Very interesting Adam. Thanks for starting such a great thread! 
Snow Science Thread
- Thread starter mtnpull
- Start date
if the graupel snowfall is followed by an extended cold spell with more snowfall, is it possible the graupel layer may not stabilize at all, leading to a prolonged weak layer?
In other words, if the weather patterns dont go through warm/cold cycles, would the weak layer remain?
Last season, this type of weather pattern happened...and many areas experienced an early weak layer that never fully stabilized, leading to many avy deaths later in the season. Not saying the weak layer was caused by graupel, but it is possible.
Thanks for starting this thread...if even on person learns something that will save a life, then it is a HUGE success!!
I think that factors will definitely contribute to how long before it stabilizes. I would say that it would be rare for it to remain real persistent like a rain crust though.
The only way to know for sure if a graupel layer has stabilized is to dig a snow pit and do some stress tests. We can talk about pits later. I recommend everybody dig a couple pits this year and look at the different layers and perform some stress tests. It is actually pretty fascinating. Although it is hard sometimes to pull away from a great day of riding to do, it is worth the education you get from doing it.
A few key points on graupel and then we will move on. Remember, because of the characteristics of graupel it tends to roll down the slope into less steep locations such as mid slope break overs, bottom of cliff bands that level out some and other less steep areas. Because of this you can trigger a slide much lower on the slope than usual.
Usually graupel only falls in short bursts leaving very little accumulation. But because it can roll down and build up in shallower areas it can still accumulate more in those areas. I have occasionally seen graupel fall in 1-2" at a time.
Unless you are on a mountain to see the graupel fall you may never know it has. It can fall in isolated areas and many times not foreseen by avy forecasters. Digging a pit is the best way to know if there has been a recent deposit of graupel.
Surface Hoar, I experienced some of this last year in NE Washington during the cold spell late January, caused by cold, clear , humid conditions. It was literally like shaved ice about 2 1/2"-3" thick and I thought at the time that if it snowed on top of this it would be very unstable, I don't know how long it would take to bond, if ever. I kick myself for not taking pictures, I haven't seen this before or since. Great thread.
I too seen this last year in the snowies. It is a frozen variation of dew. It can be really cool looking in the morning when the sun comes out and shines on it.
Both you guys are right.
Surface hoar is basically the formation of frost on top of the snow. It requires a clear cold night and humidity. In these conditions it can literally grow each night. This means that surface hoar will not grow in thick pines, but will grow in aspen groves and open meadows and on slopes.
Also air can only hold only so much water (humidity) colder air cannot hold as much as warmer air so as the nights get colder the moisture the air looses forms the frost (surface hoar) on the snow. If there is a dense fog the hoar will not grow as it does not have the clear night, however if there is a light fog it can grow like crazy. Additionally cold air sinks into valleys and meadows. This means the surface hoar will typically grow better at lower elevations. The wind can easily ruin or blow the surface hoar away so you tend to see it less at the ridge tops.
All these factors make surface hoar very difficult to forecast for. Because of the wind factor it can be prevalent on one slope and non existent on another. Let's say there is a dense fog the surface hoar will not grow in the dense fog, but just above the fog layer it can grow leaving surface hoar at one elevation and not at another (bath tub ring effect). Seeing a surface hoar layer in a pit can be very difficult even to the trained eye. This means the best way to predict a surface hoar weak layer is by observation. Making mental notes of the slopes and areas that surface hoar is prevalent and those it is not. Do this prior to snow piling up on the layer.
Surface hoar is a very weak and persistent layer. It can fail to bond the entire winter. It can fail in the snow pack in two different ways.....either by collapsing or shear. Collapsing happens when the snow falls on the hoar and the hoar remains standing. Eventually as the weight piles up the standing surface hoar collapses with and audible thud. Shear happens when the crystals are laid flat instead of remain standing. The snow piles up and it fails sliding on those crystals. It is more likely to bond to the layers if it is laid flat rather than standing. The best way to detect surface whore in the snow pack is to dig a pit and do a shear test or a compression test and look for crystals on the bottom of the snow block.
Surface hoar is basically the formation of frost on top of the snow. It requires a clear cold night and humidity. In these conditions it can literally grow each night. This means that surface hoar will not grow in thick pines, but will grow in aspen groves and open meadows and on slopes.
Also air can only hold only so much water (humidity) colder air cannot hold as much as warmer air so as the nights get colder the moisture the air looses forms the frost (surface hoar) on the snow. If there is a dense fog the hoar will not grow as it does not have the clear night, however if there is a light fog it can grow like crazy. Additionally cold air sinks into valleys and meadows. This means the surface hoar will typically grow better at lower elevations. The wind can easily ruin or blow the surface hoar away so you tend to see it less at the ridge tops.
All these factors make surface hoar very difficult to forecast for. Because of the wind factor it can be prevalent on one slope and non existent on another. Let's say there is a dense fog the surface hoar will not grow in the dense fog, but just above the fog layer it can grow leaving surface hoar at one elevation and not at another (bath tub ring effect). Seeing a surface hoar layer in a pit can be very difficult even to the trained eye. This means the best way to predict a surface hoar weak layer is by observation. Making mental notes of the slopes and areas that surface hoar is prevalent and those it is not. Do this prior to snow piling up on the layer.
Surface hoar is a very weak and persistent layer. It can fail to bond the entire winter. It can fail in the snow pack in two different ways.....either by collapsing or shear. Collapsing happens when the snow falls on the hoar and the hoar remains standing. Eventually as the weight piles up the standing surface hoar collapses with and audible thud. Shear happens when the crystals are laid flat instead of remain standing. The snow piles up and it fails sliding on those crystals. It is more likely to bond to the layers if it is laid flat rather than standing. The best way to detect surface whore in the snow pack is to dig a pit and do a shear test or a compression test and look for crystals on the bottom of the snow block.
S
Soccerd6
Well-known member
cool - I learned something new today - I did not know the name for this type of precipitation! I always assumed it would form an unstable layer - seemed like a no-brainer!
good topic!
good topic!
Thanks for this Post. I'm learning and loving it.
Would anyone who knows more about this subject than I do care to discuss the legitimacy of "snow compaction" harming the land, causing erosion, harming "snow mammals", etc?
I can't for the life of me find any substance to those claims whatsoever.
I can't for the life of me find any substance to those claims whatsoever.
I have been busy the last few days, with little time to post.
Have we covered Surface Hoar well enough? Are there any further comments or questions about it?
If so post them up.
In the mean time, let's talk about Depth Hoar.
What is it?
How is it formed?
Does it bond well?
How long does it take to stabilize?
Is it easy to forecast?
Have we covered Surface Hoar well enough? Are there any further comments or questions about it?
If so post them up.
In the mean time, let's talk about Depth Hoar.
What is it?
How is it formed?
Does it bond well?
How long does it take to stabilize?
Is it easy to forecast?
Not being familiar with this I looked it up to try to understand more about this condition, now my brain hurts. There are a lot of scientific formulas that i found here, too much for me maybe you can simplify. U. S. DEPARTMENT OF AGRICULTURE
FOREST SERVICE
ALTA AVALANCHE STUDY CENTER
Project C
Progress Report No. 2
THE FORMATION RATE OF DEPTH HOAR
J. C. Giddings
Dept. of Chemistry University of Utah
and
E. LaChapelle
Avalanche Hazard Forecaster
April 1961
U. S. DEPARTMENT OF AGRICULTUREFOREST SERVICE
ALTA AVALANCHE STUDY CENTER
Project C
Progress Report No. 2
THE FORMATION RATE OF DEPTH HOAR
J. C. Giddings
Dept. of Chemistry University of Utah
and
E. LaChapelle
Avalanche Hazard Forecaster
April 1961
Y
Yamahamod
ACCOUNT CLOSED
Depth Hoar:
Looks like:
Sparkly, larger grained, beginning and intermediate facets are square 1-3 mm, advanced facets can be cup-shaped 4-10 mm.
Feels like:
Loose, runs through your fingers, granular, crunchy when chewed.
Smells like:
The ground. (because the rapid diffusion of warm, moist air from the ground causes depth hoar)
Also called:
Temperature Gradient (TG) (but this is an outdated term) sugar snow, squares, sometimes incorrectly called "hoar frost" by old, rural geezers.
Formed:
From large temperature gradients between the warm ground and the cold snow surface. Usually requires a thin snowpack combined with a clear sky or cold air temperature. Grows best at snow temperatures from -2 deg C to -15 deg C.
Mechanical Properties:
Behaves like a stack of champagne glasses. Relatively stronger in compression than in shear. Fails both in collapse and in shear. Especially nasty when it forms on a hard bed surface. Commonly propagates long distances, around corners and easily triggered from the bottom--your basic nightmare.
Persistence:
Extremely persistent in the snowpack from several days to several weeks, depending on temperature. The larger the grain, the more persistent. Percolating melt water in spring often re-activates large-grained depth hoar. Depth hoar is guilty until proven innocent.
Distribution Pattern:
At mid latitudes, mainly on shady aspects (NW-NE). In very cold climates, forms on warmer slopes (sun exposed, near fumaroles, non permafrost areas). At arctic and equatorial latitudes, it shows much less preference for aspect.
Regional Differences:
• Continental climates: extremely common throughout the season. Often makes up the entire snowpack until about February.
• Intermountain climates: Common before about January.
• Maritime climates: Rare and usually in the early season.
Forecasting considerations:
Never underestimate the persistence of faceted snow as a weak layer. Makes large and scary avalanches. Carefully measure temperature gradients across the weak layer. Large gradients mean the snow will remain weak, small gradients mean the snow is gaining strength but it takes several days to several weeks depending on temperature.
Best Stability tests:
Explosives tests, cornice drops, Rutschblock, compression test, test slopes.
Routefinding Considerations:
Easily triggered from the bottom of a slope or from an adjacent flat area. Pay attention to what your slope is connected to. Depth hoar avalanches usually triggered from a shallow snowpack area--avoid rocks outcropping in the middle of a slope.
Watch the video here: http://www.avalanche.org/~uac/encyclopedia/depth_hoar.htm
I didn't even know such a thing happens. Good to know but all this will still not replace all your saftey gear.
Thanks
Looks like:
Sparkly, larger grained, beginning and intermediate facets are square 1-3 mm, advanced facets can be cup-shaped 4-10 mm.
Feels like:
Loose, runs through your fingers, granular, crunchy when chewed.
Smells like:
The ground. (because the rapid diffusion of warm, moist air from the ground causes depth hoar)
Also called:
Temperature Gradient (TG) (but this is an outdated term) sugar snow, squares, sometimes incorrectly called "hoar frost" by old, rural geezers.
Formed:
From large temperature gradients between the warm ground and the cold snow surface. Usually requires a thin snowpack combined with a clear sky or cold air temperature. Grows best at snow temperatures from -2 deg C to -15 deg C.
Mechanical Properties:
Behaves like a stack of champagne glasses. Relatively stronger in compression than in shear. Fails both in collapse and in shear. Especially nasty when it forms on a hard bed surface. Commonly propagates long distances, around corners and easily triggered from the bottom--your basic nightmare.
Persistence:
Extremely persistent in the snowpack from several days to several weeks, depending on temperature. The larger the grain, the more persistent. Percolating melt water in spring often re-activates large-grained depth hoar. Depth hoar is guilty until proven innocent.
Distribution Pattern:
At mid latitudes, mainly on shady aspects (NW-NE). In very cold climates, forms on warmer slopes (sun exposed, near fumaroles, non permafrost areas). At arctic and equatorial latitudes, it shows much less preference for aspect.
Regional Differences:
• Continental climates: extremely common throughout the season. Often makes up the entire snowpack until about February.
• Intermountain climates: Common before about January.
• Maritime climates: Rare and usually in the early season.
Forecasting considerations:
Never underestimate the persistence of faceted snow as a weak layer. Makes large and scary avalanches. Carefully measure temperature gradients across the weak layer. Large gradients mean the snow will remain weak, small gradients mean the snow is gaining strength but it takes several days to several weeks depending on temperature.
Best Stability tests:
Explosives tests, cornice drops, Rutschblock, compression test, test slopes.
Routefinding Considerations:
Easily triggered from the bottom of a slope or from an adjacent flat area. Pay attention to what your slope is connected to. Depth hoar avalanches usually triggered from a shallow snowpack area--avoid rocks outcropping in the middle of a slope.
Watch the video here: http://www.avalanche.org/~uac/encyclopedia/depth_hoar.htm
I didn't even know such a thing happens. Good to know but all this will still not replace all your saftey gear.
Thanks
Great post yamahamod! That is what this thread is all about.....getting people thinking about how snow works and RESEARCHING it. There is some great info out there.
I will post more about depth hoar when I have a little more time. In the mean time let's get a few more peoples insight.
I will post more about depth hoar when I have a little more time. In the mean time let's get a few more peoples insight.
This thread needs to stay near the front, not on page two.
I agree, it needs to achieve sticky status.
How do we identify these layers when they are burried in the snowpack?
I believe that's the purpose of digging a snowpit.
Usually the avalanche forecasters are pretty on top of weak layers like this and include it in their assessments. However, the best bet is to do as the previous poster suggested and dig a snow pit. It is a great way to learn about the snow and it's stability or lack thereof.
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