Three avalanche factors, the conditions (snow pack and weather), the terrain and the human factors are repetitively assessed or filtered at three different levels – at home while planning, in the area and on the individual slope, therefore the term 3x3. As a control instrument the Reduction Method defines a socially acceptable residual risk level of 1, risk being the danger potential divided by the product of the reduction factors. In this formula the danger potential is a numerical representation of the forecasted danger descriptor and the reduction factors are numerical representations of slope angle, slope aspect as well as group size and spacing. These reduction factor definitions and values come from statistical evaluation of avalanche incidents.
For novices Werner Munter has defined an Elementary Reduction Method with which the backcountry user simply renounces from traveling on certain slope angles depending on the avalanche forecast: moderate danger level: no travel on slopes steeper than 40°; considerable danger level: no travel on slopes steeper than 35o; high danger level: no travel on slopes steeper than 30°; extreme danger level: no backcountry travel at all.
Werner Munter stresses that his method cannot eliminate risk, but that by using his approach the number of past avalanche fatalities could have been cut in half. The purpose of his method is to allow practitioners to make decisions within minutes without having to dig snow pits and to avoid what he refers to as a “deadly threesome”. A “deadly threesome” would be traveling on a 40° slope with a northern aspect during considerable avalanche danger.
The methodology in its existing form is only good for the Alps, but can be adapted to other areas. In Europe Werner Munter’s 3x3 Filter and Reduction Method has shaken up and sensitized mountain guide associations and alpine clubs alike. Today his book “3x3 Lawinen” is officially recognized and recommended by the UIAGM (International Union of Mountain Guide Associations) and the UIAA (International Union of Alpine Associations) and is the basis for the avalanche training programs in all alpine organizations across the Alps.
In December 2002 the third revised edition went into print with a new Golden Rule which bases on the Reduction Method and delivers quick results without having to calculate.
Further information in German can be found under www.slf.ch/info/tour0-de.html. Book information: 3x3 Lawinen – Risikomanagement im Wintersport, Werner Munter, ISBN 3-00-002060-8.
Estimating a reduction in Avalanche Forecasts
Werner Munter
The idea behind Werner Munter’s Reduction Method is to take a basic danger level and see if—through at least three observations from three perspectives: Regional, Local, and Slope—the forecasted danger can be reasonably reduced. Hence the term, 3x3 Reduction Method.
| Avalanche Report | Avy Scale | Danger Potential |
|---|---|---|
| LOW | 1 | 2 |
| MODERATE | 2 | 4 |
| CONSIDERABLE | 3 | 8 |
| HIGH | 4 | 16 |
| EXTREME | 5 | 32 |
DANGER
Starting with the basic forecasted danger...
| Class | Reduction | Factor |
|---|---|---|
| 1st | Steepest Slope Angle 35°-39° | RF2 |
| Steepest Slope Angle 30°-34° | RF4 | |
| 2nd | Avoid N sector slope (NW-N-NE) | RF2 |
| Avoid N half sector slope (WNW-N-ENE) | RF3 | |
| Avoid Named Avalanche Slope Sectors | RF4 | |
| 3rd | Constantly skied slope | RF2 |
| Large group w/distance between each | RF2 | |
| Small Group (2-4 persons) | RF2 | |
| Small Group w/distance between each | RF3 |
÷Reduction
...see if the danger level can be reduced by accounting for these factors on your proposed tour.
DANGER ÷ Reduction
= Risk
Using the following simple formula based on the Forecasted Danger (from a local bureau) determine if the risk is acceptable or not.
If the formula yields a number greater than 1, it is advisable to stay home, or seek another location for skiing. Less than one, it is probably okay to venture out. Equal to one? You should consider more factors, perhaps head out but take a closer look at conditions in the field. Prevention is worth a pound of cure, so be conservative in estimating danger as it is easier to avoid avalanches than recover if caught by one.
Example 1: GO
The forecasted danger level is Considerable (Danger =8). You plan to ski tree'd slopes where there is normally no avalanche hazard (RF=4), with a maximum pitch below 34° (RF=4), and the slopes are skied regularly (RF=2).
8 ÷ 4 x 4 x 2 = 8/32 = 1/4 = 0.25<1 so the trip is a GO
Example 2: Maybe
Same forecasted danger (8) but this time the group decides to ski steeper slopes, up to 39° (RF=2), the group consists of three persons (RF=2), and the goal is an open slope facing ENE (RF=2).
8 ÷ 2 x 2 x 2 = 8/8 = 1 so the trip is a MAYBE
In this case there are no substantial factors reducing the avalanche risk. If you're cautious, plan a different route. If you're adventurous you will probably go anyway, but should be especially concerned with the danger on the slope you intend to ski. It would be advisable to try and ascertain the stability of the slope with more field observations before skiing it.