New Zealand Mountan Safety Council

Online Course - Conditions

Signs of unstable snow - Could the slope avalanche?

In most avalanche accidents one or more of the following signs were present. Check the Backcountry Avalanche Advisories at when planning your trip, and see if the Forecasters have mentioned any of these red flags. Note where (aspect and elevation) these dangerous conditions have been observed. Stay alert when making your own observations in the field.

Some of these clues are obvious and will be hard to miss, others need you to be alert and actively seek them out. Making sense of what you see, hear, and feel takes time. Learn to scan the terrain looking for clues as you travel. Use small rolls/slopes where there are no terrain traps and where the chances of being caught are low to get a feel for the snowpack. If moving out onto larger more exposed slopes, scan for key details and avoid common trigger points.

Communicate anything you observe to others in your group.

Recent Avalanches

The strongest indicator of unstable snow is recent avalanche activity. It tells us there is a weakness in the snowpack that has been triggered. Take note of where it occurred, as well as the type and size of the avalanche. You should treat slopes that have the same aspect and elevation as suspicious. Avoid them if you can, but if you need to use these slopes, stick to low angled sections (below 25°) that are not connected to larger slopes above.

Signs of Instability

Sudden collapsing snow (whumph sound), shooting cracks, and hollow drum like sounds are nature’s warning signs that the snowpack is unstable. When a sudden collapse occurs, you have triggered the weakness in the snowpack, but the slope may not have been steep enough to slide downhill. A whumph should make you nervous in the guts, perhaps even lower, causing a pucker. Act accordingly and back off any steep terrain you might be on.

Heavy snowfall or Rain in the past 24hours

Significant new snow or rain can overload and weaken the snowpack. Avalanches are often triggered naturally during and immediately after a storm, as the snowpack cannot adjust fast enough to the extra weight being added, and it fails. It is also common for people to trigger avalanches on the first fine day after a storm as the snowpack is still sensitive to light triggers, and needs more time to stabilise. Be patient before charging into steep terrain straight after a storm. Gather more information from local sources to see if there has been any local recent avalanche activity.


Wind can deposit snow ten times faster than snow falling from the sky. It deposits snow on lee slopes and can rapidly create dangerous ‘Slab’ avalanche conditions. Wind often accompanies a storm, but it can also build ‘slab’ conditions days/weeks after the last storm even when skies are clear. Watch for tell tale signs like cornices forming on the lee side of ridges. These will often overhang a slope that appears ‘fat’ and smooth. Remember about cross loaded slopes too.

Rapid Warming

Rapid increases in the temperature of the snowpack can weaken the bonds between snow crystals causing unstable snow conditions. Sources of rapid warming can be direct sun on northerly aspects, warm air often coming from the north or northwest, and rain. Measure this with a thermometer. Early signs can be pinwheeling/snowballing on the snow surface of steep ground (especially below rock outcrops), sticky or boggy feeling snow where equipment does not glide well. Late and very dangerous signs can be deep glide cracks that open up. These often appear where the terrain rolls over sharply and is unsupported.

Persistent Deep Weak layers

These can be difficult to identify, and you will often not know they are there unless they are mentioned in your region’s Backcountry Avalanche Advisories. You should pay special attention to where they may be located (aspect and elevation). These areas need to be avoided due to the large amounts of snow that can be involved. Avalanches failing on a deep weakness can spread over a surprisingly wide area, and are seldom survivable.  



Avalanche Forecast Regions:
Mountain Safety Council managed websites
Mountainf Safety Council websiteAdventure Smart websiteNew Zealand Avalanche Center