Featherly crystals that form on the snow surface during clear and calm conditions – essentially frozen dew. Forms a persistent weak layer once buried.

Surface hoar is a fancy name for frost. When you have to scrape your windshield in the morning, surface hoar grows on the surface of snow—hence its name. It grows during clear, humid and calm conditions and once buried, it is a particularly thin, fragile and persistent weak layer in the snowpack, which accounts for a number of avalanche deaths each season.

Surface hoar is an especially tricky weak layer because it can form very quickly. One calm, clear night—sometimes just a few hours—is enough time to deposit a thin layer on the snow surface. And once buried, it is very thin and difficult to detect, yet very weak. Also, it tends to form in a complex, hard-to-predict distribution pattern on the terrain. For instance it might form only above a certain elevation where the mountain rises above the clouds. It might form below a certain elevation where cold, humid air pools. It might form in a distinct elevation band where thin clouds form a “bathtub ring” in a confined mountain valley. It tends to form on open slopes as opposed to in trees. Also, when deposited on the snow surface, since it is so fragile, any small disturbance—especially wind—can easily destroy the layer making it very “pockety” i.e. you find it in one spot but not another. No wonder Canadian research indicated that surface hoar accounts for most unintentional human triggered avalanches triggered by professionals

Because surface hoar is so thin, it is also difficult to detect. Often you can’t see it in a snow pit wall and it only reveals itself when you get a clean shear and you look at the bottom of the block and see the flat, feathery, sparkly crystals glittering back at you. The best way to detect surface hoar is to carefully pay attention to the snow surface each day. Before the storm arrives, carefully make a mental map of where surface hoar remains intact. You can typically find surface hoar in basin bottoms and near creeks or lakes.

How it forms:
During a clear sky, the snow in the shade or at night radiates a tremendous amount of heat away and the snow surface becomes very cold. Since we know from earlier in this chapter that warm air holds more water vapor than cold air, the vapor from the warmer air above the snow will condense onto the surface of the snow, and voila, we have surface hoar. Surface hoar (frost) is simply the winter equivalent of dew.

Note: in arctic latitudes, the mid-winter sun is so weak that surface hoar grows all day long, even in the sun. You can grow HUGE surface hoar in the north-country, especially in basin bottoms and near streams.

Next, let’s take a short lesson in the second ingredient for surface hoar–humid air. Humidity, or relative humidity, is the amount of water air can hold compared to the amount it actually does hold. For instance, air at 50 percent relative humidity contains only half the amount of water vapor it could if there was an infinite supply of water around. How much water can air hold? It depends on the temperature. Remember, warm air holds much more water vapor than cold air. In other words, we can change relative humidity two ways, first, by adding or taking away water (humid air left over after a storm or humid air near streams), and second by raising or lowering the temperature. This second method, as it turns out, creates much, if not most, of the humidity that forms surface hoar. As air cools down during a clear, calm night, it becomes more humid. Often, this cold, humid air pools up into the bottoms of mountain valleys and basins, exactly where we find surface hoar.

Finally, we need the last ingredient, calm air. Too much wind will destroy the fragile surface hoar crystals, plus, too much wind doesn’t allow the cold, air to pool and become humid. Actually about 3 mph is best for surface hoar production because it’s just fast enough to bring a continuous supply of humid air to the snow surface but not too fast to destroy it.

In summary, surface hoar forms in the following conditions:

• Clear sky

• No direct sunshine, or very weak sun

• Calm or light winds (about 3 mph is best)

• Open slope exposed to a clear sky (trees or clouds can radiate their own heat and disrupt the process)

• Humid air

Distribution Pattern of Surface Hoar:
With this knowledge of both radiation and humidity in mind, let’s where we are most likely to find surface hoar after a clear, calm night. First, the snow must be exposed to a clear sky. This means that surface hoar doesn’t grow under evergreen trees where the thick branches disrupt the back-radiation process. However surface hoar grows just fine in a sparse grove of aspen trees because they don’t block much radiation.

And what about humidity? We know that cold air sinks and on cold, clear conditions, cold air will pool in the bottom of a valley or a mountain basin. When air cools it becomes more humid, thus, surface hoar tends to form more at lower elevations or especially in the bottom of mountain basins and not nearly as much on mountain tops or ridges. We also find thick layers of surface hoar near open streams because they provide such a constant vapor source.

This is a tricky situation, because normally we expect more avalanche danger the higher we go on a mountain because there’s more snow and more wind. But with surface hoar as a weak-layer there’s counter-intuitively more danger at lower elevations, which commonly surprises people who aren’t accustomed to surface hoar.

But what happens if the air in the valley bottom becomes so humid it turns into fog? Remember the snow surface has to be exposed to a clear sky to form surface hoar. So if the fog is thick enough, it prevents surface hoar from forming. But with a thin fog, surface hoar grows like crazy. Now let’s say the fog is thick, perhaps 300 m (1000 vertical feet) which is probably thick enough to prevent surface hoar from forming on the valley floor, it still forms along the top of the fog layer where we still have the perfect conditions for surface hoar. So like a bathtub ring, in the morning we often see a thick layer of surface hoar along the top of the fog layer. Often you see this same bathtub ring effect along the top layer of stratus clouds that are low enough for the mountaintops to rise above the clouds.

Once formed, surface hoar is very fragile, and even a light wind can either blow or sublimate it away. Because the wind can remove surface hoar from some areas and leave it in others, once buried, it can be devilishly difficult to detect. A snowpit in one place might show nothing suspicious while one 10 feet away may show a very fragile layer. We don’t find as much surface hoar on mountain tops not only because of the aforementioned humidity differences but because the wind blows more on mountaintops and ridges than in valleys.

Surface hoar forms much more commonly in maritime climates than continental climates because it needs humid air. In high latitudes such as Alaska and northern Canada, surface hoar grows all day long since the sun is so weak in mid winter. I have seen widespread areas of eight inch thick surface hoar crystals in Alaska, in the bottom of mountain basins.

Mechanical properties of surface hoar:
Surface hoar makes perhaps the perfect avalanche weak-layer. It’s thin, it’s very weak, it’s notoriously persistent and it commonly forms on hard bed surfaces, which are also slippery. Finally, thin weak-layers tend to fail more easily because any shear deformation within the snowpack is concentrated into a small area.

Surface hoar can fail either by collapse or in shear. It can fail in collapse if the new snow is added slowly, the surface hoar crystals remain standing up, like columns, and when critically loaded, just one thump and all the columns collapse catastrophically, like the old college trick where you can stand on an upright, empty beer can without crushing it, but one tap of a finger–and crunch!–ready for the recycle bin. In fact, this is probably the most common scenario for surface hoar, as well as other persistent weak layers: often the first or second storm on top of a surface hoar layer doesn’t weigh enough to overload it, but the third or fourth storm finally adds up to the critical weight. Whamo! Just like the college beer can experiment.

Surface hoar can also fail in shear when the first snowfall lays the surface hoar crystals over on their side; they remain as a paper-thin discontinuity in the snowpack with very poor bonding across that layer. These laid-over crystals, however, tend to bond up more quickly than the ones that remain standing on end.

Types of surface hoar:

This is getting a little fancy for mere mortals, but there different kinds of surface hoar crystals and some are more dangerous than others. I call these: needles, feathers and wedges. Different combinations of temperature and relative humidity form each kind. The take home point here is that the danger and persistence of surface hoar goes in the order of: needles, feathers and wedges–wedges being the worst.

Type of surface hoar Conditions formed under Looks like Forecasting considerations
Needles Very cold temperatures above -21 deg C. Tiny Needles Less persistent, doesn’t form thick layers
Feathers Normal temperatures Feathers Persistent, but is laid down more easily than wedges
Wedges Normal temperatures Wedges Very persistent and tends to remains upright

Forecasting Considerations:

Surface hoar crystals are notoriously persistent in the snowpack. Instabilities commonly last for a week or two. In the cold snowpacks of Montana and Wyoming, I have seen avalanches on a surface hoar layer four months after it was first deposited. The best way to deal with surface hoar is carefully map it every time it forms BEFORE new snow covers it up. Any time we have surface hoar on the snow surface and I know we have a storm on the way, I will dutifully march around and carefully notice where it still exists and where either the sun has melted it away or the wind has destroyed it, and I will document it for future reference. As you can imagine, this information literally takes on life and death importance during each successive loading event.

Another tricky situation with surface hoar: During a snowstorm, it might be snowing and cloudy when you go to bed, and still snowing and cloudy when you wake up. But during the night, unbeknownst to you, the winds die and the sky clears for a few hours, and voila, a thin layer of surface hoar forms–and you didn’t even notice it. The next day, you will notice sensitive soft slab avalanches within the new snow and you expect them to calm down after a day like usual, but instead, they last for several days. You dig to investigate and find the culprit. Darn that sneaky surface hoar!

Surface Hoar Summary:

clear sky, light to calm wind, humid air.

Looks like:
Sparkly, flat, feather-like or wedge-shaped, stepped, striated crystals–sometimes mistaken for facets or stellar snow that falls from the sky.

Also called:
hoar frost, frost, feathers

Distribution Pattern:
Open areas without trees or sparse trees exposed to a clear sky, lower elevations as opposed to upper elevations, the bottoms of mountain basins, beneath thin fog layer, the top of a thick fog layer or stratus cloud layer, shady, calm areas, near streams

Extremely persistent weak-layer–one week to months depending on temperature. Especially persistent and dangerous when on top of a firm ice crust.

Best snowpit detection method:
Shovel shear test or compression test. Look at the bottom of the block to see the crystals.

Forecasting considerations:
Carefully map the distribution of surface hoar BEFORE it is buried by subsequent snow. Be suspicious of it with each loading event. Surface hoar is guilty until proven innocent.