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Thread: Isothermal == Rotten?
03-24-2012, 06:39 PM #1
Isothermal == Rotten?
I understand that an isothermal snowpack is one with no temperature gradient, but what is the result of that condition? From the context in which I usually see the term, I'm guessing that it equates to what I call a rotten snowpack, in which my boots/snowshoes/skis easily punch through to the ground (or thereabouts). Is that correct?
03-24-2012, 06:59 PM #2
Or stable? Isothermal snowpacks are caused by spring warming of the snow until it is all at 32 degrees, whereas, in mid-winter, it could be -20 degrees one meter down. When it hits this point, water permeates the snowpack and breaks down hoar crystal layers creating a solidity in the pack. However, once portions of the snow go above 32, a whole new dynamic called wet slides begins. That's how I understand it.
03-24-2012, 11:35 PM #3
I always thought isothermal = rotten.
edit, for us here generally rotten means you are postholing everywhere everything. 0 degree (C) snowpack top to bottom.
I would agree though that technicaly a snowpack can be isothermal but not rotten.
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03-25-2012, 12:36 AM #4Registered User
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- Oct 2003
Rotten usually refers to some sort of weakness in the snowpack, often facets or depth hoar. An isothermal snowpack by definition means there is no temperature gradient, in practice it is always at 0C throughout. At 0C there is free water in the snowpack as Splat alluded to. Overtime this freewater will often undergo a melt-freeze cycle as it is cooled overnight, and thus strengthens and rounds everything out.
The largest hazard comes from the first time the winter snowpack goes isothermal in the spring. At that time weak layers that have been buried all winter are warmed up to the freezing level. This melts the bonds between crystals in these weak layers and may cause them to once again be reactive. If there has been buried depth hoar you may see full climax avalanches. It is often a 2 or 3 day spring cycle. Once the snowpack has had time to adjust to this new warming the risk of slab avalanches may decrease.
To some degree it is semantics, perhaps "rotten" carries a different connotation elsewhere. It is not a standardized term.
03-25-2012, 09:45 AM #5Registered User
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- Jan 2006
I've never understood why this term hasn't been put out to pasture. What people mean when they say isothermal is that the snowpack has reached a certain saturation point that has rendered it cohesionless (or rotten). The amount of water in the snow is far more important than the fact that its nearly uniform in temperature. Snow can be isothermal, but still mostly solid.
Come on snow nerds, put isothermal out to rest with TG and ET.