Is it Rime Ice or Hoar Frost?
Thanks to a combination of freezing fog and excess moisture in the atmosphere last weekend many Wisconsinites were treated to a picturesque winter wonderland.
While the dazzling display of crystalline beauty was due to the freezing fog, many assumed the frosty display was hoar frost. However, the tree branches, pine needles, and dried fall foliage flocked with spiky crystals were actually covered with rime ice.
According to DTN meteorologists, rime ice forms when supercooled droplets freeze and attach onto an exposed surface. In order for the droplets to form into ice.
When supercooled droplets from fog freeze and attach onto an exposed surface, you get rime ice. All objects impacted need to be at 32°F or below causing the liquid to freeze instantly.
Rime ice can be classified as either hard or soft and will generally depend on the wind strength. Hard rime ice is very dense and trees appear to be flocked with the crystals. This occurs due to freezing fog when wind speeds are high, and temperatures range between 17-28°F.
DTN meteorologists point out that soft rime is a bit more delicate. On closer inspection you will notice individual crystal spikes. This usually happens when winds are calmer during a freezing fog.
The crystals of rime ice typically build up on the windward side of objects, building a somewhat spongy, porous layer of heavy ice.
According to the National Weather Service, hoar frost is defined as a "deposit of interlocking crystals formed by direct sublimation on objects, usually those of small diameter freely exposed to air, such as tree branches, plants, wires, poles, etc."
The development of hoar frost typically occurs on relatively clear nights when air with a dewpoint below 32° is brought to saturation by cooling. Hoar frost also needs calm air that allows those complex lacy deposits of crystals to form.
In other words, the moisture in the air skips the droplet stage and goes straight to crystallizing, similar to dew but when temperatures are below freezing.
Hoar frost derives from the old English word “hoary,” meaning getting on in age. With this thought in mind, many trees, especially evergreens did present a "hair-like" appearance resembling white, feathery beards. Unfortunately warmer weather and rising winds caused the trees, powerlines and other objects to lose their wintry finery by mid-afternoon.
It's also interesting that hoar frost is mentioned several times in weather folklore. According to the Old Farmer’s Almanac hoar frost on May 1st indicates that a good harvest is forthcoming. Another saying associated with hoar frost is that rain will be in the forecast six month following a morning of hoar frost.
Despite the differences between the two weather phenomena, it was a boon to winter weary souls.