4 November 2010: More Paradise climax avalanches.

"However, when slope usage is taken into account ( number of people ), aspect as a predictor is too weak. If anything, the risk is slightly higher on east than west aspects, and north-south aspects form a mirror image of each other in this Canadian data set."

OK this is getting interesting now.
So CM please explain the above statement in layman's terms and how it relates to the real world... I'm not quite undersanding it.

citing research by Grimsdottir...

The paper that was derived from the same Master's research is here . There's a wealth of nifty work here by the same group.

The data set accumulated in Grimsdottir/McClung was from a heli-ski outfit that was trying to avoid avalanches. It's a testament to their skill, that, after correcting for usage patterns, the guides manage to get the risk of a slide down to a constant (and hopefully acceptable) level on all aspects. To me, those data suggest that, once you know what you're doing, your probability of getting caught in a slide is independent of aspect.

Using that information to suggest broadly that aspect isn't a useful predictor of instability is a great way to get someone into trouble.

Aspect matters a lot to my decision making in the winter. Which aspects are least safe, however, can change in ten minutes.

It's a mellow cloudy early morning in February, and you're on an alpine North-South ridge at 6000', 18 hours after a typical storm that dropped 2-3' of snow on level ground at 4,000' in the Central Cascades. Both the East and West sides of the ridge are treeless and pitched at a sweet 38 degrees. Telemetry showed winds to 30+ mph at 4,000' overnight out of the WSW, and the fresh cornices draping the East side of the ridge agree. You're guiding a party that has expressed a strong preference for minimum risk (never mind how you got here). Knowing only these facts, which side of the ridge would you rather ski? East, West, or both?

My first statement was in response to aspect as a predictor of avalanche activity.

Statement 1

To me, those data suggest that, once you know what you're doing, your probability of getting caught in a slide is independent of aspect.

Statement 2

Aspect matters a lot to my decision making in the winter. Which aspects are least safe, however, can change in ten minutes.

Yes, as I have already stated several times, aspect is in general a poor predictor of avalanches.

Statement 3

Using that information to suggest broadly that aspect isn't a useful predictor of instability is a great way to get someone into trouble.

Which is it? Statement 1, Statement 2, or Statement 3? You agree that the relationship between aspect and instability may change rapidly-which means aspect is a poor predictor of avalanches. In the same post you go on to say that it's unsafe to suggest that aspect is a poor predictor of avalanches. You can't have it both ways.

Also, I haven't suggested it ... it's suggested by The Avalanche Handbook.

Further, I didn't cite the paper you referenced, I cited The Avalanche Handbook because a lot of research gets published but not all of it ends up in the "bible".

It's a mellow cloudy early morning in February, and you're on an alpine North-South ridge at 6000', 18 hours after a typical storm that dropped 2-3' of snow on level ground at 4,000' in the Central Cascades. Both the East and West sides of the ridge are treeless and pitched at a sweet 38 degrees. Telemetry showed winds to 30+ mph at 4,000' overnight out of the WSW, and the fresh cornices draping the East side of the ridge agree. You're guiding a party that has expressed a strong preference for minimum risk (never mind how you got here). Knowing only these facts, which side of the ridge would you rather ski? East, West, or both?

I wouldn't choose either of those slopes in those conditions when a party has expressed their preference for minimum risk.

So CM please explain the above statement in layman's terms and how it relates to the real world... I'm not quite undersanding it.

This means that, after removing certain assumptions and biases from the analysis, it was clear that aspect is a poor predictor of avalanches.

It doesn't mean aspect is useless, but on the other hand all of the following are true:

Instability lingers on cold aspects.
Cold aspects are less prone to thaw instability.
South facing slopes are prone to crust formation.
Radiation recrystallisation is problematic on south aspects.
Surface hoar and depth hoar form independent of aspect.
Wind loading patterns are chaotic and cannot be resolved simply by evaluating the prevailing wind direction.

Here are three visualisations of wind directions on separate days at Crystal Mountain. These images were created for a project I undertook to satisfy my own curiosity.

How would you forecast wind loading?







The French have done some very cool research on this, but I believe they have concluded that the prevailing winds are a poor indicator of loading patterns because of channeling that occurs when wind hits rough mountain terrain.

Which is it? Statement 1, Statement 2, or Statement 3? You agree that the relationship between aspect and instability may change rapidly-which means aspect is a poor predictor of avalanches. In the same post you go on to say that it's unsafe to suggest that aspect is a poor predictor of avalanches. You can't have it both ways.

I can. I believe aspect is an excellent predictor of instability in combination with other factors that, in general, vary with time. I do agree that aspect alone is not enough. It's misleading to new folks reading this forum to dismiss the importance of aspect entirely because its predictive power isn't simple. A season-long average misses a lot of information at the hourly, daily, storm-cycle, and monthly scales.