How Weather Affects
Avalanche Risk
Weather drives nearly every aspect of snowpack evolution — from the load added by a storm to the weak layers created by cold and clear skies. Part 5 breaks down the four key weather factors and teaches you how to read them before and during your day in the mountains.
View Full Course →The snowpack is not a static object — it is a dynamic system shaped entirely by the weather events that built it. Understanding how to test it in the field is essential, but equally important is understanding why it behaves the way it does — and weather is the answer to that question.
Snowfall, wind, temperature, and rain each interact with the snowpack in distinct ways. Rarely does a single factor create dangerous conditions alone — it is almost always a combination, layered over days or weeks, that sets the stage for an avalanche cycle.
"The most dangerous day in the backcountry is often not the day of the storm — it's the 48 hours that follow."
1. Heavy Snowfall — Stress on the Snowpack
Leading Cause of Avalanche CyclesHeavy Snowfall
Fresh snowfall is one of the primary drivers of avalanche cycles because it rapidly adds weight to the snowpack, loading weak layers beneath before they have time to adjust. The snowpack needs time to settle and bond — and that window of adjustment is when risk peaks.
Critical Thresholds
Red Flags After Snowfall
What an "Upside-Down" Snowpack Means
When a storm delivers heavy, dense snow on top of lighter, lower-density snow already on the slope, it creates what forecasters call an upside-down snowpack — the heavier layer sitting on top of the weaker, less dense layer beneath. This is structurally unstable and highly prone to slab formation.
Practical Example
You're touring two days after a 50 cm storm. The fresh snow feels heavy and dense underfoot, and your skis break through a soft layer beneath. This is a classic indicator of a loaded, unstable snowpack.
→ Action: Stick to slopes below 30° and avoid all avalanche terrain until the snowpack has had adequate time to settle. Check La Chamoniarde or Météo France for the current danger rating before making any terrain decisions.
2. Wind — Nature's Snow Sculptor
Redistributes Snow · Creates Wind Slabs · Builds CornicesWind
Wind is a force multiplier for avalanche danger. Even without new snowfall, wind alone can redistribute existing snow from windward slopes to leeward aspects, rapidly building wind slabs — dense, poorly bonded layers that are among the most common triggers for slab avalanches.
How Wind Reshapes the Snowpack
- Wind slabs form on leeward (downwind) slopes where wind deposits snow unevenly. These slabs are dense and heavy, often sitting on top of weaker, unconsolidated snow beneath. They fracture suddenly under pressure — including the pressure of a skier crossing them.
- Cornices build along ridgelines where wind deposits snow on the leeward side. They are unstable, often extend further than they appear from above, and can collapse and trigger avalanches on slopes below — even without a skier or climber as a trigger.
- Windward slopes are scoured down to harder, icier layers — lower slab risk, but icy conditions create their own hazards.
Red Flags in the Field
Practical Example
After a storm with 50 kph winds, you're evaluating a steep leeward slope. Digging a pit reveals a dense, hard slab sitting on a weak, sugary faceted layer beneath — the classic wind slab over weak layer structure.
→ Action: Avoid all slopes showing signs of wind loading. After any storm with significant wind, treat leeward aspects as high-risk until confirmed otherwise by snowpack testing.
3. Temperature Changes — The Silent Killer
Rapid Warming Weakens Bonds · Cold Preserves InstabilityTemperature Changes
Temperature changes are one of the least visible but most impactful influences on snowpack stability. Both warming and prolonged cold create dangerous conditions — through opposite mechanisms.
Red Flags in the Field
Practical Example
You're descending a south-facing slope on a warm spring afternoon. The snow feels wet and heavy underfoot, and small loose snow avalanches are beginning to release naturally around you.
→ Action: Leave sun-exposed slopes before temperatures peak. Time your descents on south-facing terrain for early morning when the surface is still firm. Never linger on steep, sunny slopes in the afternoon.
4. Rain — A Heavy Burden
Adds Weight · Saturates Layers · Triggers Immediate InstabilityRain on Snow
Rain is one of the most rapidly destabilising weather events in the snowpack. It simultaneously adds significant load and saturates weak layers — the combination of which can trigger avalanches almost immediately, both during and after the rain event.
How Rain Destabilises the Snowpack
- Added weight: Rain adds significant load to the entire snowpack almost instantly — far faster than snowfall allows the pack to adjust.
- Lubrication: Water percolates through the snowpack until it reaches a dense or ice layer, where it accumulates and creates a slippery interface. Entire layers can then slide on this water film — even on relatively low-angle terrain.
- Wet avalanches: Rain-on-snow events are the primary driver of wet avalanche cycles. Wet avalanches are slower but incredibly dense and destructive — and they leave debris that sets like concrete.
Red Flags in the Field
Practical Example
After a rainy night, you're skinning onto a slope where the snow feels waterlogged and heavy. Small snowballs are rolling downhill on their own — a reliable indicator of wet instability.
→ Action: Avoid all avalanche terrain during and after rain-on-snow events. This is a non-negotiable retreat condition — the snowpack is actively destabilising and slopes can release at any moment, including on angles you would normally consider safe.
5. Combining Factors — The Avalanche Recipe
Dangerous avalanche conditions are rarely created by a single weather factor. It is the combination of events — layered over hours and days — that produces the most serious risk. Understanding how these factors compound is essential for reading a forecast and making go/no-go decisions.
Example — A Three-Factor Avalanche Cycle
A common and dangerous sequence in the Chamonix area. Each factor alone would elevate risk — together, they create conditions where even a skier's weight is enough to trigger a large slab.
Result: A complex, multi-hazard avalanche cycle — both dry slab on leeward aspects and wet slab on sun-exposed slopes. Even a light skier crossing a convex roll on a loaded slope is enough to trigger a large release. → Stick to terrain below 30°. Avoid leeward and sun-exposed aspects entirely.
6. Reading Weather Reports for Avalanche Safety
Knowing what to look for in a weather forecast — and how to connect it to snowpack behaviour — is a core planning skill. These are the four data points that matter most.
❄ Snowfall Totals
How much snow fell in the last 24–48 hours? Over 30 cm is a threshold event. Check whether snowfall was intense (>2 cm/hour) — rapid accumulation gives the snowpack no time to bond.
💨 Wind Speed & Direction
Strong winds during or after a storm mean wind slabs on leeward aspects. Know which way the wind was blowing — that tells you which slopes are loaded.
🌡 Freezing Level
A rising freezing level indicates warming — the higher it climbs, the more of the snowpack is above 0°C and actively melting. Afternoon freezing levels above 2,500 m in the Chamonix area are a serious wet avalanche signal.
🌧 Rain Forecast
Any rain at elevation is a non-negotiable flag. Rain-on-snow events can spike danger to Extreme within hours. If rain is forecast above your intended elevation, do not enter avalanche terrain.
Key Takeaway — Chapter 5
Weather drives avalanche risk — but rarely through a single factor alone. Heavy snowfall loads weak layers. Wind redistributes that snow into unstable slabs on leeward aspects. Temperature changes either weaken bonds rapidly through warming or preserve buried weak layers through prolonged cold. Rain on snow destabilises the entire snowpack almost immediately. Learn to read these factors in the forecast before you leave, and continue reading them in the field throughout your day. Part 6 covers the avalanche danger rating scale — and why "Moderate" does not mean safe.
- Part 1: What Are Avalanches? Types, Causes, and Risks
- Part 2: Essential Avalanche Equipment: Your Lifeline in the Backcountry
- Part 3: Recognising Avalanche Terrain: A Critical Skill for Backcountry Safety
- Part 4: Field Observations and Snowpack Testing
- Part 5: How Weather Affects Avalanche Risk YOU ARE HERE
- Part 6: Understanding Avalanche Danger Ratings
- Part 7: Best Practices for Avalanche Safety
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