by Melanie B. Fullman, US Forest Service

The other morning, we awoke to a wondrous sight: half-inch hoar frost that clung to everything – tree branches,  bird feeders, last summer’s grass stalks, the door to the wood shed – and SPARKLED in the early morning sun. It was amazing!

So what IS Hoar Frost?

If you recall that most compounds can be found in 3 forms – as a gas, a liquid, and a solid – you might also remember that if you cool a gas, you get a liquid, and if you keep cooling the liquid, you get a solid. In the case of water, that solid is ice. Hoar frost is a result of the direct conversion of water vapor (a gas) to ice crystals without going through the liquid phase.
 
The temperature at which air is totally saturated is called the dew point. When water vapor condenses into liquid, the result is raindrops and dew. If water vapor condenses directly into ice, we get snowflakes and frost. If the dew point is above freezing, dew forms. But if the air is sufficiently dry that the dew point is below 32°F, the water vapor converts directly into ice, creating hoar frost.
Hoar frost is very different from frozen dew. Frozen dew is dew that has frozen after it was formed. So frozen dew looks like frozen water droplets, whereas hoar frost is a delicate icy structure.

Furry Ice

Hoar frost is basically an intricate build-up of interlocking ice crystals – a white, 'furry' form of ice. These crystals come in a huge variety of shape – needles, cups, plates, fern, and feather – depending on the temperature at which they developed. The size of the crystals depends on the time, temperature, and amount of water vapor available.
Hoar frost also has different names, depending on where it forms. Surface hoar are crystals that grow on snow, ice, or already frozen surfaces. Air hoar is a deposit of hoar frost on objects above the surface, such as tree branches, plant stems, wire, etc. Depth hoar refers to cup shaped crystals formed in dry snow, beneath the surface.
The spectacular hoar frost we experienced last week was mostly air hoar. Air hoar forms on tree branches, plant stems, leaf edges, wires, poles, etc. The longer the conditions that favor hoar frost, the longer the crystals. You’ll need to look  quickly though: air hoar tends to be the least durable of the frosts, susceptible to light winds and the slightest bit of sunshine. As soon as the crystals or the air around them warms up, evaporation from the crystal surfaces dissolve them away.

Beautiful, Dangerous? Crystals

The most common form of hoarfrost is surface hoar. This consists of ice crystals that form overnight on top of snow. The sparkles we often see on a clear morning here in the North Woods are the reflections off the facets of surface hoar crystals. This type of frost occurs when our snow is warmed during the day, then cooled at night. Since the night air cools the surface of the snow more than the inside, water evaporates from inside the snow and re-crystallizes on the surface. By morning, the snow surface is covered with a layer of sparkling ice crystals.

Given their delicate appearance, it might surprise you to learn that surface hoar is a common cause of avalanches. If cold clear nights with light/no winds follow the formation of surface hoar, the crystals remain intact. Buried by subsequent snows, the new upper layer is essentially laying on a very weak layer of ice crystals. The stage is set, then, for a skier or snowmobiler to trigger an avalanche. The best way to prevent a tragedy is for backcountry adventurers to dig a quick snow pit and examine the layers of snow that have been deposited. The surface hoar crystals, now frozen in place, are still easy to identify. Turns out, a quick look for beauty can save the day.

Other Forms of Frost

Rime is a deposit of ice that forms when the water droplets in freezing fog or mist stick to the outer surfaces of objects. Unlike hoar frost; rime is created by the water vapor first condensing to liquid droplets and then freezing on contact with anything it touches. Not surprisingly, areas prone to heavy fog are also prone to rime.
“Soft” rime formations look similar to hoar frost, with white ice needles and scales. Soft rime is similarly fragile and can be easily shaken off objects. Hard rime is formed under the same conditions as soft, but with a harsher wind. Hard rime has an icy solid appearance, sometimes in bizarre, wind-driven formations.
Window frosts are those lovely, fern-like creations that we’ve probably all seen on the inside of windows. Window frost forms when a pane of glass is exposed to below-freezing temperatures on the outside and moist air on the inside. Water vapor from the air inside the building condenses as frost on the inside surface of the window. Window frost often makes elaborate patterns as the crystal growth is strongly influenced by the window surface. Scratches, residual soap streaks, etc., can change the way the crystals accumulate and grow.
 
Although the result is undoubtedly pretty, the cause is considerably less attractive. Window frost is technically an indicator of a poorly insulated window. Window frost was more common in the past, when most houses had single-pane windows. Newer, double-pane windows are much better insulated and thus not so cold. If you have fern frost on your windows, you’re probably paying a price for them…

The formation of frost flowers, or "ice flowers," can occur in the late fall or early winter when the ground is not yet frozen. As freezing sap in the stem of some plants expands, it causes long, thin cracks to form along the length of the stem. Water is then drawn through these cracks and freezes upon contact with the air. As more water is drawn through the cracks it pushes the thin ice layers further from the stem, causing a thin "petal" to form. In the case of woody plants and tree branches, the freezing water is squeezed through the pores of the plant into long thin strings of ice that look like hair - i.e. "frost beard".

Clear Moon, Frost Soon

Clear skies generally help all kinds of frost form. When a night is cloud free, heat escapes from the earth quicker, because it isn’t being ‘blanketed’ by clouds. This allows temperatures to drop fast enough to form frost. Beautiful, amazing frost.

Sure, our mornings are dark and cold. But there’s so much to ‘frost’ than just scraping if off a windshield - bundle up, take a walk, enjoy.