The Sun's Influence!

With sunny and mild weather forecast for the coming week, it's useful to take into account which aspect of slope to ride on- and when. We came across this rather in depth article which really shows what a marked difference the sun can have on different orientated slopes....

The compass direction a slope faces
(i.e. North, South, East, or West.)
Slope Aspect with respect to the sun:

The direction a slope faces with respect to the sun (aspect) has a profound influence on the snowpack. It often takes several years of experience in avalanche terrain before most people appreciate the importance of aspect. If you don’t know your north from south, then you had better learn, because someone who doesn't know the aspect has missed one of the most important pieces of the avalanche puzzle. Buy a compass. Use it often and work on developing an intuitive feel for slope aspect. No excuses on this one.

The influence of aspect with respect to the sun is most important at mid latitudes, say from about 30 degrees to around 55 degrees--from about the southern U.S. border to about the northern British Columbia border. At equatorial latitudes, the sun goes almost straight overhead, which shines equally on all slopes. At arctic latitudes, in the winter, the sun is too low on the horizon to provide much heat and when it finally gets high enough in the spring and summer, it just goes around in a big circle anyway, shining on all the aspects with nearly the same intensity. Thus, in the arctic spring, aspect has some influence but not nearly as significantly as in mid latitudes. Therefore, the importance of aspec is primarily at mid latitudes.

At mid latitudes in the northern hemisphere:

• North facing slopes receive very little heat from the sun in mid winter. Conversely, south facing slopes receive much more heat. Therefore, a north facing slopes will usually develop a dramatically different snowpack than a south facing slope.

• South facing slopes tend to be warmer and often develop thin ice crusts. Because these crusts tend to grow weak layers around them from near-surface faceting, be careful not to assume southerly aspects are safer.

• How about east and west? East facing slopes catch sun only in the morning when temperatures are colder while west facing slopes catch the sun in the warm afternoon. Consequently, east facing slopes are colder than west facing slopes.

• A cold snowpack tends to develop more persistent weak-layers than a warm snowpack A cold snowpack commonly develops notoriously fragile weak-layers such as facets and surface hoar. Largely because of this, the lion's share of avalanche accidents occurs on north and east facing slopes, partly because that is where we find the best snow and people tend to trigger more avalanches there, but mostly because they exhibit more persistent weak layers.

• In wet snow conditions due to strong sun, it's just the opposite of a dry snowpack: south and west facing slopes will usually produce more wet avalanches than the more shady slopes.

• During prolonged cloudy or stormy conditions when the sun seldom shines on the snow, there will be very little difference between sunny and shady slopes.

• Remember that in the Southern Hemisphere it's just the opposite. South facing slopes are colder than north facing ones.

Human Factors:

Seemingly, subtle differences in slope aspect can have a huge effect on the stability of the snow. I can't count the number of accidents I have investigated in which people start snowmobiling, skiing or snowmobiling in a bowl on a safe aspect, but as they use up the snow, they not only gain confidence, but they tend to slowly work their way around the bowl onto the progressively more dangerous aspects, until someone finally triggers an avalanche.

Additional Terms:
Anchors Hard Slab Avalanche Slide
Aspect High Danger Sluff
Avalanche High Marking Snowpit
Avalanche Path Isothermal Soft Slab Avalanche
Avalanche Transceiver Layer, Snow Stability
Bed Surface Leeward Stability Test
Collapse Loading Starting Zone
Concave Slope Loose Snow Avalanche Stepping Down
Considerable Danger Low Avalanche Hazard Sun Crust
Convex Slope Melt-Freeze Snow Surface Hoar
Cornice Metamorphism, Snow Sympathetic Trigger
Corn Snow Moderate Danger Temperature Gradient
Couloir Persistent Weak Layers Terrain Trap
Cross Loading Point-Release Track
Crown Face Probe Trigger
Danger Ratings Propagation Trigger Point
Deep Slab Avalanche Rain Crust Upside-Down Storm
Density, Snow Remote Trigger Weak Layer
Depth Hoar Rime Weak Interface
Dry Snow Avalanche Runout Zone Wet Snow Avalanche
Extreme Danger Sastrugi Windward
Faceted Snow Settlement Wind Loading
Fracture Ski or Slope Cut Wind Slab
Glide Skinning, Skin Track Whumpf
Graupel Slab


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