A 30-degree slope...
Why is it important to highlight slopes above 30° on winter maps?
Since a few days ago, the “Show slopes over 30°” option is available on the Bluerando website, to better plan routes on snowy terrain. Soon, the same feature will be available on the Bluerando App as well, so you can check the slopes themselves even during the trip.
Slopes with inclination greater than 30° are colored pink, adopting the same standard used by SwissTopo, the world's top digital cartography (and beyond...).
By the way: on Bluerando, when you are in Switzerland, you can choose to view the very map published by SwissTopo, in summer or winter mode. You can also download it to your phone via the “Offline Maps” feature, so you can use it even when there is no Internet connection.
But back to the topic of this article. At the moment, the areas covered by this new service are: the Italian and French Alps, the Jura Massif and, for Switzerland, Valais, Ticino, Graubünden and the Canton Uri. Gradually, other mountain areas will be included to complement them.
To see slopes of more than 30° you must check the “Show slopes over 30°” option near the lower right corner of the chosen map. The more you zoom in on the map, the better you will recognize the colored slopes. This feature is still in the experimental stage, and evolving. The terrain models used are very accurate; 25 meters for Italy and France, even 2 meters for Switzerland. It is therefore a rather “heavy” model; be patient and give the page itself time to display the entire colored texture. Avoid scrolling and zooming madly, so as not to overload the browser's memory. In case of difficulty, try turning off the option, framing the area you are interested in, and then turning it back on. Also so as not to overload the browser's memory, the colored texture is only visible when the framed area is not too large, approximately when the map shows an area of 8 x 4 kilometers or less.
We are working hard, to maintain the quality of the result with a lighter model, by optimizing the data available to us.
First, let's clarify the terms in order to avoid unpleasant misunderstandings:
Slope is the change in elevation between two points and their horizontal distance and is given in %; inclination, on the other hand, is the angle between the line indicating the average slope obliquity and the horizontal plane. It's given in degrees.
But back to the question; the choice to indicate slopes of more than 30° stems from the fact that this value is commonly used to indicate the critical angle of snow stability on a slope; for the record, 50% slope corresponds to just under 27°.
This value is valid for spontaneous avalanches and still has at least 3 exceptions: dry and very light snow avalanches, wet snow avalanches, and, most importantly, wind slabs.
On slopes with a slope angle greater than 30°, the force of gravity tends to cause the snow to move downward, a movement that continues until a slope angle less than the kinetic friction angle is reached.
Very basically, snow on very steep slopes (over 45°) tends not to pile up, and on less steep terrain (under 30°) it tends to stay in place.
In between lies the real problem.
In fact, it's the slopes between 30° and 45° that give the most satisfaction to prepared skiers, but they are also the ones where slabs of dry or wet snow are most likely to form; hence the decision to color slopes over 30°.
But beware of two fundamental aspects:
The first, and most important, is that the slope should be measured or estimated at the steepest point of the slope and not at its average; for this reason an accurate reading and measurement of contour lines will be very useful to us, beyond the generic coloring provided by the map.
The second aspect is that, like all rules, this too has the exceptions we saw earlier accentuated by the presence of one or more people, whether uphill or downhill, which can be a trigger for avalanche release.
To conclude, the coloring on the map of slopes between 30 and 45 degrees, serves first of all for the preparation of the outing at the desk so as to know in advance the potentially most dangerous stretches to face them, if the case, with the necessary precautions. Safety distances in groups or between individuals, choice of times, stopping points, loopholes, alternatives...
Of course, this analysis can and should also be done during the outing, but if one knows in advance the critical slopes, it will be easier to behave correctly and opt in advance for the best choices.
In fact, often, caught up in the eagerness of the climb, fatigue, the pleasure of the descent or more simply bad weather, we risk finding ourselves suddenly and too late in the middle of critical slopes.
In a future blog we will look at how to recognize or measure the slope angle directly on or near the slope.
Happy hiking!
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Ernesto Bassetti
Ernesto is one of the leading snow and avalanche experts in Italy and beyond.
By passion and by profession, he has been involved in mountain safety for over 40 years, with particular attention to the avalanche issue.
He is co-founder, and until 2020 Vice President, of MND Group; important international player in the field of mountain infrastructure and world leader in controlled avalanche release systems.
He is a ski instructor, National Ski Mountaineering Instructor, former director of the Mario Righini Ski Mountaineering School.
He was promoter, co-founder and soul of the "Servizio Valanghe Italiano" (Italian Avalanche Service), of which he held the Presidency for 18 years.
