Hoar Frost and Rime Ice: What’s the Difference?
The New Year dawned cold in the badlands of South Dakota. Temperatures in the low single digits at sunrise on January 1 warmed to a balmy 30°F by midday. The sunny, calm conditions were perfect for my first hike of 2013. I hadn’t walked fifty feet from the trailhead, however, before the sun glinting off the surface of the snow captured my attention. I commonly see delicate sparkles on the snow in the morning sun, but these were bold flashes coming from platy ice crystals the size of my thumbnail.
I knelt to see better, and exclaimed in delight. Even with my naked eye, I could see fine growth ridges running parallel to the edges of each plate, forming beautiful facets. My first thought was surprise that such big, perfectly hexagonal snowflakes could have persisted since the last snowfall, several days ago. But then I realized that the ice crystals weren’t old snowflakes at all: they were a beautiful example of surface hoar.
Surface hoar is a kind of frost that forms when humid air comes into contact with very cold snow on the ground. It tends to form on cold, clear nights with little wind. Why? The lack of cloud cover means that there’s no insulating atmospheric blanket to slow heat loss, so snow on the ground gets very cold very fast. (Trees and shrubs provide a similar blanketing effect, so surface hoar is much less common in forests, say, than in open areas.) If you have air with high moisture content, the molecules of water vapor that bump into the cold snow surface can suddenly have so much heat sucked out of them that they freeze in place—without ever going through the liquid phase.
The challenge with hoar frost is that, in order for it to form, there has to be some seriously humid air around. And, as our chapped lips and cracked fingertips remind us daily in the winter months, cold air is usually dry. Very cold air simply can’t hold as much moisture as warmer air can. So where does the water vapor come from?
One potential source is within the snowpack itself. When snow on the ground warms enough during the day to start to melt, some of the liquid water evaporates, increasing the humidity within the snowpack. When night falls, the surface of the snow cools rapidly, but the interior of the snowbank doesn’t cool as fast. The humid air that works its way towards the chilly surface can feed the growth of hoar frost crystals.
I noticed while I was hiking that the surface hoar tended to be present in places where there was grass, or on gentle, sheltered slopes. Patches of snow that were flat and exposed hadn’t grown the big plates that had caught my eye. I suspect this is because the grasses and slopes created pockets that protected the snow from the desiccating effects of the wind. Where winds could pull moisture from the snowpack, it became too dry to support the high concentration of water vapor necessary for surface hoar development.
Surface hoar often vanishes once the sun has risen and warmed the surface of the snowpack, so it’s most easily observed in the morning. But at higher latitudes, where the sun’s rays hit the snow at a low angle, the hoar frost crystals can sometimes continue growing even during the day. This can create some truly stupendous crystals:
Hoar frost doesn’t have to occur on the surface of the snow, which is why the particular form that I’ve been discussing is more properly referred to as “surface hoar.” Hoar frost that forms on trees, for example, commonly has a feathery or dendritic appearance.
That brings us to rime ice. Rime ice can often be told apart from hoar frost by appearance, but not always. Take, for instance, these three-inch needles of ice on a tree here in Badlands:
At first glance, this looks a lot like hoar frost. One clue that it isn’t comes from the fact that the frost spikes don’t uniformly encircle the branches, as you can see even better in a picture taken the same day that shows the whole tree:
Rime ice forms when liquid water droplets in the air freeze onto a surface, growing into combs, needles, or feathery forms. In the two photographs above, the rime grew over the course of many hours, as a freezing fog lingered for most of a night and all of the following morning.
A freezing fog occurs when a low-lying cloud is cooled to temperatures below the freezing point of 32ºF (0ºC). Without anything solid to nucleate around, the tiny, suspended water droplets remain liquid, even as they cool to temperatures where you’d expect them to be frozen. (If it gets cold enough, a water droplet will freeze even without a nucleus, so freezing fogs are unlikely when it’s truly frigid.) Water droplets can remain in this supercooled state until they bump into something solid, and then they freeze on contact.
If there’s a gentle wind blowing, the first supercooled water droplets to bump into, say, a tree branch will be deposited on its windward side. Later deposition continues on the same surface, so that needles or combs of ice grow outward, into the wind. Rime ice generally won’t form an even coating all the way around a tree branch.
The clear, uniform coatings of glaze ice, for comparison, are deposited by falling rain that freezes on contact with a chilled surface. Both rime and glaze can be destructive if enough heavy ice is deposited on tree branches or powerlines, but glaze is the more common problem in most areas.
So, what’s the difference between hoar frost and rime ice? It all has to do with how the ice crystals formed: hoar develops when water vapor freezes, going directly from the gaseous state to the solid, while rime forms where supercooled liquid water droplets freeze on contact with cold surfaces. If it goes from gas to solid, it’s hoar frost. If it goes from liquid to solid, it’s rime.
Whatever you call it, it’s pretty.Sources:
Ahti, K., & Makkonen, L. (1982). Observations on rime formation in relation to routinely measured meteorological parameters. Geophysica, 19(1), 75-85. Elsom, D. M. (1980). Wind patterns from rime deposits. Weather, 35: 86–89.
Heinrich, B. (2011). Personal communication.
Shea, C., & Jamieson, B. (2009). The role of moisture in surface hoar growth. (PDF from Applied Snow & Avalanche Research, University of Calgary.)