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Hoar Frost and Rime Ice: What’s the Difference?

surface hoar on snow

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.

surface hoar on snow in Badlands National Park

A closeup view of the surface of the snow in Badlands this afternoon. The big, platy ice crystals projecting upward are surface hoar.

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:

Photo by Amanda Taylor, from the Wayne Flann Avalanche Blog. Surface hoar crystals can grow to great size if there’s sufficient moisture, cold temperatures, and no wind for long periods of time. This example is from Cerise Creek, British Columbia; I’m guessing the nearby creek was the source of the abundant water vapor necessary to grow such impressive 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.

Frost around an animal's burrow.

These hoar frost crystals grew around the mouth of a burrow where an animal tucked itself away for a cold winter night in the Maine woods. Its warm, moist breath provided the necessary water vapor for the crystals to grow. January 2011.

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:

soft rime on twig

Long, spiky needles of soft rime ice formed during a freezing fog in the Badlands in January 2010.  Unlike the surface hoar illustrated above, you can see that each needle of rime is not a single crystal.

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 from freezing fog

Another view of rime ice on a tree in Badlands National Park, after a persistent freezing fog in January 2010. The longest of the rime needles reached three inches!

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.

glaze ice and waves

Glaze ice deposited by the splashing of waves along the shore of Lake Champlain. Burlington, Vermont, January 2012.

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.

frost on stem

Frost crystals on a thorny stem in the Maine woods, January 2011.

Sources:
Ahti, K., & Makkonen, L. (1982). Observations on rime formation in relation to routinely measured meteorological parameters. Geophysica19(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.)
19 Comments Post a comment
  1. WHOA. Like you say…. they’re both pretty. :) But really fascinating. That’s one chilly hike.

    January 2, 2013
    • The calm air (that allowed the surface hoar to stick around all day) actually made it quite lovely out. The sun was shining and we were moving–I was a bit overdressed, in the end! The omnipresence of the wind here in South Dakota makes almost any non-windy day a pleasure.

      January 3, 2013
  2. Very interesting! I’ve seen lots of the glaze ice; I’ll have to keep a close eye out for hoar and rime now. Thanks!

    January 2, 2013
    • One thing that occurred to me, as I was writing the post, was that knowing how these different things form is a real help for figuring out where to find them. Happy hunting!

      January 3, 2013
  3. That last photo from Maine is spectacular–love it!

    January 2, 2013
  4. Cathy, I enjoy the pictures and the education. Winter is usually the only time I backpack (I’m not a fan of heat, poison ivy, and spider). It’s the time of the year I can actually see what’s happening in the woods. Thanks for sharing your art and your knowledge.

    January 15, 2013
    • Thank you! Winter is a wonderful time to be outdoors, especially in the woods. The snow is a such a wonderful canvas for animal tracks, among other things. It’s always eye-opening to see such obvious animal sign, especially from the abundant small mammals like mice and voles that I don’t see in the flesh very often.

      January 15, 2013
  5. George #

    Interesting Blog Cathy and nice blog site. For a few days a couple of weeks ago we had some interesting ice crystals landing on cold surfaces. The weather was clear, in the single digits, and Rime on the tree branches. In the early morning just after the sun came up crystals were floating in the air, flittering here and there reflecting the sun as they did so. Eventually, some would settle on cold surfaces (like an iron railing where I photographed some). They looked pretty much like snowflakes but hard and glassy looking (not soft and fluffy). Would these be Hoar? or something else?

    January 30, 2013
    • Oooh, that’s a great question, George! I’ve seen this phenomenon on rare occasions in the geyser basins in Yellowstone in wintertime, where the geysers and hot springs pump lots of moisture into air that can be very cold and very still in the wintertime.

      I don’t have a good understanding of what’s happening in that situation. I think that the otherwise very dry air gets pockets of much higher humidity. These high-moisture pockets, if left very still while the temperature drops, could effectively become supersaturated with water vapor. The result might be a relatively warm, moist layer of air that gets trapped near the ground surface, overlaid by a layer of cold, dry air. At sunrise, the heat input could cause the air to begin to mix, and the surplus moisture would freeze into sparkling crystals.

      That’s really all speculation, though. You may be able to get a more definitive answer from the folks at Caltech’s Snow Crystals lab. If you do, I’d love to hear what you find out!

      Thanks for reading–hope to see you here again.

      January 30, 2013
      • George #

        Thank you for your in depth response. Now you have got me really curious. Your hypothesis reminded me that I forgot to mention a rather important variable. I live (and the phenomenon occurred) at an elevation where the inversion interface forms. So I am thinking that this ice crystal formation may have only been developing at the interface, ie. where warm saturated air suddenly cooled and became supersaturated. I didn’t really pay attention to where the inversion was that day but the temperature at my elevation (~4400 feet) was colder than what was being reported a couple hundred feet lower. As you said, the sun and perhaps the morning stoking of the heaters and fireplaces may have warmed the hypothermic layer to form sufficiently to begin mixing with the cooler air at my elevation; likely in pockets, as you suggest. Does that make sense?

        Thank you for the reference too. The crystals I saw would be dendrites tending towards fern dendrites which is consistent with the temperature I observed. I skimmed the article “The Physics of Snow Crystals” but will have to get a big cup of coffee and spend some serious time with it.

        In spite of all the interesting technical stuff, it was sure a pretty event. Hope it happens again. For now I am going to call it “Clear Sky Snow”.

        January 31, 2013
        • Nifty! I was thinking about inversion layers as I wrote out my guess, so I’m pleased that you mention that may have been a factor.

          On the subject of the prettiness of snow–are you familiar with the book The Snowflake: Winter’s Secret Beauty? It’s a stunner, with beautiful snowflake photography and good technical information (that I never seem to read because I’m too busy looking at the pictures).

          January 31, 2013
          • George #

            Thanks for the reference. I ordered it. Actually, that was what I was doing on the I noticed these crystals. I had just got my new extension tubes and was looking for something to photograph. Now all I have to do is make them look as good as the one;s in the book. ha.

            January 31, 2013
  6. Totally awesome !

    December 15, 2013
  7. This morning in Rapid City, I saw the kind of ice I have known and loved since childhood. A thin layer of ice forms on a water surface, then the water below runs away, leaving the thin ice layer over a pocket of air. Today, this phenomenon occurred in the street gutter lining my driveway. The same ice forms over edges of creeks and other places. It is so much fun to walk on, breaking the ice with a crackling sound whether you are a young child or a seventy-year-old man. Do you know a name for this type of ice? One would think someone would have named it in the past, but searching the internet so far has been fruitless for me.
    David

    April 3, 2014
    • What a great question! No, I’ve never heard an official name for this sort of ice. I’ve always thought of it as “shelf ice” and I agree – it’s great fun to step on it!

      April 4, 2014

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