How safe is a crown?

**Aceman** · 01-02-2010, 11:07 PM

[ame="http://www.youtube.com/watch?v=sq8OlTzXYGY"]YouTube- Cornice Ridge Avalanche crown[/ame]

Watching this video I was wondering how safe it is to be underneath the crown? Is it any safer than before it slid? Aceman

**Jethro** · 01-02-2010, 11:49 PM

My take is to never trust a cornice. Their makeup is different than the snowfield on their lee side. A cornice is a large overhanging mass of usually a very dense snow. Being below a crown is a different matter. The snow has released its energy, kind of like a rubber band snapping. The snow inclined to slide has slid and what remains is the relatively save snow. If someone was to ski above you while you were inspecting the crown you could be in for a world of hurt though. It is possible to crack off some snow above the crown. If you are going to inspect a crown, be sure no one will be affectinfg the same snow from above.

**CookieMonster** · 01-04-2010, 12:26 AM

Originally Posted by ZGjethro

Being below a crown is a different matter. The snow has released its energy, kind of like a rubber band snapping. The snow inclined to slide has slid and what remains is the relatively save snow. If someone was to ski above you while you were inspecting the crown you could be in for a world of hurt though. It is possible to crack off some snow above the crown. If you are going to inspect a crown, be sure no one will be affectinfg the same snow from above.

Whether or not a crown is safe to travel below or inspect depends largely on current conditions and the amount of snow that remains above the crown line.

Recent avalanches are an absolute indicator of high instability. Your assumptions that "the snow has released its energy" and that "the remaining snow is not inclined to slide" are laughably dangerous. Have you considered that the snow remaining above the crown line is unsupported?Sometimes the snow above the crown line is no longer dangerous, but other times the snow above the crown line certainly IS dangerous.

I would say, if you need a rule of thumb, then assume that fresh crown lines below any significant amount of "hangfire" are most certainly *NOT* safe to inspect or travel below. Old crown lines should be approached cautiously and/or respectfully.

**Jethro** · 01-04-2010, 12:58 AM

Good points, but not really too different than what I stated. I think i mentioned that it is possible (maybe probable?) to cause a second slide on the same slope. If the snow slid and left a crown, then there was a tensile, not compressive force on the snow pack. That does not mean that the hanging snow is safe by any means.

When I mentioned that the snow was not "inclined to slide", I was referring to whatever triggered the initial avalanche. Obviously some snow moved and some stayed. The snow that stayed above the crown I would say was "inclined to not slide". With another trigger force, it could easily go. That is why I said you could be in for a world of hurt if you were below a crown and some one triggered the slope above you. You could also be the trigger from below

**Jethro** · 01-04-2010, 01:35 AM

I re-read my original post and I called the snow above the crown "relatively safe". While a lot of things are relative, this snow is NOT safe. It is a hangfire that is not as touchy as the snow that slid but It could be barely staying up there. I do not know what I was thinking when I posted that. CookieMonster was right to jump my case about that.

**CookieMonster** · 01-04-2010, 02:41 AM

Thanks for the additions. I don't jump on people's cases, and I apologise if my post seemed harsh - that was absolutely not the intent.

**CookieMonster** · 01-04-2010, 02:48 AM

Originally Posted by ZGjethro

If the snow slid and left a crown, then there was a tensile, not compressive force on the snow pack. That does not mean that the hanging snow is safe by any means.

Your above-quoted statement does not make sense. Tensile cracks form when, after a failure, the slab starts to move and tears away from its surroundings. I don't see how this is related to compressive forces? Do you mean "tensile force" or do you mean shear force?

Can you provide additional clarification?

**inthemtns** · 01-04-2010, 09:57 AM

The two guys in the video look to be at the top of a ridge. The slide looks like it triggered near the ground. I would think that they could rule out a propagating compression failure that would trigger anything above them. So that leaves the real danger in 1) a natural slide 2) someone triggering above. They obviously didn't think these dangers were problamatic, since they are standing directly under the crown.

What do you all look for when deciding to check out a crown?

Cheers to the very polite thread^^^^^^

**Jethro** · 01-04-2010, 10:11 AM

I was not talking about the snowpack in a vertical column, which is usually tested with compressive force, or shear(shovel test). I am thinking of the forces that result in a sliding motion. There would be a shear force between different layers, and a tensile force within the same layer. When I speak of a tensile force I am meaning gravities pull on the whole slab. At the crown that was the point where the slab snapped and the most slide prone pieces moved down slope

This is my amateur take on this. I am not a snow pro by any means

**_Aaron_** · 01-24-2010, 10:49 PM

I try to avoid skiing under crowns. Its not definitely safe or unsafe (it all depends on the case) but seriously, who wants to ski bed surface anyways?

**slo-steve** · 01-24-2010, 11:22 PM

Hi cookie monster,

In general, is there an agency who regularly measures large avalanches, or maybe just avalanches that cause fatalities? And if so, how do they measure the crowns, from below with a probe or tape measure, or from a distance with suveying equipment, or something else? I'm curious because one report about last weeks huge slide on Mt. Hood said the crown ranged from 4-10' tall. Do you know if someone actually climbed up wy-east to inspect and measure the crown? Thanks,

Steve

**inthemtns** · 01-25-2010, 02:14 AM

Originally Posted by slo-steve

Hi cookie monster,

In general, is there an agency who regularly measures large avalanches, or maybe just avalanches that cause fatalities? And if so, how do they measure the crowns, from below with a probe or tape measure, or from a distance with suveying equipment, or something else? I'm curious because one report about last weeks huge slide on Mt. Hood said the crown ranged from 4-10' tall. Do you know if someone actually climbed up wy-east to inspect and measure the crown? Thanks,

Steve

The guys that put out the local avi report here in Bozeman video their inspection of the crown for most of the avalanches in the report area. None of the slides this season have been on the scale of the Hood slide, though.

Better them than me. I avoid crowns all together.

**wra** · 01-25-2010, 07:42 AM

Originally Posted by slo-steve

Hi cookie monster,
How do they measure the crowns, from below with a probe or tape measure, or from a distance with suveying equipment, or something else? I'm curious because one report about last weeks huge slide on Mt. Hood said the crown ranged from 4-10' tall. Do you know if someone actually climbed up wy-east to inspect and measure the crown?
Steve

I triggered this slide walking the ridge the other day.

Hopped off the crown, since it was near the ridge, lacking hang fire and measured

with my ski pole.
Didn't try measuring this part

because of stress fracturing and hang fire.
Best to let the stress fractures sit for a day and settle out.

**inthemtns** · 01-26-2010, 10:02 AM

Originally Posted by wra

I triggered this slide walking the ridge the other day.

Hopped off the crown, since it was near the ridge, lacking hang fire and measured

with my ski pole.
Didn't try measuring this part

because of stress fracturing and hang fire.
Best to let the stress fractures sit for a day and settle out.

Wow that is big. Was it a giant wind slab? Was the one below a secondary trigger.

**wra** · 01-27-2010, 10:13 AM

It's part of the same slide. Slide was full depth, total winter accumulation.
This one's bigger.

and is also the entire winter snow. Second go around, much larger than the first.
The photo shows the traverse in, left side, to hop off the crown.

If a person is interested in snow, you can stand at the top and wonder or, figure out how to get out on the slide for learning.
Passive or aggressive, your choice.

**RambleOn** · 01-28-2010, 01:52 PM

holy shit man where was that?

**Grizzly Adams** · 01-28-2010, 04:56 PM

Originally Posted by wra

I should think you'd be pretty safe with that pole propping up the crown.

**wra** · 01-28-2010, 11:48 PM

Originally Posted by RambleOn

holy shit man where was that?

West Monitor bowl, Park City ridgeline.
How about this one,

Grizz?

**Skidog** · 02-01-2010, 02:47 PM

So weird to see the pockets popping out all over the place. Its like that sugar near the ground just pops out and leaves a big hole....Scary stuff out there right now...