3x3 filter method & reduction method

The Swiss avalanche expert Werner Munter has developed effective methods that freeriders can use to drastically reduce the ever-present risk of avalanches. He uses the 3x3 filter and reduction methods, which are also known to many as the Munter methods.

The 3x3 filter method for assessing avalanche danger

The aim of the 3x3 filter method is to reduce the risk of triggering an avalanche as much as possible. In practical avalanche science, the aim is to find out whether this slope will hold when you ski it. To do this, the three avalanche-forming factors:

1. Snow (snow and weather conditions)

2. Mountain (terrain)

3. People (skiing on a slope)

viewed from three different distances (perspectives). Namely:

1. Regional You always start with a regional assessment of the avalanche danger. Thanks to the avalanche situation and weather report, you know the danger situation in the region.

2. LocalIn the local assessment, in the area, you try to recognize the possible danger spots around you through constant observation.

3. Single slope checkIn the zonal single slope check, you search the slope you want to ski for possible danger spots.
   If you come to the conclusion 3x3 = 9 x that the conditions are favorable, you can ski down. If, on the other hand, you come to the conclusion that the conditions are unfavorable or dangerous once or more than once, you must not ride - otherwise your life will be in danger!
   The 3x3 filter method corresponds to a system of three superimposed safety nets. The smaller the area to be assessed or the closer the assessment distance, the finer the mesh size of the safety net - and ultimately the smaller the risk. The sequence of the assessment is decisive, i.e. it must start with the regional assessment (preparation), followed by the local assessment (in the area) and finally the individual slope check.

The reduction method

With the reduction method, you try to assess the overall risk and then reduce it to the lowest possible residual risk by adapting your behavior to the respective risk. Hence the name: reduction method. Werner Munter developed the reduction method on the basis of many years of statistics and probability calculations in order to establish a uniform benchmark for the acceptable residual risk. The [avalanche] risk is calculated as shown in the diagram on the right. *The value 1 is the limit of the acceptable residual risk. The risk can never be zero in the mountains. If you want to risk more, e.g. residual risk 1.33, you can do so. But no winter sports enthusiast can claim that they were unaware of the risk.

With the reduction method

...mountain guides, freeriders and ski tourers can come to a decision within a short time. The aim is always to make a yes/no decision: "to go or ... not to go".
The avalanche danger is no longer assessed on the basis of many variables, such as static friction or shear stress, but should be assessed with the help of the constants, i.e. slope inclination, terrain, map. The avalanche situation report provides information on the general avalanche danger and the snowpack structure. This information is always accessible to everyone and, with a little practice, easy to understand: the higher the avalanche risk assessment in the avalanche situation report, the higher the risk potential in the area. The hazard levels - low to high - are assigned hazard potentials in the reduction method. These hazard potentials describe the entirety of the [probable] hazard points in an area. The higher the hazard potential, the greater the probability of triggering an avalanche when freeriding. Research into snowpack stability has shown that the danger potential [= sum of danger spots] doubles with the next higher warning level on the avalanche danger scale:

Avalanche Situation Report warning levels

* Low avalanche danger, warning level 1 = danger potential 2
* Moderate avalanche danger, warning level 2 = danger potential 4
* Significant avalanche danger, warning level 3 = danger potential 8
* Major avalanche danger, warning level 4 = danger potential 16 and above...

You can select any intermediate values to independently assess the avalanche danger. For example: The avalanche situation report indicates "moderate" snow slab danger, but your own observations in the area suggest that the danger could be higher. The hazard potential is then assessed as moderately significant = 6. There are precautionary measures [= reduction factors] that can be combined [multiplied] with each other. The result of this must be at least as great as the hazard potential [= avalanche risk].
The reduction factors are divided into three classes according to their significance - for triggering an avalanche: first/second and third class. As the steepest part of the slope is ultimately decisive for triggering an avalanche, these reduction factors are first class!
Reduction factors (RF) = precautionary measures

Important!

• If the slope is "considerably" steep, rocky slopes are taboo: in other words, never exceed 39°! Avoid cornice jumps and cliff jumps.

• If it is dangerous in all exposures, the reduction factors no. 4-7 are invalid! This is often the case when the critical amount of fresh snow has been exceeded or the snow cover is soaked.

• Only moderately steep terrain (less than 30°!) can be skied at danger level "large".

• The maximum steepness to be skied is always the steepest part of the slope.

The reduction method - an example

The avalanche situation report reports "considerable" snow slab danger. This corresponds to hazard potential 8.

• The slope to be skied has a steepest point of approx. 34° >> first-class reduction factor 4

• Exposure: north-east >> no second-class reduction factor possible

• Participants: 3 experienced freeriders = small group >> reduction factor 2 or 3 if everyone keeps large distances.

This results in a residual risk of:
>> 8 : (4 x 2) = residual risk 1

If the group rides with relief distances, the residual risk is much more favorable:
>> 8 : (4 x 3) = residual risk 0.66

Result: The residual risk of triggering an avalanche is acceptable. If the slope is skied without relief distances, the limit has been reached! The greater the safety reserves, the better. As it is safer and the descent is more fun: always keep large distances.

If you use the reduction method consistently, you will have to give up a slope every now and then - which may be skied by some adrenaline freaks a little later. However, the reduction method gives you a better chance of enjoying powder slopes for many years to come. And it's so much fun - you should treat yourself to it your whole life".