Measuring Slope Angle

Slope angle is the single biggest factor in avalanche likelihood.  Being able to identify slope angle is a crucial skill to help us assess avalanche danger and make route choices.  The vast majority of avalanches occur on 30 - 45 degree slopes.  At steeper angles the snow tends to slide regularly enough not to form big slab problems.   Over time we develop an eye and instinct for it, but there are some useful techniques we can use to measure the angle.


Inclinometers

You can carry an inclinometer with you as shown in the image below.  They are small, light and don't require any batteries!  When using them it's best to lay down your ski or pole along the fall line, and place the inclinometer on top of it to ensure you get an accurate angle.  You can also hold them up to estimate a distant slope - but it can be tricky, as you have to make sure you view the slope exactly side-on.

photo credit: Avalanche.ca / NICOLE MATEI


Mobile Phone Apps

Many modern mobile phones have position sensors that will allow you to download a free app to measure the incline.  A quick search on your phone's app store will come up with several options (the one shown in the image below is Bubble Level by NixGame by  for android).  

A quick and easy way to measure the slope you are on, is to position your ski straight up the fall line.  Open up the app and and place your phone flat on your ski.

photo credit: Nico BENARD

To measure a distant slope you can also hold up the phone in landscape, align the lower edge with the distant slope, and read the angle. (see the title picture at the top of the article to get an idea).  As with sighting a distant slope with an inclinometer, it can be hard to get an accurate reading if you are not viewing the slope exactly side-on.


Ski Pole Methods

There are several ways in which to use your ski poles to measure the angle of the slope you are on.  As we look at these methods, keep in mind that the vast majority of avalanches occur on 30 - 45 degree slopes - with 37.5 degrees the [un]sweet spot!

Right angle to slope - this method makes use of the known ratios of a right-angled triangle.  It gives you an easy way to identify 3 useful slope angles: 27, 37 and 45 degrees.  Use the following steps:  

  1. Start by holding the top of your pole with two fingers, and allowing the pole to dangle straight down. With the help of gravity this should get you fairly close to vertical.
  2. Stick the first pole into the snow up to the basket, as near to vertical as you can manage.
  3. Bring your second pole as horizontal as you can, with the basket against the vertical pole, and the handle just touching the slope. To make sure the second pole is horizontal, make sure the angle between the two poles is a right-angle (90 degrees).
  4. You can now get an idea of the slope steepness by reading where the horizontal pole meets the vertical pole. If the horizontal pole's tip meets the vertical pole:
    • right at the top, you have a 45 degrees slope.
    • about 3/4 way up the vertical pole, you have a ~37 degree slope. The most prone slope angle
    • about 1/2 way up the vertical pole, you having a 27 degree slope. At this angle and below, avalanches still occur, but it's unlikely.
photo credit: earnyourturns.com

Equilateral Poles

This is another quick method that uses the known angles of equilateral triangles.

  1. place one pole in the snow, with the tip pointing straight up the fall line, and ensure it makes a visible mark in the snow.
  2. pick up and hold this pole by the handle, but keep it's point in the snow where it made the mark
  3. now add the second pole, hold it's handle in the same hand, but let it dangle straight down
  4. keeping your first pole in the snow at the top mark, move your hand down-slope until the point of your second vertical pole makes contact with the slope. If the vertical pole's tip meets the slope:
    • right at the bottom of the mark in the snow, the slope is exactly 30 degrees.
    • if it meets the slope higher up, the slope angle is less. As a rough rule of thumb, half way up the mark in the snow will be 15 degrees slope angle
    • if it meets the slope further down than your original mark, the slope is steeper than 30 degrees. A rough rule of thumb in this case, is half the length of your pole mark again, would indicate 50 degree slope angle
Equilateral Poles: in this case the vertical pole meets the slope higher than the bottom end of the pole mark, so this slope is less than 30 degrees. Since half way up the pole mark would have been 15 degrees, we can estimate this at about 20 degrees.

With a bit of practice these methods are a quick and easy way to guesstimate your slope angle, and inform your decision making.

Safe Trails!