Spectacular hoarfrost
Last Friday night, February 15, we had unpredictable precipitation dancing between sleet, freezing rain or rain, with temperatures hovering around freezing. Exposed surfaces froze, yet the road surfaces remained warm enough not to create widespread hazardous conditions. It could have gone in any direction – snow, sleet, freezing rain, or just plain rain. Some ice accumulated on trees and my front porch was treacherous. We were spared from the worst of it.
Saturday morning I drove to Fayetteville and for about a five mile stretch on Hwy. 23, from about Harold’s Storage to Turpentine Creek, ice had accumulated on the trees at the higher elevations, with a fog of moisture sitting in atop then. It was a beautiful site.
I went about my business, then on the way home from Fayetteville around 3 p.m., much to my surprise, the ice remained on the trees and the fog hovered at treetop level. It was about 33°F. Then I noticed something else was going on. The surfaces are not merely covered with ice from the previous days freezing rain, the ice was in fact growing on every surface into sublime crystalline spikes.
Beautiful hoarfrost formed on every exposed surface in the air from barbed wire fencing to remnants of roadside weeds from the previous season. It was an ephemeral fairyland of nature’s beauty.
What is hoarfrost? I turned to the website of the National Snow and Ice Data Center for a definition. “Hoarfrost [is] a deposit of interlocking ice crystals (hoar crystals) formed by direct sublimation on objects, usually those of small diameter freely exposed to the air, such as tree branches, plant stems and leaf edges, wires, poles, etc., which surface is sufficiently cooled, mostly by nocturnal radiation, to cause the direct sublimation of the water vapor contained in the ambient air.”
Hoarfrost, therefore, is like dew, except when it’s cold enough outside to freeze water (that is, when it’s 32°F or 0°C), and there’s moisture in the air, then hoar crystals (flat crystals that interlock together), form from the moisture in the air when it comes in contact with the edge of the object that is below freezing.
Usually you see hoarfrost in the morning after a cold night, but the perfect combination of freezing surfaces, with just above freezing, moisture-laden air, combined to produce a rare spectacular hoarfrost display on scattered Ozark hilltops.