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I'm reading up on winter technique. The book im reading states:

Most (slab) avalanches happen on slopes between 30 and 45 degrees

I think I've seen this before but ; how do I recognise (easily) a slope in this range?

  • 45° is 1/2 half way between horizontal and vertical, 30° is a 1/3 of the way between horizontal and vertical. They're probably the two easiest angles to eyeball, but some people just don't have the same eye for estimating angles as others. – ShemSeger Dec 9 '14 at 4:52
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    @ShemSeger I think it's tough to estimate when not looking from the side. Your point of view distorts the scenery. I think estimating the steepness of a slope needs a lot experience. – Wills Dec 9 '14 at 6:25
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    @EverythingRightPlace - I think you're right, and I really like your pole method in your answer. – ShemSeger Dec 9 '14 at 18:25
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    I the key word here is "Most" - meaning some happen outside this range. Many slopes change angles, and what is happening above you or below you may be prime slab terrain that you cannot see clearly Although answers to this Q will be useful, be very careful about getting obsessed with the angle, and learn to read the snow. – user5330 Apr 26 '15 at 0:28
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If you don't have a compass or other instruments, it is possible to measure the slope with your two (ski) poles solely.

Just hold one pole vertically by using gravity and stick it into the snow. Hold the other one horizontally until it reaches the slope with one end and the first pole with the other end. Now you check the height in which the poles contact each other. If it is at the top of the first pole, your slope is 45°, if it is approx. in the half of the first pole size, you are facing a 30° slope.

You will understand this better, if you have a short look here:

enter image description here

or more in detail here:

enter image description here


Because you asked exactly for 30 and 45 degree, the previous mentioned method should work pretty well for you.

Still there is another method to get an approximation which I like even more. There you can (in a typical range) get some useful measurements of your slope with an accuracy of approx. 3 degree. I wont try to explain it, just found a great video explanation here:

https://www.youtube.com/watch?feature=player_detailpage&v=vlCiJma_rpA#t=111

  • This was the one I was after specifically, though all answers are good and useful! – user2766 Dec 9 '14 at 6:50
  • There is also a way to measure a 30° slope "exactly". Mark a line of one pole length in the snow. Now build an equilateral triangle using this line and your 2 poles. If one side of the triangle is vertical, the slope is exactly 30°. – anatolyg Dec 10 '14 at 20:06
  • @anatolyg Yep, please see the video I linked to. This is the method to measure 30° (equilateral triangle) for the "measuring pole" being vertically. Then for each displacement of 10cm you add or subtract three degree. Of course this is just an approximation for a regular pole length. – Wills Dec 10 '14 at 21:19
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To put it simply, carry a compass with you that has a clinometer to measure a slope's angle, set one of your poles down on the slope and place the compass on top of the pole to get a solid reading. If you spend enough time in one area you'll start to become familiar enough with the terrain to remember roughly what the angles are and which routes are the best. But if you're exploring new areas, take the time to get good measurements. Taking risks in avalanche country isn't the way to go.

This site has a lot of good information. http://www.skiingthebackcountry.com/skiing-resources/how_to_use_inclinometer

Also, get this book if you haven't: http://www.amazon.com/Staying-Alive-Avalanche-Terrain-Tremper/dp/1594850844

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    LOL +1 for "How NOT to use your inclinometer"! – Wills Dec 8 '14 at 21:43
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Two other answers have given methods for measuring this on-site. The trouble is that there's a lot of behavioral and sociological research showing that this doesn't really work. Once you get to the location where the activity is planned, you'll tend to go ahead anyway because you feel committed, and because there is a strong psychological need to show other people that your actions are consistent with the way you present yourself. See McCammon, "Evidence of heuristic traps in recreational avalanche accidents," http://www.snowpit.com/articles/traps%20reprint.pdf .

For these reasons, it's better to determine the slope angle at home, using a topo map, before you go. Use a ruler to measure the distance between two topo lines on the map, and use the scale on the map to convert this to a horizontal distance x in units of meters. Let y be the vertical spacing of the topo lines in meters. (If your map has elevations in feet, then determine both x and y in feet.) Divide y by x. If the result is between 0.6 and 1.0, then the slope angle is between 30 and 45 degrees.

Personally, I just use the following ultra-simple checklist:

a) Is the slope 30-35 degrees or more (or is the area bare of old-growth trees)?

b) Has there been more than 6 inches of snow in 48 hours? I can determine this online, e.g., here's info for a mountain near where I live.

If both of these factors exist, I figure that out at home, and I don't go.

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    This is a valid point and yes, your tour always starts at home with preparation. Still you should be able to assess the conditions on-site AND you really should be able to trust and follow your assessment. There are other objective dangers like the weather, cornices, ice and stone fall. If you aren't able to value and re-estimate outdoors, you will never be able to be a good and flexible mountaineer. – Wills Dec 9 '14 at 5:08
  • In the UK, the uplands very rarely have trees. Most of the upland in the UK are hill sheep farms. In the summer, the sheep eat almost all undergrowth including tree saplings. – user2766 Dec 9 '14 at 16:17
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    Conditions can get significantly worse overnight without any snowfall. Look at the Avalanche Bulletin for my area, recent wind and fluctuating temperatures have created a considerable hazard for wind slab avalanches. The weather can give you a hint of what to expect, but it's important to look at avalanche warnings before you venture out into the hills. If there aren't any bulletins for your area, then you need to learn how to conduct a snow study. – ShemSeger Dec 9 '14 at 18:46
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You can use the common technique with the poles to measure the slope angle like discribed by @EverythingRightPlace.

There is also a ski-pole-sticker that you can attach to your pole and then simply read the angle with a single pole (found on this website).

There are also several smart phone apps for your smart phone to measure the angle of a slope. These Apps are only an additional help for the existing physical methods and are not completely reliable.

for WindowsPhone

for Android

for iPhone

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    Thx, also something nice to have! Of course a mobile isn't completely reliable though. – Wills Dec 10 '14 at 7:05
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    @EverythingRightPlace I've added a sentence to describe your given point. – ibex Dec 10 '14 at 7:13
  • Thx for the info about the kickstarter ski-pole-sticker. Can't vote +2 though ;) – Wills Jan 14 '15 at 20:12

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