverbank early in the morning.  Later  when I downloaded them and looked closely, I realized that these were not frost flowers but clusters of tiny frost ferns that had grown up just that morning, as water vapor from the river froze on the cold ice.

Like dew, hoarfrost forms when air becomes supersaturated and has to give up moisture.  This water vapor can collect as dew on a cool summer morning or as hoarfrost when temperatures are below freezing.  (Note -- frost is frozen water vapor not frozen water.)  Hoarfrost develops through depositional growth as water vapor continues to freeze on the growing ice crystal.  Snowflakes form the same way, and like snowflakes, hoarfrost crystals come in many different shapes.

The hoarfrost sculptures on the Laramie River were indeed ferns -- fernlike stellar dendrites.

Air temperature and degree of saturation (humidity) determine the type of hoarfrost crystal or snowflake that forms.  According to the following chart, conditions along the Laramie River that morning were sufficiently cold and humid to produce elaborate stellar dendrites.  (Dendritic growth forms are those that branch repeatedly.)

Needle clusters form in warmer conditions with less humidity.  Here's one shot of several taken by tuxable.com in the swamps and rivers of Alberta.

Continuing down the river I found more of Jack's beautiful arrangements of ice, hoarfrost, water and ice floes.

The hoarfrost was short-lived, the sculptures started to melt as the sun warmed the air and were soon gone.  But it is only early December, more arctic air masses will come our way, and Jack will be back.