Yesterday I posed the question, “What are the conditions that promote the growth of Surface Hoar”. There were many correct answers. Let’s dive a little deeper into the conditions and take it beyond the level 1 understanding.
In my AIARE 1 avalanche courses I teach students to remember the three “C’s” that promote the growth of surface hoar.
Cold- Unsurprisingly it needs to be below freezing (32°F, 0ºC) for surface hoar to form.
Clear- The mechanisms that drive the growth of surface hoar require rapid radiative heat loss from the snowpack. This only occurs on clear nights. Cloud cover has an insulating effect on the earth and pretty much prevents the type of heat loss needed for this process to occur.
Calm- Surface hoar crystals, once formed, are vulnerable to moderate wind speeds. The ice forms have very little horizontal strength, and if you find a pocket of decent sized surface hoar you can test this easily by getting close to the crystals and giving them a hearty blow of air from your own lungs.
That is pretty much the level 1 type knowledge I instill in my students. Now let’s dive deeper into other important considerations that contribute to the snowpack’s ability to form surface hoar.
Humidity & Dewpoint– Basically surface hoar is winter’s equivalent of “dew”. There must be a high level of humidity (over 70%) in the air right at ground level.
Air movement– Since the crystal growth of surface hoar is removing vapor (moisture) from the air as it grows that moisture must be replaced for the crystals to keep growing. The air movement required is so light that an observer would record “calm” for wind speed.
Temperature Inversion/Gradient– A strong inverted temperature gradient is needed right at the snow surface. This gradient is best achieved by the rapid radiative heat loss scenario promoted by cold clear nights. Even a light breeze will remove this ground level inversion and inhibit growth.
Now that we’ve gotten the sciencey weather stuff out of the way let’s get into some practical terrain considerations for the topic.
Sheltered/Lee– Since we know any noticeable wind inhibits surface hoar growth we can expect to find it in areas protected from the wind, i.e. lee.
Canopy– Because surface hoar requires rapid radiative heat loss any tree canopy will inhibit growth. Surface hoar is often found in clearings that are not obstructed by any tree cover.
Aspect– It is reasonable to assume northern aspects may be more likely to grow surface hoar as they are colder aspects. In areas with predominant west winds (White Mountains) you may have more luck finding surface hoar on your North to East aspects.
Elevation– Since above treeline conditions rarely promote the growth of surface hoar this weak layer is more common below treeline, and in the right conditions middle elevations.
Slope Angle– Surface hoar has what is called “anisotropic” structure. What this basically means is it is strong on the vertical axis and very weak on the horizontal axis. This property is the main reason it is such a reactive weak layer when buried by new snow. Its ability to withstand gradual increases in load until it reaches its breaking point is why in certain snow climates it’s the primary issue for human triggered avalanches. It is also why you would not expect this in steeper start zones. In the rare case of this forming on a 40 degree start zone of an alpine gully any cohesive snow that falls on top of it will quickly overwhelm its horizontal strength. Field observations show this is a common layer for remotely triggering avalanches on flat terrain. Twenty five degree rollovers are also commonly triggered on surface hoar.
Slope Shape– Research shows that concave slopes inhibit growth. This is due to the reduced radiative heat loss on concave slopes. Conversely, convex slopes may see more progressive growth as they can accelerate rapid radiative heat loss and a stronger surface left temperature gradient. Yet another reason convexities on a slope are common trigger points.
Proximity to Water– Because the ground level humidity needs replenishment you can often find this growing next to creeks, streams, and steam “vents”. These “vents” are pretty cool, even on a negative temperature day if you stick your thermometer in one of them you’ll find the air temp hovering around freezing (32°F, 0ºC).
Snow Climate– The prior mentioned weather conditions are most common in Continental (Rocky Mountain) snow climates. Surface hoar can, and does, form in any snow climate, but it’s most prevalent in Continental snow climates.
Local Examples– I have found a few places in Mount Washington Valley that I consider “surface hoar farms”.
The most reliable spot I have found is just south of the trestle cut at the top of Crawford Notch where some sheltered steam vents on the west side of the tracks often have surface hoar crystals around them. Unfortunately the railroad is active this winter and no trespassing signs are posted so I don’t recommend heading there.
The field just outside the AMC Highland Center has had some brilliant surface hoar displays over the years through they are usually short lived as calm clear weather doesn’t last long at the top of the notch.
The base of the Cog Railway has small slopes that have produced some of the largest surface hoar crystals I have ever seen in person, some up to 4 centimeters in size!
I’ll often find small pockets on the sides of the Tuckerman Ravine Trail but the spot on the east side of Mount Washington I’ve had the most luck is on the Summer Lion Head trail, right as the trail steepens and crosses an old avalanche path there are a couple small steam vents on the left side that have a clear view of the sky (thanks to old avalanche activity).
Summary– Well that’s about it for my brain dump on surface hoar. Hope you’ve learned something new about what is likely my favorite type of ice formation. For more reading you can check out The Avalanche Handbook. It’s where most of my information is derived and a great resource for those who really want to dive into the minutiae of avalanche formation.
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See you in the mountains,
Northeast Alpine Start
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