...The Avalanche Triangle: Weather...

Huge, dense, frequent snowfalls and mild temperatures characterize Maritime--or coastal--climates. The depth of the snow, warm air and relative lack of air space within the snowpack result in a tendency toward strengthening over time. Avalanche cycles generally run during and immediately after storms and the snowpack tends toward homogeneity as layers get mashed out.

The Cascades of the Pacific-Northwest and the Sierra-Nevadas of California are good examples of mountains with Maritime snowpacks. "Sierra Cement" describes stereotypical Maritime snow.

"Scary" is a good way to describe Continental snowpacks. Shallow snow cover, light fluffy snow, cold temperatures and long dry spells between storms result in a richly textured snowpack whose layers tend toward weakening over time. The fabulous powder snow associated with this climate comes with a stiff price: avalanche cycles are not tied to storms, but can occur anytime that conditions are favorable.

You will find Continental conditions in Moab's LaSal Mountains and just about anywhere in Colorado.

Transitional snowpacks--like that of Utah's Wasatch Mountains--exhibit characteristics of both Maritime and Continental snowpacks, generally starting out weak and friable early in the winter and gaining strength as the season progresses.

In any climate, weather--past present and future--plays a huge role in the evolution of snow on the ground. The thing to remember is that snow hates rapid change: the snowpack can absorb gradual changes but can't adjust to abrupt ones.

These are the 10 Weather-Related Factors which affect the stability of the snowpack:

FACTOR:	SIGNIFICANCE:	POSSIBLE EYEBROW RAISERS:
1. Old snow depth	Helps to identify the type of changes at work in the snowpack and the degree of anchoring occuring.	Covered anchors; shallow snowpack; faceted "sugar" snow.
2. Old snow surface	Will the new snow bond or will this become a weak area in the snowpack?	Icy or hard crusts; light density snow; surface hoar; big temperature difference between old and new snow.
3. New snow depth	Is there enough new snow to cause overloading?	12" of new snow or 6" with wind.
4. New snow type	Will the new snow crystals form a slab or a buried weakness?	Graupel; needles; heavily rimed crystals.
5. New snow density	How heavy is the new snow? Will it stick?	Heavy snow on top of light snow; very dense snow.
6. Snowfall intensity	Did the new snow come in too fast for the snowpack to adjust to the added weight?	1" per hour or more for 6 or more hours.
7. Precipitation intensity	Is the weight of the water in the new snow too much for the snowpack to handle?	1" or more of actual water-weight in 12 hrs or less; rain.
8. Temperature	Is the air temperature hurting the snowpack stability?	Rapid changes; warming during the storm; persistent cold; excessive heat.
9. Wind	Is the snow being transported into deep drifts and packed into slabs? Is the wind forcing air up and over the mountains, causing heavier snowfall (orographic lifting)?	15-50 mph winds
10. Settlement	Is the snow settling and compacting?	No settlement; snow in trees.