The season
| Month | Suitability | Conditions |
|---|---|---|
| June | Early season | Snow possible above 2,400 m. Some cols may require microspikes. Some refuges not yet open. |
| July | Peak season | Best weather. Warmest temperatures. Most crowded. |
| August | Peak season | Slightly higher storm probability. UTMB race week causes trail and town congestion. |
| September | Late season | Cooler, fewer crowds, shorter days. Some refuges close mid-September. First snowfalls possible. |
| October | Off-season | Many refuges closed. Snow on cols. Not recommended without alpine experience. |
Source: autourdumontblanc.com; La Chamoniarde.
The practical TMB window is late June through mid-September. Earlier and later dates are possible but require checking refuge opening dates, col snow conditions, and carrying microspikes.
Month-by-month
Late June
The earliest TMB start date for most trekkers. Pros: some refuges already open, wildflowers, long days. Cons: snow patches on north-facing cols (Col des Fours, Fenetre d'Arpette, Grand Col Ferret), not all refuges open, trail markings may be buried under residual snow. Check La Chamoniarde conditions reports before departure.
July
Peak season begins. All refuges are open. Trails are snow-free on the standard route. Weather is generally the most stable of any month, though afternoon thunderstorms remain common. Crowds are heavy, especially in the first two weeks after French school holidays begin. Refuge booking 6-9 months in advance is essential.
Early August (pre-UTMB)
Similar to July but with marginally higher storm frequency. The valley is fully operational. Crowds remain heavy. This is the most popular window for international visitors.
UTMB week: August 24-30, 2026
The Ultra-Trail du Mont-Blanc race week transforms Chamonix. Key events:
| Event | Date | Start | From |
|---|---|---|---|
| PTL | Mon Aug 24 | 08:00 | Chamonix |
| TDS | Mon Aug 24 | 23:50 | Courmayeur |
| OCC | Thu Aug 27 | 08:15 | Orsieres |
| CCC | Fri Aug 28 | 09:00 | Courmayeur |
| UTMB 100M | Fri Aug 28 | 17:45 | Chamonix |
Cutoff: 46h 45min — final finish by 16:30 Sunday August 30.
Impact on trekkers:
- Chamonix enforces strict restrictions on private vehicle mobility during race week
- Trails are shared with thousands of runners
- Accommodation across the entire valley sells out
- Start zones, checkpoints, and municipal sectors operate under restricted access
- Organizers reserve the right to modify routes without notice
Unless you specifically want the UTMB festival atmosphere, avoid the TMB during this window. If you are on the trail already: be aware of runner traffic, respect course markings, and expect crowded refuges.
Source: UTMB World Series — 2026 Calendar; Alps2Alps — UTMB Chamonix 2026 Guide.
September
The quiet season. Most refuges remain open through mid-September, though some begin closing after the first or second week. Temperatures are cooler (10-15 C daytime at 2,000 m), days shorter, and first snowfalls possible on high cols. The compensating advantage: far fewer people. September is the connoisseur's month for the TMB.
The afternoon thunderstorm rule
This is the single most important weather pattern for Chamonix trekking and it applies to every day of every month from June through September.
Pattern: Thunderstorms build from approximately 13:00, peaking 15:00-17:00. They develop rapidly over exposed ridges and cols. Lightning above treeline is a serious, potentially fatal hazard.
Strategy: Start each day early (06:00-07:00). Aim to be at the highest point of the day — the col or the ridge — by late morning. Begin descent before 13:00. This means breakfast at the refuge by 06:00, on trail by 06:30-07:00.
Typical conditions (July-August, 1,500-2,500 m):
| Metric | Value |
|---|---|
| Daytime high | 15-25 C (valley); 5-15 C (cols above 2,000 m) |
| Nighttime low | 8-12 C (valley); 0-5 C (refuges above 2,000 m) |
| Rain days per month | 10-14 |
| Summer snowline | ~3,200-3,500 m |
Source: La Chamoniarde / OHM; general Alpine meteorology.
Haute Route glacier window
The Walker's Haute Route (Chamonix to Zermatt) shares the TMB's season: mid-June to mid-September. The ski touring / glacier version has a separate window: mid-March to early May, when glacier snow bridges are stable and days are long enough for high-altitude crossings.
For the glacier version, the window is critical. Too early: short days, extreme cold, avalanche risk from fresh snow. Too late: snow bridges thin, crevasses open, afternoon melt destabilizes slopes. April is generally considered the optimal month.
See Haute Route guide for details.
Refuge availability by month
| Month | French refuges | Italian refuges | Swiss refuges |
|---|---|---|---|
| June | Some open mid-June; check individually | Most open by late June | Most open by late June |
| July | All TMB refuges open | All TMB refuges open | All TMB refuges open |
| August | All open (UTMB week: fully booked) | All open | All open |
| September | Most open through mid-Sept; some close after 15th | Similar to France | Some close mid-Sept |
| October | Most closed | Most closed | Most closed |
The booking portal montourdumontblanc.com shows opening and closing dates for each refuge.
Rockfall and permafrost: the new variable
Warming permafrost is destabilizing rock faces across the Mont Blanc massif. This is not a future projection — it is a current and accelerating phenomenon that affects trail access every season.
Recent incidents:
- June 2025: 523 m3 rockfall on the Aiguille du Midi south face. Cracking sounds reported in the Kohlmann/Clair de Lune area suggest further collapses.
- August 2025: Fatal rockslide on the Egratz Viaduct between Chamonix and Passy — two fatalities on the road, not on a trail.
- December 2025: Rockfall near Trient on the Chamonix-Martigny road.
Source: recency.md; Gripped.
For trekkers, the practical implication is to check La Chamoniarde before every stage. Trail closures and route diversions are announced there, sometimes at short notice. The old gondola path to the Mer de Glace is permanently closed due to instability. The Fontaine Froide to Sur le Rocher trail was closed May 11-June 12, 2026. The Eaux Rousses "du Clu" trail has been closed since October 2024 with no reopening date announced.
Rockfall risk is highest in the afternoon when solar heating expands rock and melts ice in cracks. This reinforces the early-start rule: be moving by 06:30, clear exposed terrain before midday.
2026 infrastructure closures
Beyond rockfall, several key installations have scheduled closures:
| Facility | Closure dates (2026) |
|---|---|
| Aiguille du Midi cable car | November 2 - December 18 |
| Panoramic Mont-Blanc gondola | Until July 4 |
| Grotte de Glace | May 11 - June 5 |
| Mer de Glace gondola (Montenvers) | May 11 - May 29 |
Source: Chamonix.net — Lift dates.
The Aiguille du Midi closure is in the off-season and affects few trekkers. The Panoramic gondola closure until July 4 affects the Vallée Blanche cable car connection to Italy. The Mer de Glace closures affect anyone planning a late-May or early-June visit to the glacier.
The Mont Blanc Tunnel (Chamonix-Courmayeur) also has periodic night closures through August 2026. If you plan to transfer between the French and Italian sides of the TMB by vehicle, use daytime hours. Real-time status: tunnelmb.net.
When to book
| Timing | What you get |
|---|---|
| October (portal opens) | First pick of dates and refuges. Essential for peak-season July/August. |
| January-March | Still possible for July/August, but popular refuges (Bonatti, Lac Blanc) may be full. |
| April-May | Shoulder season (late June, September) still available. Peak season: limited. |
| June+ | Last-minute availability exists but itineraries become inflexible — you take what is left. |
The 2026 portal opened October 15, 2025. Peak-season refuges sold out within days of launch. If you are reading this and planning for July-August 2027: set a calendar alert for mid-October 2026.
The UTMB paradox
The Ultra-Trail du Mont Blanc is simultaneously the TMB's greatest marketing asset and its greatest logistics headache. The race — and the brand — has made the Tour du Mont Blanc one of the most recognized treks on Earth. Trail runners who complete the UTMB become TMB evangelists. The event generates global media coverage that reaches audiences who would never otherwise encounter a French alpine trail.
But the practical cost is a full week of degraded trekking conditions. Trails are designed as shared-use paths, not as race infrastructure. When 2,000+ runners pass through a col over a 24-hour period, the experience for hikers on the same path is fundamentally altered. Refuges are booked for race logistics. Chamonix's parking, restaurants, and accommodation operate at maximum capacity with race-week pricing.
The informed choice is simple: if you want to watch the race, UTMB week is spectacular. If you want to walk the TMB in relative peace, book a different week. September, in particular, offers quieter trails, cooler temperatures, and the first hints of autumn colour on the lower stages.
Weather forecasting resources
Check these sources before each stage:
| Resource | URL | What it provides |
|---|---|---|
| La Chamoniarde (OHM) | chamoniarde.com/en | Daily conditions, trail closures, rockfall alerts |
| Meteo France (mountain) | meteofrance.com | Regional forecasts, thunderstorm warnings |
| SLF (Swiss side) | slf.ch | Snow conditions, avalanche bulletins (Haute Route) |
La Chamoniarde is the most useful single source. It is operated by the Office de Haute Montagne, the same organization that coordinates with PGHM rescue teams. Their conditions reports are written by professionals who work on the mountain daily.
Most refuges post weather forecasts on a chalkboard or printout in the common room each evening. Ask the gardien (warden) about conditions for the next stage — they communicate with neighbouring refuges by radio and have granular knowledge of trail conditions between huts.
Sources
- autourdumontblanc.com — Season, weather overview
- montourdumontblanc.com — Refuge opening dates, booking
- La Chamoniarde / OHM — Mountain conditions, daily forecasts
- UTMB World Series — 2026 Calendar — UTMB dates, schedule
- Alps2Alps — UTMB Chamonix 2026 Guide — Impact on valley logistics
- Wikipedia: Haute Route — Glacier season, ski touring window
- recency.md — 2026 season specifics, closures
- HiiKER, "Tour du Mont Blanc Insights" — Visitor patterns
- Outdoor Pilgrim, "TMB By the Numbers" — Traffic data
- Responsible Vacation — Overcrowding context