Winter Wonderland: Hoarfrost greets Minnesotans today

Minnesotans awoke to a December winter wonderland today.

A blanket of fog froze on contact with trees, fences and other objects this morning as impressive hoarfrost deposits accumulated. Hoarfrost occurs when super-cooled water vapor freezes directly on objects.

Here's a more detailed definition of hoarfrost from the Glossary of the American Meteorological Society. (AMS)

hoarfrost

A deposit of interlocking ice crystals (hoar crystals) formed by direct deposition on objects, usually those of small diameter freely exposed to the air, such as tree branches, plant stems and leaf edges, wires, poles, etc.

Also, frost may form on the skin of an aircraft when a cold aircraft flies into air that is warm and moist or when it passes through air that is supersaturated with water vapor. The deposition of hoarfrost is similar to the process by which dew is formed, except that the temperature of the befrosted object must be below freezing. It forms when air with a dewpoint below freezing is brought to saturation by cooling. In addition to its formation on freely exposed objects (air hoar), hoarfrost also forms inside unheated buildings and vehicles, in caves, in crevasses (crevasse hoar), on snow surfaces (surface hoar), and in air spaces within snow, especially below a snow crust (depth hoar). Hoarfrost is more fluffy and feathery than rime, which in turn is lighter thanglaze. Observationally, hoarfrost is designated light or heavy (frost) depending upon the amount and uniformity of deposition.

Hoarfrost accumulation favors the upwind side of trees and other objects. Light southeast winds this morning created a delicate and beautiful coating of ice crystals on the upwind side of trees.

Slippery too

Unfortunately for drivers hoarfrost accumulates on roadways as well. That made for some slick spots as an icy layer of crystals deposited on roads this morning.

Weather conditions were perfect for hoarfrost this morning. Temps close to the dew point, and misty fog provided the ideal recipe for a frosty frocking.

Check out snowcrystals.com for more hoarfrost and all things ice crystals.

Forecast: Thaw on track

Our well advertised December thaw kicks in over the next 36 hours. Temps approach freezing late today, and cross the 32 degree threshold tomorrow afternoon. It still appears several days above freezing are ahead, and temps should peak near 50 degrees this weekend. The Twin Cities could stay above freezing, even at night for about 80 hours this weekend.

Enjoy the hoarfrost and the scant snow cover while it lasts.