[harpo-the-skier]

Does tree cover effect surface hoar formation?

Here in Tahoe, we have had a long cold spell and I have oberserved much surface hoar. However, I have been mostly touring in open areas. We are expecting a big storm over the next couple of days, so I was worried that the SH would form a weak layer when buried. I was wondering if it would be any safer in the trees. My understanding is that you need radiational cooling to cool down the snow surface for SH to form. I was thinking that tree cover would prevent this radiational cooling. I will dig a pit before going up the slope, but I was wondering what to expect.

[Rebob]

Tree cover might protect surface hoar, once it's formed, but it will not likely prevent its formation.

Surface hoar results from clear skies--which allows long range radiation to escape the snow, humid air--which supplies the water vapour to create surface hoar crystals, and a little movement of air--which brings a constant supply of water vapour to the cooler snow surface. In our area, subalpine tree complexes tend to protect the surface hoar from higher velocity winds after it has formed. Often, our larger surface hoar are found in treed glades near treeline.

Also, surface hoar can be difficult to detect as a buried layer, even when you know how much new snow has fallen. One trick you can use is to use your snow saw to create a thin vertical lens of the snow layers and look for the surface hoar using backlighting. Even then, they can be tough to isolate on your crystal card.

[harpo-the-skier]

Thanks for replying Rebob,

I was guessing that the tree cover would interfere with the long range radiation, but I guess not.

[funhog]

I can't explain any physics but I know when I park my rig underneath a tree, its less likely to have frost in the morning than when I park it out on the open.
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It's all about fun....

[Ava Blanche]

So this business of radiation has always fascinated me. Now I'm really a dirt person, not a sky person, so I am sure someone else can provide an even better explanation, but I'll take a chance and go first and let someone else fill in the details and make the corrections.



1. For all practical purposes, we can assume that heat energy from the sun arrives at the earth's surface as short wave radiation.

2. Snow reflects most of this arriving radiation, but some is absorbed and heats the snow.

3. But snow, being white, also loses heat through long wave radiation.

4. During a winter day, the sun is lower in the sky, and the short wave radiation is spread over a greater area on flat ground, and provides less heat energy to the snow. Of course, on slopes oriented towards the sun, there is less loss. However, long wave radiation continues, and so there can be a net loss of heat from the snow.

5. Reflected short wave and the long wave radiation from the snow normally escapes into space. But if clouds are present, some of this radiation is returned to the earth; this is why it is colder when it is clear, and not so cold when it is cloudy.

Now things get a bit more complicated.

6. First, the snow-covered top of the tree canopy behaves the same as the ground. However, some of the reflected short wave, and more of the long wave radiation from the snow, is absorbed by the underside of the tree canopy and causes the immediate local area under the tree canopy to be slightly warmer.

7. The warmer it is, the more water vapour the air can hold, and the less is available to condense onto the snow surface as hoar crystals.

Bottom Line: you are less likely to have hoar crystals forming under the trees, and therefore there is likely to be less future avalanche hazard there.
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There are no easy solutions, only intelligent choices

[Happy Bear]

Trees inhibit the growth of surface hoar (sh), as Ava Blanche described above. It's easy to observe--find a glade with big sh, then ski towards the trees and watch as the grains get smaller and smaller. Keep this in mind, because trees will also shelter the sh in openings and glades from wind and sun, as Rebob discussed. The sh on open slope can be destroyed before burial, so its no longer a problem, and be preserved in glades, leaving pockets of instability.

[Rebob]

I suppose part of the issue here will be based on the amount of tree cover present in the area. The area I'm thinking of in this discussion is that region close to treeline where glades predominate. In these regions, trees do not inhibit sh formation, as they do not prevent the longwave radiation from radiating into the troposphere.

As a timely example, I've included a portion of the CAA avalanche bulletin for the current time frame. It points directly to the issue under discussion:

[Rebob]

Happy Bear wrote:

[Ava Blanche]

[Rebob]

I appreciate Ava's last point on the post.

On the subject of tree cover, there are some bits and pieces out there...

From the CAA:

[Ava Blanche]

My, the politeness here continues to be stunning. Thank goodness the environmental/political board is there to draw away the more rambunctious crowd.

Anyway, the publication you refer to is really worth having: Snow Management in Forested Terrain by Peter Weir (2002) put out by the British Columbia Forest Service. Highly recommended!



The best part is that you can download the whole thing for FREE at http://www.for.gov.bc.ca/hfd/pubs/Docs/Lmh/Lmh55.htm

But if you're going to try printing a copy, it might be better to order a hard copy instead at http://www.publications.gov.bc.ca/ I think the cost is about $20 Canadian.

The paper you refer to is also excellent; there has been a lot of superb work done on the topic of forests and avalanche by David McClung and some of his grad students, like Geoff Anderson.
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There are no easy solutions, only intelligent choices

[Ava Blanche]

But to the main point I wanted to make: what all this talk about trees means is that people in resort areas where logging is contemplated should be careful about accepting claims that partial cutting is being done to reduce the chances of creating new avalanche start zones; like above a highway or a housing development, for example.

There is no research that I am aware of that partial cuts, or gladed runs produced by some heli-skiing companies, reduce avalanche hazard. There are certainly things that we can do to minimize the probability that forest openings will create new start zones, but partial cuts may not be the answer.

Here is an example of a logging development deliberately designed to minimize the probability of creating new avalanche start zones.



This is on Brohm Ridge near Squamish, B.C.. Originally, the whole area was slated for clear cutting, but I pointed out that this would increase avalanche hazard to the point where snowmobilers using the road would be subject to extra hazard. Of course, at that point everyone said "let 'er rip!"

So we left the middle, steepest part of the block above the road intact- no trees were harvested, and only small opening were placed on the moderately steep ground on either side. The true partial cut at the top was not developed to reduce avalanche hazard, but rather to meet visual objectives.
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There are no easy solutions, only intelligent choices

[snotsicle]

I think there are two issues here. First, trees inhibit growth of SH, as mentioned above. Go out on a sunny day after a clear frosty night, and you'll find the biggest surface hoar in clearings, especially in low spots such as frost hollows and concave low spots on slopes (cold air drainages).

But, secondly, trees also inhibit the destruction of SH crystals by solar heating. During that sunny day, the SH in the clearings will burn off, but in the shade the SH will persist. Suppose it starts snowing later that evening. Now you have buried SH in the trees and none in the clearings. Unless that clearing was on the wrong aspect, or shaded by a cloud, etc. when the sun was out.

I think one of the reasons why there is often so much spatial variability associated with SH instabilities is that there are interrelated two factors, each with their own spatial patterns.

[Gary Brill]

I think one definitely needs to differentiate between a canopied forest and gladed or open trees near or below treeline.

A canopied forest should block surface hoar formation because it prevents radiational cooling at the snow surface and the strong vapor pressure gradient that is necessary for crystal formation. I think another factor is that the dendritic branches of conifers also act as freezing nuclei to remove water vapor from the surrounding air (there is usually more rime in the trees than on the snowcover - this effect was discovered years ago by , I believe, a Swedish or Norwegian meteorologist who advanced an early theory on supersaturation and the formation of precipitation).

In addition, a canopied forest acts as a reservoir in stopping the fall of a good deal of the new snow to the underlying ground. Then, on warm-up or in windy conditions, the stored snow drops from the treetops, compressing and strengthening the underlying snowpack. Fallen logs in old growth forest also disrupt windflow patterns except in very deep snowpacks. For all of these reasons, it is more difficult to initiate slabs in forest.

The larger the openings between the trees, the more likely surface hoar is to form in the glades, and up to a point, the more likely it is to persist. One of the biggest effects in destroying surface hoar is atmospheric mixing, since humidities are likely to fall below saturation. Mixing of the atmosphere evaporates surface hoar. In glades still air manages to maintain a layer of cold surface air adjacent to the snow surface maintaining higher humidities and either maintaining or growing surface hoar. As has been mentioned, shade is also a factor on surface hoar persistence.

rebob:

[Rebob]

Gary Brill wrote: