Avalanche Awareness: Part 5: How Weather Affects Avalanche Risk

In the first part of this 7-part course on avalanche safety, we introduced the types of avalanches, their causes, and the risks they pose. In the second part, we discussed essential avalanche equipment and the importance of knowing how to use it effectively. The third part focused on recognising avalanche-prone terrain, while the fourth section took a deep dive into field observations and snowpack testing to assess stability in real time.

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With this foundational knowledge, we now turn to one of the most dynamic and impactful factors influencing avalanche danger: weather. Weather drives nearly every aspect of snowpack evolution, affecting its stability and often creating the conditions that lead to avalanches.

From snowfall to wind, temperature changes, and rain, understanding how weather interacts with the snowpack is critical for predicting hazards and planning safer routes. In this section, we’ll explore how different weather events influence avalanche risk and teach you how to incorporate this knowledge into your backcountry decision-making.

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1. Heavy Snowfall: Stress on the Snowpack

Why It Matters:

Fresh snowfall is one of the leading causes of avalanches because it adds weight and stress to the snowpack, often overwhelming weak layers beneath.

Key Points to Remember:

• Timing: Most avalanche accidents occur within 24–48 hours of a storm. This is the period when the snowpack is adjusting to the new load.
• Rate of Snowfall:
• More than 30 cm (12 inches) of snow within 24 hours significantly increases avalanche risk.
• Intense snowfall (>2 cm/hour) doesn’t give the snowpack enough time to bond, increasing instability.

Red Flags in the Field:

• Drifting Snow: Indicates high winds during the storm, which can form unstable wind slabs.
• “Upside-Down Snowpack:” When heavy, dense snow falls on top of lighter snow, it creates poor bonding and instability.

Practical Example:

You’re skiing two days after a storm that dumped 50 cm of snow. While skinning uphill, you notice the fresh snow feels heavy and dense, and your boots sink into a soft layer beneath. This is a classic sign of a weak snowpack.

Action: Stick to slopes under 30° and avoid avalanche terrain.

2. Wind: Nature’s Snow Sculptor

Why It Matters:

Wind doesn’t just move snow—it reshapes the snowpack, creating slabs that can fracture and slide under pressure. Even without new snowfall, wind can redistribute existing snow into dangerous formations.

Key Points to Remember:

• Wind Slabs:
• Form on leeward (downwind) slopes, where wind deposits snow unevenly.
• These slabs are often dense, heavy, and poorly bonded to the layers below.
• Cornices: Strong winds can create overhanging masses of snow along ridge lines. These cornices are unstable and can collapse, triggering avalanches below.

Red Flags in the Field:

• Snow that feels hard and slabby underfoot, often with a hollow sound.
• Ridges with wind-blown textures or visible cornices.

Practical Example:

After a storm with 50 kph winds, you’re evaluating a steep slope. The leeward side is loaded with fresh wind-blown snow. Digging a pit reveals a dense slab sitting on a weak, faceted layer.

Action: Avoid slopes that show signs of wind loading.

3. Temperature Changes: The Silent Killer

Why It Matters:

Temperature plays a critical role in how snow layers bond or weaken. Rapid warming or freezing can drastically alter the snowpack’s stability.

Rapid Warming:

• Effect on Snowpack:
• Melts surface snow, adding water that percolates through the layers.
• Weakens bonds between snow layers, often causing wet avalanches.
• When It Happens:
• After a sudden rise in air temperature.
• On sunny days when solar radiation warms the snowpack (especially on south-facing slopes).

Cold Snaps:

• Effect on Snowpack:
• Preserves instability by preventing weak layers from settling or bonding with the layers above.
• Promotes the formation of faceted snow—a weak, sugary layer caused by strong temperature gradients within the snowpack.

Red Flags in the Field:

• Wet, sticky snow on sunny slopes (indicating melting).
• Hard, icy layers after a freeze-thaw cycle.

Practical Example:

You’re descending a south-facing slope on a warm spring day. The snow feels wet and heavy, and small loose avalanches are starting to release naturally.

Action: Avoid steep, sunlit slopes in the afternoon.

4. Rain: A Heavy Burden

Why It Matters:

Rain drastically increases avalanche risk because it adds significant weight to the snowpack and saturates weak layers. Rain-on-snow events are particularly dangerous in midwinter or spring.

Key Points to Remember:

• Increased Weight: Rain adds a tremendous load to the snowpack, often causing immediate instability.
• Lubrication: Water from rain can percolate through the snowpack, creating a slippery interface that allows entire layers to slide.
• Wet Avalanches: Rain often triggers wet avalanches, which are slower-moving but incredibly destructive due to their density.

Red Flags in the Field:

• Water pooling on the snow’s surface.
• Slushy, saturated snow that feels heavy and hard to move through.

Practical Example:

After a rainy night, you’re touring on a slope where the snow feels waterlogged. Small snowballs are rolling downhill, a sign of wet instability.

Action: Avoid avalanche terrain entirely during or after rain-on-snow events.

5. Combining Weather Factors: The Avalanche Recipe

Avalanche conditions are rarely the result of a single weather factor. It’s the combination of factors—heavy snowfall, strong winds, rapid warming, and rain—that creates complex and dangerous snowpacks.

Example Scenario:

1. Heavy Snowfall: A storm dumps 40 cm of snow over 24 hours.
2. Wind Loading: Strong winds during the storm deposit snow on leeward slopes, forming unstable slabs.
3. Rapid Warming: The next day, temperatures rise sharply, weakening bonds in the snowpack.

Result: A perfect storm for slab avalanches. Even a skier’s weight can trigger a large slide.

Action: Stick to safe, low-angle terrain and avoid leeward slopes.

How to Use Weather Reports for Avalanche Safety

Understanding how to interpret avalanche forecasts and weather reports is essential for planning your trip.

What to Look For:

1. Snowfall Totals: How much snow has fallen in the last 24–48 hours?
2. Wind Speeds and Directions: Strong winds and leeward aspects often mean wind slabs.
3. Freezing Levels: High freezing levels indicate warming, which can destabilise the snowpack.
4. Rain Forecasts: Rain increases the risk of wet avalanches and snowpack saturation.

Where to Find Information:

• Use resources like Météo France’s Avalanche Bulletin or local forecasting services in your area.

Key Takeaway

Weather is the driving force behind avalanche risk. By understanding how snowfall, wind, temperature changes, and rain interact with the snowpack, you can better predict instability and make safer decisions. Always combine weather knowledge with field observations and terrain awareness for a comprehensive approach to backcountry safety.

Ready to follow onto the next lesson? Click here for chapter 6 - Understanding Avalanche Danger Ratings and Why “Moderate” Isn’t Safe