By Jenny Neyman
Poets penning odes to winter wonderlands would have found plenty of inspiration on the Kenai Peninsula over the weekend.
“Winter-Time” by Robert Louis Stevenson: “Black are my steps on silver sod;
Thick blows my frosty breath abroad; And tree and house, and hill and lake, Are frosted like a wedding cake.”
A thick coating of hoar frost bloomed overnight Friday, covering trees, signs, Christmas decorations, even Dumpsters and derelict cars in sparking, crystalline glory. Saturday dawned cool and clear, with the sky saturating into a deep blue that offered a stunning backdrop to the delicate frost forms, while sunlight glinted off the tiny facets.
“It’s been extra gorgeous,” said Marion Nelson, who organizes the Central Peninsula Garden Club. “I know why people flock trees for the holidays, but, geez, give it up. The real thing is so much better.”
Hoar frost is the marriage of moist air and cold surfaces.
“It occurs when you have a fair amount of moisture in the air and then you get a lot of things around it that are really cold,” said Dr. David Wartinbee, a professor at Kenai Peninsula College’s Kenai River Campus. “When you get foggy weather like we’ve been having and it’s really cold over lakes and so on, there’s a lot of moisture coming up. That in turn then causes the moisture in the air. It condenses or freezes onto a hard object, like leaves. Or sometimes you see it if you go out on a lake where the lake is frozen with little sticks coming up out of it with all kinds of frost on it. That’s where moisture has come up from the edge of the lake and hit something that’s solid and cold and it just freezes on it. It can form in all kids of neat crystals, and can vary. Sometimes it looks like big fuzzballs. It’s really interesting.”
All frost is a combination of cold and moisture, but there are different kinds that form under different conditions. Rime, for instance, is a quick-forming frost that comes from heavily saturated air under windy conditions, like in the Arctic Ocean. The moisture that forms frost rime generally goes through an actual liquid form — like ocean spray on ship rigging — and condenses quickly into a solid ice that coats everything that was wet.
Hoar frost, on the other hand, is moisture in the air that condenses straight into a solid ice state — skipping the liquid phase. It generally forms on cool, clear nights. Without an insulating cloud layer, heat is lost rapidly into the air, causing objects losing heat to become colder than the surrounding air. When that happens, moisture in the air condenses on the objects. Unlike frozen dew, which is condensed water that freezes into tiny droplets, hoar frost is ice crystals. As with all crystals, they grow on top of themselves, starting with a tiny seed crystal and blooming out into intricate, interlocking shapes. They can be spikes, tiny needles or delicate leaflike structures.
Some objects make better hosts for hoar-frost crystals than others. Metals that cool quickly and stay cold accumulate frost quickly, for instance. Air movement also has to be ideal for impressive displays like this weekend’s to occur. A little air movement circulates humid air around cooler objects and feeds crystal growth. Too much wind and the crystals are destroyed.
In Kenai on Saturday, hoar-frost crystals grew out from road signs and light posts in a northeasterly direction, indicating a light wind had been blowing in off Cook Inlet overnight while the frost was forming.
When the frost is sufficiently disturbed, by friction or wind, the crystals break off their perches and float to the ground, looking like snow even if it’s a clear, blue-sky day. They can also be melted in some spots more quickly than others, for instance in spring when sunlight’s rays carry enough heat, leaving one side of a frosted object bare and
the shaded side still covered.
In general, frost can be damaging to plants, if it comes before plants are sufficiently prepared for winter. This time of year, though, even the heavy, shardlike hoar frost of the weekend shouldn’t do much damage.
“I’ve been in Alaska for 25 years and the frost is really heavy on the trees, but I’ve never seen it impact their survivability,” said Julie Riley, a horticulture agent with the University of Alaska Fairbanks Cooperative Extension Service.
The bigger danger would come from snow falling on top of frost-covered objects, since they could accumulate a larger load.
“If we have a heavy snow, because there’s more surface area there, that would increase the weight of snow on the branch. Trees might be able to hold more snow,” Riley said. “It’s the heavy snows, when it’s close to 32 degrees and the snow carries a lot of weight, or if people have trees that have weak branching angles.”
Most likely, though, the crystalline coating won’t do any damage, as any plants hardy enough to make it through a winter in Southcentral Alaska shouldn’t be bothered by their sparkly coating of bling.
“I would just encourage people to enjoy the beauty and not be worried about it at all,” Riley said. “It has just been spectacular. Just the frost itself makes the landscape more enjoyable.”