|From Central Park in Winter by Thomas Nast, 1864.|
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.