Characteristics of the slope
Besides the surface and the presence of trees or bushes, slopes distinguish themselves in the following:
- Course of the fall line
These all influence the choices you make when on the mountain. First the angle. The angle of the slope is very important because slab avalanches happen on slopes of 30 degrees or steeper. But how steep is 30 degrees? And how do you measure that? Let's start with the following question: how steep is the slope in the photo below?
The correct answer is 44 degrees. Did you get it right? Don't worry if you guessed wrong. Getting the angle right takes practice. Angle is a frequent discussion point among skiers and snowboarders: ‘Well I skied a run of 60 percent, that is steep! 35 degrees is nothing, man! 70 percent, that is steep.’ Sound familiar? It is vital that you know what the angle is so you know if you are in avalanche terrain.
Degrees instead of percentages
We measure an angle in degrees and not in percentages. For percentage lovers is here a table so you know what we are talking about and can join the conversations in the bar. These are all cases of the steepest parts of the runs. At the Harakiri for example, it is only a small part which is truly steep. The rest of the run is significantly less steep, while the Le Mûr has a sustained steep pitch.
|Name ski run||Angle||Percentage|
|Le Mûr, Avoriaz||34,2||76%|
|Mont Fort, Verbier/Nendaz||34,65||77%|
|Gamsleiten 2, Obertauern||39,6||88%|
A run of 45 degrees is equal to a run of a 100%. A run of 30 degrees is already steep for a groomed slope. In powder that’s very different. In paragraph 3.11 ‘Conditions for a slab avalanche’ we already wrote:
When riding powder a steepness of 30 degrees is not that bad. The fun only really begins at 27 degrees. At a lower angle you are mostly focussing to keep your speed and not to get stuck on the flatter parts.
For most of us powder snow becomes fun between 30 and 39 degrees. You feel the resistance underneath your feet, you can pick up speed and the snow rustles past your face. Unfortunately this is also the angle that is most suceptable to avalanches.
In the world of off-piste riding, degrees are more important than percentages. The number of degrees shows the steepness of the slope.
- An average steep slope is less than 30 degrees
- A steep slope is between the 30 and 35 degrees
- A very steep slope is between 35 and 40 degrees
- An extremely steep slope is steeper than 40 degrees
How do you measure the angle?
You can of course measure the angle while on the slope, and nowadays there are many apps on your smart phone that can help. But imagine this scenario: you are on the backcountry slope, you measure the angle and it's too steep. You cannot ride the slope. It would have been better to prepare at home. You can do this in two ways. First, you can measure the angle using the contour map. Or, if the route is mentioned in a guidebook or online, that might mention the slope angle.
The SnowCard is the ideal instrument for measuring the angle. It contains an angle indicator and a ruler to measure the angle on the contour map. In addition to this there are many apps which you can use to measure the angle. In any case, practice measuring the angle regularly, at home and on slope. By doing this frequently you develop a better feeling for steepness and will you be able to estimate the steepness of a slope by looking at it.
Measure the steepest part
Whether you measure on a map or on slope, always measure the steepest part of the slope. On a contour map look for the part where the contour lines are closest to each other. On the slope it is important to be able to recognize the steepest part.
Another characteristic of a slope is the direction it faces. Tools to determine the aspect are a compass or the combination of your watch and the position of the sun. And the aspect can also be read on maps.
1. The compass
In the past a compass was always an extra thing to carry, nowadays most smart phones and many watches come with a built in compass. Whether it's actual compass, your watch or smart phone, the arrow always points north. Stand with your back to the slope. Find north with the arrow and make sure the ‘N’ is also pointing that direction. Now you know which aspect the slope is facing.
2. Sun and time
It is also possible to determine the aspect using the sun. The sun rises in the east, is at its highest point in the south and sets in the west. In the winter in the Alps the sun reaches its highest point around noon. In France this will be bit later than in eastern Austria, but that's the general idea. On the day daylight savings time starts, the sun will reach its highest point around 13.00 (1 pm). Once you know that you can get started. Find the sun, look at the time and determine on which slope you are standing. It works like this:
3. Aspect on the contour map
On a contour map the upper side of the map is usually north. Because the ski lifts are marked on the map you can find, with some practice, your favourite off-piste runs on a map of your favourite area. And because you know the upper side of the map is facing north, you can determine the aspect of these slopes pretty easily.
The angle is always measured in the so-called fall line. The fall line is the straight line downwards. Basically it is the shortest route an object would follow if it would fall down. As skiers, we can make our own, creative lines. The term fall line is often used to direct freeriders. ‘Stay in the fall line’ means that you should not venture too much to the left or right of your starting position, but rather try to ride down as straight as possible.
The shape of the slope
The form of the slope means the transition from steep to flat terrain and vice-versa. When the terrain goes from flat to steep, the risk of avalanches increases. This could be because the wind deposited drift snow here, or because the snowpack is thinner there and the weak layers are closer to the surface, increasing the chances of hitting one. This is called a concave or a convex slope.
Bulging – Convex
With a convex the bows outwards. The snowpack here is stretched, so to speak, and your chances of triggering a slide are greater.
Hollowing – concave
A concave slope is comparable to the inside of a bowl. The slope bows inwards. On a concave slope the snowpack is often supporting the snow on the slope above, so the slide will start above you.
- The steepness of the slope is expressed in degrees, not in percentages
- The steeper the slope, the more interesting it becomes, but also the higher the chance of avalanches
- Slab avalanches can occur on slopes steeper than 30 degrees
- An angle indicator, app, the naked eye and the combination of a contour map and the angle ruler on your snowcard are tools to measure the angle
- The aspect determines the amount of sun a slope receives during a winter day
- The amount of solar radiation determines how quickly the snowpack changes
- Tools to determine the aspect are the compass, combination of position of the sun and a watch, and a good map
Shape of the slope
- Transitions from steep to flat terrain and the other way around mean a heightened avalanche risk
- These transitions are often starting points for avalanches
- The slope is bulging on a convex slope. By applying pressure the avalanche starts right where you are standing
- The slope is hollowing at a concave slope. By applying pressure the avalanche starts above the point where you are standing