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Avalanche Engineers - the film - and a question.

post #1 of 6
Thread Starter 





Now notice a couple of things - the type of crystals they're creating in the lab sure sounds like

surface hoar


but the conditions they create are somewhat different than the conditions NAC describe on their page.

Movie:

cold sunny calm day (followed by storm)

NAC:


clear + humid calm day, forms in the shade  (followed by storm)


So...do the primary driving factors for surface hoar vary by location (one thing in Montana, another in California, frex?)   Or is the movie actually talking about faceted snow, the sun creating a gradient > 1C per 10cm?

If #2 - then does faceted snow "fill in" the sunny zones where surface hoar didn't occur?


Edited by cantunamunch - 6/12/15 at 7:15am
post #2 of 6
I think tat you're talking about apples and oranges to a degree. Surface hoar and faceted snow (aka depth hoar) aren't the same thing.

Surface hoar forms on the (duh) surface, under the described conditions but becomes a weak layer when buried.

Faceted snow/depth hoar forms under the surface, transforming the already buried snow and creating a weak layer.

I may have misunderstood your question.
post #3 of 6
Thread Starter 
Quote:
Originally Posted by Bob Lee View Post

I think tat you're talking about apples and oranges to a degree. Surface hoar and faceted snow (aka depth hoar) aren't the same thing.

 

Yes, I know.     Now look at the video at around 5:20 onwards.    Which is that?   It's at the surface.   It needs to be buried to be dangerous.     But the conditions described are those that create faceted snow.     

 

Quote:
Originally Posted by Bob Lee View Post


Faceted snow/depth hoar forms under the surface, transforming the already buried snow and creating a weak layer.
 


I will repeat it using different phrasing:   can the -same- temperature and insolation conditions create both types of layers (surface hoar and whatever it is they're showing us in the video - on different parts of the mountain, surely).      Can the same cart carry both apples and oranges?


Edited by cantunamunch - 6/14/15 at 1:49am
post #4 of 6
The problem is, this is a lab. So, what they are doing, as I understand it, is simulating different types of snow conditions via temperature and humidity control. The brief glimpse of surface hoar is only talking about potential conditions at the surface, while below something else is happening. It's not the type of snow they are discussing at that point, but only the external forces. The type of facet they are talking about is just below the surface ("near surface hoar"), then more snow shows up, pushing it lower relative to the whole.
Edited by sibhusky - 6/14/15 at 10:02am
post #5 of 6
Thread Starter 
Quote:
Originally Posted by sibhusky View Post

The problem is, this is a lab. So, what they are doing, as I understand it, is simulating different types of snow conditions via temperature and humidity control. The brief glimpse of surface hoar is only talking about potential conditions at the surface, while below something else is happening. It's not the type of snow they are discussing at that point, but only the external forces. The type of facet they are talking about is just below the surface ("near surface hoar"), then more snow shows up, pushing it lower relative to the whole.

 

Notice that they did generalise and tell us that clear sunny days cause this (I'm going to call it Type 3) weak layer in the broader environment, outdoors.    If they're right, and I have no reason to doubt them,  the external forces do not match the external forces needed for surface hoar.   Yes, it needs to be buried by later storms (which makes the layer unlike depth hoar).   

So we apparently either have yet another mechanism for weak layers, one not listed on NAC's website.      Or this is a subspecies of faceting, with the weird property of not happening in shady spots and needing to be buried by later storms.       

post #6 of 6
Quote:
Originally Posted by cantunamunch View Post

I will repeat it using different phrasing:   can the -same- temperature and insolation conditions create both types of layers (surface hoar and whatever it is they're showing us in the video - on different parts of the mountain, surely).      Can the same cart carry both apples and oranges?

th_dunno-1[1].gif. There's so much that I don't know/understand about the creation of weak layers, and I'm not a snow scientist. I'm more focused on finding weak layers, and the incredible amount of variation in conditions that affect snow within even a small area makes it really difficult to forecast weak layers based solely on weather conditions.

While I appreciate snow science like in the video, a lot of skiable slopes aren't like laboratory conditions - variations in topography, aspect, etc. That's my excuse. tongue.gif
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