Avalanche Basics
The hows and whys.
How and Why?
There are three things that need to be in place for an Avalanche to happen.
1- You need a weak layer in the snowpack.
2- You need a slope to be steep enough.
3- You need a trigger.
Weak Layers
Weak layers in the snowpack are caused by poorly bonded sections beneath the surface of the snow. This can be caused by a multitude of reasons, such as the impact of the weather on a previous top layer - which has now been covered by new snow-, general snowpack temperature changes, rain, sun… the list goes on.
Slope Angle
For an avalanche to happen the mountainside needs to be steep enough for the snow to slide. Imagine two magazines lying flat on top of each other, you pick up the bottom one at one end a lift slowly, eventually, the top one will slide off.
The general rule is that an avalanche won’t occur on slopes under 30˚ or above 45˚. Under 30˚ isn’t steep enough for snow to slide, while snow naturally falls off of slopes above 45˚ - this is called sluffing- therefore on very steep slopes, there’s not a large enough build-up of snow to cause an avalanche. But remember these are just general rules and of course, avalanches travel and flat zones below steep slopes can be extremely dangerous too.
Triggers
Triggering an avalanche is caused by an additional load on the snowpack, probably you! Over 90% of avalanche victims trigger it themselves, so best practise while in the backcountry is essential. Some natural triggers are an additional weight of new snow, weight from rain, or an increase in the water content within the snowpack caused by warm temperatures.
Avalanche Bulletins and Warnings
The general ratings across the world are normally flagged in a 1 to 5 system. 5 is an extremely dangerous unbonded snowpack, with large naturally triggered avalanches likely, with 1 being a well-bonded snowpack, where avalanches possible but likely only to be triggered by very heavy loads and predicted to be small in size.
The descriptions on the ratings slightly change between Europe and North America, so make sure you know what each rating actually means.
Avalanche Rose
Some forecasts like to use an Avalanche Rose, which is a great tool as it highlights the more dangerous aspects and elevations of the latest forecast. There are many designs so make sure you take time to familiarise yourself when in a new region north is always north but the zones/rings of the rose can mean different things from elevation to treeline. Most use a traffic-light system where green in low risk, red is high.
Weather
You need to not only understand the avalanche forecast but also the implications of the weather too. Obviously, you need to know the current and predicted weather, so as to be prepared for your day timings wise and what equipment you’ll need. But you also need to look at the weather of the past too.
New snowfall, especially 20cm and above is a big red flag, as this is enough to put a new weight load on the snowpack, but snow isn’t stationary and even a relatively light wind can transport dry snow – it takes a higher wind to move wet snow- depositing often deep areas of new snow on the leeward side.
Aspect
70% of avalanche accidents happen on slopes facing between west and east, with a massive 56% on slopes between north-west and north-east. The reason for this is northern facing slopes get less sun and staying cold slows the snowpacks bonding. We all love cold dry snow, so often choose north-facing slopes to ride on but it certainly adds to our risk, and remember 30% of avalanches still happen on southern aspects so south is no green light. Also, keep in mind that the forecast on high-risk days often refers to all aspects.
