Introduction
Following on from last month’s tip, this article will explain how to estimate slope angle by feel – that is, with reference to familiar everyday objects.
Importance of Estimating Slope Angle
Tip #68 explained the importance of the angle of a slope, with regard to avalanche risk. It explained how 98% of avalanches occur on slopes of 27° or more. It explained how you can quickly identify these slopes from a map. For example, on a 1:50,000 map if the 50m index contour lines are closer together than 2mm, then the slope is 27° or more.
Estimating Slope Angle in the Mountains
Using the map is an excellent way to understand slope angle during the planning stage, in the warmth of an indoor environment.
But what happens when we are outside faced with an actual mountain slope?
There are a number of improvised measurement techniques in common use. They all have their strengths and weaknesses – for example using ski poles to create an equilateral triangle.
There are also various apps which make use of a smartphone’s sensors and camera, and act as clinometers. While potentially very accurate, you must take care to be sure that you are actually measuring the right thing. Minor fluctuations in the slope angle or the position taken to make the measurement can all distort the result.
Using Familiar Slope Angles
While ‘feeling’ the slope angle is a much cruder approach, only giving rough estimations, it is nevertheless a handy technique to have in the toolbox. Rather than a ‘measurement’ technique, it is a technique to tune your senses, so that you become more subconsciously aware of what sort of slope angle you are standing on. It will also help sound an instant alarm bell when you hit a 27-30° slope.
Here is a list of slope angles that we all know intuitively from everyday life – but perhaps have never thought about it in this way:
Ramps
We use ramps to move (generally) wheeled objects up or down a height difference. But at a certain point a ramp becomes too steep and we would lose control of that object.
- A wheelchair ramp is typically around 4°.
- A steeper ramp in some large supermarkets that locks the shopping trolley in place is around 12°.
Steps
Once a ramp becomes too steep, a better solution to aid movement is a set of steps. The very shallowest sets of steps can be around 15-25°. These are often the sort of steps that can be a bit annoying to walk on because they are so gentle. They are often one-and-a-half paces along for a very small upward step and interrupt the normal walking pattern.
Staircase (standard domestic)
The main staircase in your house is likely to be 30-38°. This is enormously helpful in this context as it’s easy to recognise.
Staircase (steep)
A noticeably steep set of stairs going to a floor that is not usually habitable, such as a cellar or attic, is usually 39-50°.
Stepladder
A set of step-ladders typically cover the range of 50-75°.
Ladder
A ladder that needs to be leant against a wall tends to be at an angle of about 75-80°.
Summary
There are many ways for us to understand slope angles to plan a safe route through avalanche terrain. This includes using a map (and apps) during the planning stage. It also includes using improvised tools (such as ski-poles) and phone apps to measure a slope in the field
However, it is also handy to be able to estimate a slope angle intuitively with reference to everyday objects that we know the feel of.
As soon as you feel a gradient roughly as steep as the main staircase in your house you are most likely on a slope of about 30°. Stop – and think carefully if you really mean to be there.
