Precautions
Glide Block Test
This test provides a useful measure of the shearing potential of snow on a slope. It is a good indicator of avalanche danger. The tester creates a freestanding column or block of snow that is then stress-tested for shearing capability. The procedure is as follows:
- Find an area that is similar the slope you will be traveling on in slope angle. Choose a location distant from trees or a ridge.
- Dig a vertical trench that is at least three feet deep to create a three-sided rectangle around the block you intend to test. The breadth of the trench is not important, but should be wide enough to facilitate digging. You can use a shovel, a snow saw, a snowboard tip or a ski to dig the trench. The rectangle should be as long as your ski or snowboard in the axis parallel to the slope. On the axis of the slope, the rectangle should be about four feet across (the height of the average ski pole). The trench should surround the snow block on the down-slope side.
- Using a snow saw or ski, cut a line on the upslope side of your block that connects the two free ends of the trench to complete the rectangle.
- Have a person on skis or a snowboard step carefully onto the center of the block. If the block does not slide down hill, the tester should perform a series of jumps with his or her skis or snowboard. If the block remains in place, the tester can leap on it without skis or a snowboard.
Interpret the results of the procedure using the following stability scale:
Very Unstable
- Block fails during excavation
- Block fails when tester attempts to stand on it
Unstable
- Block fails as the tester flexes for jump
- Block fails with one jump (questionable)
Relatively Stable
- Block endures several hard jumps
Stable
- Block endures
- Block fails only without skis or board
Snow Pit Test
This observation is most useful in conjunction with the Glide Block Test. It can be done during the excavation of the trench for the block or in a separate location. This test searches for inconsistencies in the layering of snow that might indicate instability. The procedure is as follows:
- Find an area that is similar the slope you will be traveling on in slope angle. Choose a location distant from trees or a ridge.
- Dig a pit that is at least three feet deep using a shovel, snow saw, snowboard tip or ski.
- Using a finger or fist, test the various layers of snow for relative hardness or softness by pushing against them.
Very hard or soft layers of snow bind poorly to the other layers and indicate potential avalanche danger. Use the following chart to gauge the hardness of a layer of snow:
Tilt Column Test
This test searches for instability particularly in the upper levels of the snow. It is not useful to distinguish weaknesses deeper in the snow (more than two shovel widths). It can be performed on a flat surface. The procedure is as follows:
- Cut a cube of snow out about the dimensions of your shovel blade.
- Excavate around it down to a little more than a shovel blade in depth.
- Slide the shovel under the cube and lift it up.
- Tap on the bottom of the shovel as you tilt the shovel at a progressively larger angle until it fails.
The angle of failure will correlate to the strength of the snow pack. The steeper the angle of failure, the more stable the snow is. Be especially wary if the cube shears cleanly as it fails instead of crumbling.
Ski Pole Test
This test is very easy to use. It can be performed dozens of times a day. It is best to use this test in conjunction with the Glide Block Test, as this test will not detect deeper weak layers. The procedure is as follows:
- Just push your ski pole into the snow.
- For harder snow, use the handle of your pole.
Instability is indicated by differences in resistance as you push your pole through. Drastically different resistance in nearby layers indicates potential danger.
Hand Shear Test
This test is easy to perform and can be repeated throughout the day. The test demonstrates how well the surface snow is bonded to lower layers. It will only detect weaknesses near the surface. The procedure is as follows:
- Cut a small square of snow out using your hand.
- Using your hands, pull on the square of snow until it comes out
Beware of clean shearing of the square from the underlying snow. This indicates poorly bonded snow and potential for avalanche.
Obvious Warning Signs
- You can see evidence of recent avalanches
- You see shooting cracks in the snow
- Evidence of recent high winds or snow drifting
- Snow collapses in a wide area around your weight (i.e. a hard shell over softer snow)
- Recent heavy snow fall
- There is a forecast for rain during your outing
- Sun balls (baseball size snowballs that form as loose chunks of snow roll down a slope—imagine a self-assembled snowman ball)
References:
Mountaineering: The Freedom of the Hills, 6th edition, edited by Don Graydon and Kurt Hanson, Seattle 1997
http://www.couloirmag.com/articles/avy/tremper_series/part1.asp April, 2005