Snow Science Thread
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As the pic Wade presented is correct...the reason a Deep slab lets go, is due to a weak layer under the slab, whether it is surface hoar..depth hoar..old sun crust etc. Usually the Deep slab is older layers that have already started to bond together. That would leave you with a thick slab resting on a unstable weak layer.
Nice pic Wade
In the Flathead we get these. Do to, our storms come in waves and can dump 6" to 12" twice a day, or for days. Or none for a couple days in the hills, allowing that snow pack to settle and then get hit again.
So we can see weird patterns in the storms and snow accumulation. Through variant temp changes we get..you can count on consolidation in the snow pack forming Deep slabs.
Nice pic Wade
In the Flathead we get these. Do to, our storms come in waves and can dump 6" to 12" twice a day, or for days. Or none for a couple days in the hills, allowing that snow pack to settle and then get hit again.
So we can see weird patterns in the storms and snow accumulation. Through variant temp changes we get..you can count on consolidation in the snow pack forming Deep slabs.
Great pic. One thing about a deep slab is that it typically, as phantom stated breaks on a weak layer. That weak layer is usually a persistent weak layer. One that sticks around for a while. That is why it allows for multiple layers to build up and start bonding together. It can also be dangerous because the slab itself may be very strong and give you the sense that the slope is "safe" when in reality it lures you out onto the slope and then breaks well above you giving you little chance of escape.
Speaking of persistent weak layers, what are some of them? Which are the most common? Are there any that we have not discussed yet?
Speaking of persistent weak layers, what are some of them? Which are the most common? Are there any that we have not discussed yet?
well we talked about surface hoar and depth. Did we make any mention of just a simple rain or ice crust?
Does the temp at which the snow falls and it's moisture content make a big difference for stability? I mean how does the wet heavy snow of 32 degrees compare to the light fluff ball snow of 0 degrees?
Does the temp at which the snow falls and it's moisture content make a big difference for stability? I mean how does the wet heavy snow of 32 degrees compare to the light fluff ball snow of 0 degrees?
Good. You hit a couple, rain or ice crust, sun crust is another. There is also faceted snow which is pretty much depth hoar found within the snow pack instead of just near the ground.
Usually wetter or damp snow actually bonds well to other layers, but it could pose a few problems. First, if there is a persistent weak layer underneath, the weight of that heavy, wet layer could cause failure. Or there are some storms that come in warm at first and then get colder. This is how we want it. That means the storm is right side up, meaning the snow on the bottom is heavier and the snow on the top is lighter. Other times storms come in cold and get warmer, this leaves real light, fluffy snow buried by some heavier, wetter snow. This is called and inverted snow pack and is cause for concern.
There are also wet snow avalanche, but they are usually not caused by wet snow, in the sense that if it is snowing heavy and wetter water content snow. Instead it is usually caused by extremes like rain saturating the snow, or rapidly warming trends. These types of avalanches vary greatly from dry snow avalanches. The don't trigger the same, are much slower moving and have way less fatalities associated with them. Consequently they get studied less and there is less info available on them.
Aspect is the direction of the face of the mountain .
It plays a huge roll in avys..it is important to understand which side the sun is hitting the slope. Whether it is North..South..East..West. Which means that if the sun is hitting the south facing slope the snow will start to consolidate it's pack quicker than the north facing slope. This means that the side the sun is not contacting (leeward side) is going to have a colder snow pack. With this in mind, the leeward side will have more of an avalanche danger than the the sun side. Reason being...the colder side will tend to have more cold days and will probably keep more unstable layers and also create more unstable layers. Whether it be from new snow fall..old weak layers not metamorphing or faceted snow.
Sun aspect will be more consolidated than the leeward side making it more stable. Leeward side will still have all the basics to form slab avalanches and will be more dangerous. The leeward side needs to be looked at with OPEN eyes and studied before you ride it.
Now..you have to look at wind..it is also as important as sun. The wind will pick snow up on one aspect of the slope and deposit it on the leeward side (the opposite slope). So, you have to look at the aspect and determine the leeward side to understand what aspect is being loaded. As stated above in this thread..wind loading is very unstable due to the amount of snow being deposited...where the snow is being deposited and how fast it is being deposited.
I left some blanks in there so someone else can fill in the rest
It plays a huge roll in avys..it is important to understand which side the sun is hitting the slope. Whether it is North..South..East..West. Which means that if the sun is hitting the south facing slope the snow will start to consolidate it's pack quicker than the north facing slope. This means that the side the sun is not contacting (leeward side) is going to have a colder snow pack. With this in mind, the leeward side will have more of an avalanche danger than the the sun side. Reason being...the colder side will tend to have more cold days and will probably keep more unstable layers and also create more unstable layers. Whether it be from new snow fall..old weak layers not metamorphing or faceted snow.
Sun aspect will be more consolidated than the leeward side making it more stable. Leeward side will still have all the basics to form slab avalanches and will be more dangerous. The leeward side needs to be looked at with OPEN eyes and studied before you ride it.
Now..you have to look at wind..it is also as important as sun. The wind will pick snow up on one aspect of the slope and deposit it on the leeward side (the opposite slope). So, you have to look at the aspect and determine the leeward side to understand what aspect is being loaded. As stated above in this thread..wind loading is very unstable due to the amount of snow being deposited...where the snow is being deposited and how fast it is being deposited.
I left some blanks in there so someone else can fill in the rest
PhantomCat, I like your aspect answer.
To further the discussion, here is what troubles me about avy education and snow science....very few things are true ALL of the time. So not having a full understanding of the issue can really bite you. For instance it was stated that the southern aspect gets more warming from the sun so the snow consolidates more quickly leading to a generally more stable snowpack, which is all absolutely true.... EXCEPT later in the spring when the sun is more intense and creates greater melting in the snowpack. When that happens the southern aspects can become dangerously unstable in the mid-late afternoon while the Northern aspects are still safe. This can especially be an issue in Alaska and Northern Canada where you can ride well into May or even June and have 18+hrs of sun. Everytime I think that I have a concept wired someone will throw an exception at me. It keeps me a little scared and wary of the avalanche dragon, which I suppose is a good thing.
This is a great thread. Thanks to MtnPull and others for putting in the time and knowledge required here.
To further the discussion, here is what troubles me about avy education and snow science....very few things are true ALL of the time. So not having a full understanding of the issue can really bite you. For instance it was stated that the southern aspect gets more warming from the sun so the snow consolidates more quickly leading to a generally more stable snowpack, which is all absolutely true.... EXCEPT later in the spring when the sun is more intense and creates greater melting in the snowpack. When that happens the southern aspects can become dangerously unstable in the mid-late afternoon while the Northern aspects are still safe. This can especially be an issue in Alaska and Northern Canada where you can ride well into May or even June and have 18+hrs of sun. Everytime I think that I have a concept wired someone will throw an exception at me. It keeps me a little scared and wary of the avalanche dragon, which I suppose is a good thing.
This is a great thread. Thanks to MtnPull and others for putting in the time and knowledge required here.
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to further that, or the idea of how its dangerous on one aspect MOST of the time, but when it reverses, it can be VERY dangerous.
we have a very common play area that is south south/west facing and is normally rather stable, but it is notorious for getting ice layers from the sun where protected aspects stay nice and dont get all icy.. so, it may not be as common, but when it happens can be very dangerous.
I have been very busy and kind of let this thread slide a little. Let's get it started back up. Lot's of good post's and comments.
Let's talk about triggers.
What is a trigger?
A trigger point?
A sympathetic trigger?
A remote trigger?
When answering any of these, please explain briefly about it.
Let's talk about triggers.
What is a trigger?
A trigger point?
A sympathetic trigger?
A remote trigger?
When answering any of these, please explain briefly about it.
A trigger is anything that causes a snowpack to release, starting an avalanche. The trigger point is the spot at which the trigger actually initiated the avalanche. A trigger can be natural, or it could be a person or animal.
S
snengineer
Well-known member
Trigger points are weak points in the snow or in the slab layer. For example we say a slab breaks above us while on a hill, this is due to the fact that the slab of snow you are riding on is blown thin at the crest or near the ridge of a hill, this crest or ridge can be a slab trigger point. Another popular trigger point occurs around rocks that are near or above the snow surface. Trees and other vegetation can cause avalanche trigger points, snow does not bond to rocks trees and vegetation like it bonds to more snow. Once you add a sledder to the mix this causes these weak points to fail = avalanche.
Here in Utah the majority of our avalanches occur the first 4-6 weeks of the snow season (Dec-Jan). during this time there are MANY trigger points exposed in the snow pack, the snow is not settled and bad things happen. usually if we can make it to new years without incident things look much better throughout the mid-late winter.
....I love snow
Here in Utah the majority of our avalanches occur the first 4-6 weeks of the snow season (Dec-Jan). during this time there are MANY trigger points exposed in the snow pack, the snow is not settled and bad things happen. usually if we can make it to new years without incident things look much better throughout the mid-late winter.
....I love snow
Mtnpull, analyze the current weather system affecting Washington state right and the possible scenarios that could come of it. (if you would)
What should we as sledders be watching for once the snow flies following the current high pressure event.
I believe I know the answer but this may be helpful as a scenario we can learn from.
)What should we as sledders be watching for once the snow flies following the current high pressure event.
I believe I know the answer but this may be helpful as a scenario we can learn from.
Very interesting topic!
Josh Zieske
Josh Zieske
Trigger points are weak points in the snow or in the slab layer. For example we say a slab breaks above us while on a hill, this is due to the fact that the slab of snow you are riding on is blown thin at the crest or near the ridge of a hill, this crest or ridge can be a slab trigger point. Another popular trigger point occurs around rocks that are near or above the snow surface. Trees and other vegetation can cause avalanche trigger points, snow does not bond to rocks trees and vegetation like it bonds to more snow. Once you add a sledder to the mix this causes these weak points to fail = avalanche.
Here in Utah the majority of our avalanches occur the first 4-6 weeks of the snow season (Dec-Jan). during this time there are MANY trigger points exposed in the snow pack, the snow is not settled and bad things happen. usually if we can make it to new years without incident things look much better throughout the mid-late winter.
....I love snow
Great post. I do want to expand a little on your post. Especially where you mentioned trigger points around rocks and outcroppings. Big rocks, trees and vegetation can sometimes be anchors in the snow pack, in a sense holding the snow in place. When we have a trigger near exposed rocks it is usually due to a weak layer like depth hoar and the snow pack is thin in those areas next to the exposed rocks. Because of the thin snow pack it makes it easier to trigger that weak layer buried below near those exposed rocks. The scary thing about this is that some slopes may sustain a load of sledders running up and down it without failing, but somebody gets to the right spot (maybe near some rock outcroppings where the snow pack is thin) and game over, the slope slides.
Mtnpull, analyze the current weather system affecting Washington state right and the possible scenarios that could come of it. (if you would
What should we as sledders be watching for once the snow flies following the current high pressure event.
I believe I know the answer but this may be helpful as a scenario we can learn from.
I think with me being in Utah I would be very un-useful. I would stay closely tuned to your avy forecasters in the area. They will be very on top of your conditions.
M
MVR
Well-known member
This is a great thread by the way...very useful info so far.
We teach a theory in our courses that can help guard against folks getting bit by the misunderstanding of snow science, we call it "terrain avoidance".
When you think about all the things that happen when you're out on a ride, the only thing you can control is the terrain that you travel through or near. Keeping this in mind, we first:
Have the student become very familiar with the avalanche forecasting service for their chosen riding area.(check it every day, even those that are not riding days...this way you will know the trends)
Next: they should understand exactly how the avalanche danger scale works and what the implications of each level means to someone traveling through that condition.
Armed with these two tools and basic knowledge of the anatomy of an avalanche, a rider can go out and have a "safer" day with little to no knowledge of snow science...Example- There may be a forecast which doesn't indicate that a rider should avoid any terrain, vs. the forecast that indicates that the rider should stay away from some or all avy terrain or stay home and work on the honey-do list. Again, an understanding of what the danger ratings mean is key.
In the circumstance mentioned in a previous post, when the conditions switch on you, the forecast center will indicate the change and provide the proper warnings...this you can use to aid in your decision.
For anyone who hasn't used one before, a forecast will give a general danger rating for the different aspects, elevations, and areas of a region. This information will be based on a mix of topics including; recent weather, snow pack trends, wind, observed slides, etc. Basically, the avalanche forecaster does the science part so that you don't have to. He/she gives you a finished product that you can use as a piece in your decision making process when deciding what terrain you should avoid on your ride in order to stay safe.
For anyone who is uneasy/unsure about making go/no go decisions in the mountains, I would recommend that they get with someone who has it dialed...practice gathering info and making preliminary plans (pretrip), then go with that experienced person into the field and put your plan into play under their supervision...this will hopefully protect the learner from the harsh reality of making the wrong decision in the early phases of their learning process. Keep in mind that this "process" is ongoing, and ever-evolving, throughout the trip.
Lets recap:
1. It is possible to be "safe" in the mountains without being steeped in snow science. Basic knowledge of avalanche anatomy and proper use of a current forecast is a good start. Snow science has it's place and is fun to delve into once you have a solid understanding of the decision making process.
2.Avalanche forecast centers will be up on the most current general conditions and even if your talking seasonal changes from a cold snow pack to spring freeze/thaw conditions, they will be implementing their warnings accordingly.
3.Practice and patience is required in order to get handy with this. Someone once told me: "The more I know about avalanches, the more I realize that I don't know". Never let your guard down, always assume that you have missed something and leave yourself an out.
*I put safe in quotes up above because we feel that every individual's interpretation of "safe" is different. Put individuals in a riding group, and the interpretation changes again...
Some interesting facts:
1. Most avalanche fatalities happen under an avalanche danger rating of Considerable. (Not many people out when the rating is high or extreme, and when it's low, well it's low) We like to think of Considerable as almost high...and so use more caution.
2.If you are of the mind to completely brush off the avy educators of the world and go out willy-nilly into the mountains to ride whatever, whenever- remember this proven fact: If only one person is exposed to danger at a time, lives will be saved. I will say this another way; if you allow only one person to climb the hill at a time, only one person is exposed. Look at the case studies and you will see in multiple burial accidents all the party would have had to do is climb one at a time with the rest of the crew in a safe area with eyes on the climber and they would have saved lives.
Mtnpull has a great thread going here, encourage your friends to read up!
Thanks for reading.
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