• David Roberts

The Numbers Game

Whenever we check an avalanche forecast, the first thing we see is usually a number. This is the hazard, rated on a scale of 0 to 5. Across Europe, there is common definition of what these numbers mean. The definitions in North America are very similar. However, just knowing the number doesn't really tell us much about what is actually happening out on the mountain.


Another problem is that 3/5 sounds like halfway along the hazard scale. In reality it isn't. The table below is from Swiss data but it is a typical distribution for mountain areas worldwide.

Table of avalanche hazard levels per day of the winter

We can only have zero avalanche hazard if there is no snow whatsoever. A level 5 hazard results in ski-resorts being evacuated, roads & railways being shut down and headlines about Switzerland on the BBC. In real-life, day-to-day use, the avalanche hazard scale ranges from 1 to 4 (just!). A rating of 3 is effectively near the top, not half-way up.


This is the reason for some forecasters moving away from the numbers and only using the adjectives that are associated with each level on the scale. A level 3 hazard means that there is a "considerable" danger. Each level has a set of descriptors, summarised in the chart below.

Real life can rarely be described in discrete steps, like the ones in the previous table. Avalanche hazard increases in an analogue manner. This means that it is important to read the whole forecast. The written description can include elements from higher risk categories or hint that the danger is only just enough to warrant a particular level. Compare these two level 4, "high" forecasts for 23rd December 2019 from SLF.

Areas A & B 23rd Dec 2019

Although the second one is shorter, the last two sentences imply a level of avalanche danger in excess of that in the first forecast. Part of the skill of an avalanche forecaster is to pull the data together, make estimates, choose a level and write a forecast. Just like weather forecasters, avalanche forecasters will never have a complete set of data to from which to make their predictions.


Different people have different biases as to what they emphasise in a written description. To try to get greater consistency, some services use a menu of set phrases for the forecasters to choose from, rather than letting write things freehand. Switzerland is one example of this.


Summary


Knowing the number or adjective that describes the avalanche danger for the day isn't enough. There are increments within the levels. The hazard can also vary with altitude, aspect, time of day and changes in the weather. Add to this the variations in terrain that you can find on a map. Then add what you can see and hear and feel whilst you're out on the mountain and you're beginning to have enough information to choose where to ski.


The average page view time for the Scottish Avalanche Information Service is 45 seconds. Many people just look at the number on the board at the bottom of the ski-lift. Perhaps we should all be spending a bit longer and taking in the details of the avalanche forecast?

Matt Login, ski-touring above Siviez on a level 3 hazard day, December 2019
Matt Login, ski-touring above Siviez on a level 3 hazard day, December 2019

Further Reading


https://www.slf.ch/en/avalanche-bulletin-and-snow-situation/about-the-avalanche-bulletin/danger-levels.html


https://www.sais.gov.uk/how-to-interpret-avalanche-reports/

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