Sleeping bag for bivouacking in the French Alps: actual temperatures by mountain range and sizing
The French Alps host the majority of technical bivouacs practiced in France. They also constitute the terrain where errors in sleeping bag sizing are most frequent and significant. The thermal variation between the valley floor (1,000 m) and high-mountain zones (2,500-3,000 m) reaches 8 to 14°C on a single summer night. Climatic differences between the massifs—humid Northern Alps, dry and continental Southern Alps, and temperate pre-Alpine massifs—require distinct sizing strategies that generalist guides do not distinguish.
This guide draws on data from high-altitude weather stations and Météo-France climatological records to provide real and precise nighttime temperatures by massif and altitude, explains the climatic mechanisms that generate the most demanding bivouac conditions, and translates these data into concrete recommendations for bag sizing. All the models cited in this article, as well as other references, are available in the sleeping bag catalog from Duvet Nomade.
Understanding the nocturnal alpine climate: the mechanisms behind cold nights
The altitudinal thermal gradient
Temperature decreases with altitude at an average gradient of 0.65°C per 100 meters under normal conditions (saturated pseudo-adiabatic gradient). In dry and anticyclonic conditions—typical of beautiful alpine summer nights—this gradient can reach 0.8 to 1°C per 100 meters. The direct consequence: a night at 15°C in Grenoble (220 m) corresponds to a night at 3-4°C on the Plateau d'Emparis (2,000 m) and -1 to -3°C on the Col d'Ornon (1,371 m, northern side) during the same night. This gradient makes it necessary to completely recalibrate your thermal sizing for every 500-meter altitude interval.
Nocturnal radiative cooling
Cloudless nights are thermally the coldest in the mountains, counter-intuitively compared to experience in the plains. The reason is radiative cooling: in the absence of cloud cover, the Earth's surface emits its infrared radiation into space without a local greenhouse effect. The surface temperature of rock or grass can drop 3 to 6°C below the ambient air temperature measured in a standard meteorological shelter. On granite exposed at 2,500 m in July during a clear night, the rock surface can reach -1 to -2°C while the air measures 3-4°C.
This phenomenon is particularly marked in the Southern Alps (Écrins, Queyras, Mercantour), where the dry continental climate generates cloudless skies much more frequently than in the Northern Alps. The "beautiful and cold" nights of the Queyras in summer are a climatological reality, not an anomaly: the lack of atmospheric humidity removes the nocturnal thermal buffer effect.
Nocturnal thermal inversions
In alpine valleys, a thermal inversion phenomenon regularly occurs during good weather: cold, denser air descends from the slopes toward the valley floor and accumulates there, creating a cold layer below 1,200-1,500 m that is sometimes colder than intermediate altitude zones (1,500-2,000 m). A bivouac at 1,200 m in a deep valley bottom can be colder than a bivouac at 1,800 m on an open slope on the same night. This inversion generally dissipates within the first 30 minutes after sunrise.
⚠️ Nocturnal thermal inversions on the valley floor below 1,200 m can make the night colder than at 1,800 m on an exposed slope. Local topography changes real temperatures by 3 to 6°C.
Real nighttime temperatures by massif and altitude
Northern Alps: Chartreuse, Vercors, Belledonne, Aravis
The Northern Alps are subject to Atlantic influence via the Rhône Valley and frequent northwest disturbances. Atmospheric humidity is structurally higher than in the Southern Alps, which mitigates nocturnal radiative cooling but generates more condensation on the sleeping bag.
Indicative climatological data for July-August. At 1,000-1,200 m (pre-alpine zone, northern Vercors, Chartreuse): minimums of 10 to 16°C, rarely below 7°C. At 1,500-1,800 m (high hiking zones, lower Belledonne): minimums of 7 to 12°C, dropping to 4-5°C during cold spells. At 2,000-2,500 m (high zones, Col de l'Arc, Plateau des Lacs in Belledonne): minimums of 3 to 9°C, with nights at 0-1°C during northwest depressions even in July. The Chamrousse station (2,250 m) records average July minimums of 5.4°C with a 10th percentile at 1.2°C.
Mont-Blanc and Vanoise Massifs
Mont-Blanc and Vanoise constitute the glacial heart of the French Alps. Nighttime temperatures there are influenced by the radiation from large glacial surfaces that cool the adjacent air. Above 2,500 m in these massifs, July nighttime minimums regularly range between -1 and 4°C, with nights at -5°C possible during depressions, even in the middle of summer.
The presence of glaciers also generates nocturnal katabatic breezes: air cooled on glacial surfaces flows down the slopes and can lower the perceived temperature by an additional 3 to 5°C in areas located immediately below the glaciers. Bivouacs in basins below seracs (Bionassay, Miage, Argentière) experience these glacial breezes regularly. The Pointe du Léchaud (2,800 m) records July minimums of around -2°C.
Southern Alps: Écrins, Queyras, Dévoluy, Mercantour
The Southern Alps present a radically different climatic profile. Continentality is high, summers are dry, and skies are clear during 80 to 90% of nights in July-August (compared to 50 to 60% in the Northern Alps). This absence of cloud cover considerably amplifies nocturnal radiative cooling.
In the Écrins at 2,500 m, July nighttime minimums range between -2 and 4°C, with clear nights regularly at -3 to -5°C on exposed plateaus. The Queyras, which is even more continental, presents similar minimums but with an even higher occurrence of clear cold nights. The Mercantour, on the Mediterranean border, is slightly milder: July minimums at 2,000 m range between 4 and 10°C, with cold nights being rarer.
The dryness of the Southern Alps is an advantage for non-hydrophobic down bags: nocturnal condensation is much less intense than in the Northern Alps, and a standard down bag without hydrophobic treatment stays dry longer between two bivouacs.
Pre-Alpine massifs: Vercors, Chartreuse, Bauges, Aravis
These limestone massifs of low and medium altitude (800 to 2,400 m) present moderate summer nighttime temperatures. At 1,500 m in July, minimums range between 10 and 15°C in open summit areas, dropping to 6-8°C during disturbances. A bag certified with a comfort rating of 5-7°C covers the vast majority of July-August bivouacs in these massifs below 2,000 m. This is the ideal terrain for a bag like the Aegismax Leto trekking (642 g, 7°C comfort, 700 cuin) or the Nano trekking (544 g, 10°C comfort, 700 cuin) complemented by a silk liner.
Model cited in the article → 
Model cited in the article →
Seasonality: how conditions evolve from spring to autumn
June: persistent winter conditions at high altitude
In June, snow is still present above 2,000-2,500 m depending on the year. Nighttime minimums at 2,000 m range between 0 and 6°C in the middle of the month, with nights regularly at -2 to -4°C in high mountains (above 2,500 m). For a bivouac in June in the Alps above 1,800 m, a bag with a comfort rating of 0°C is the reasonable minimum, and a bag with a -5°C rating is recommended for routes regularly above 2,500 m.
July-August: the optimal bivouac window
July and August are the ideal period for alpine bivouacking. Conditions are the most predictable, days are long, and routes are in their best condition. However, nighttime variations remain significant depending on the massif and altitude, as detailed in the previous sections. The correct sizing for the majority of alpine bivouacs in July-August: bag with 0°C comfort rating for routes regularly above 2,000 m, bag with 5-7°C rating for routes below 1,800 m with additional clothing layers for cold nights.
September: the rapid transition
The transition from summer to autumn is abrupt in the Alps: nighttime minimums drop by 4 to 7°C between the beginning and the end of September. Above 2,000 m, nights at -3 to -7°C become common by mid-September. The first snowfall in the high mountains (above 2,500 m) occurs regularly from the beginning of September. For a bivouac in September in the Alps above 1,500 m, a bag with a -5°C comfort rating is the appropriate size, and -7 to -8°C for routes in the high mountains.
At high altitude in the Alps, above 3,000 m in winter, nights can drop below -15°C: in these conditions, a model from the extreme cold sleeping bag collection, with a certified comfort range between -10°C and -30°C, is necessary where a 3-season bag reaches its limit.
💡 In September in the Alps, a day at 20°C in the valley can precede a night at -4°C at 2,200 m. Radiative cooling under clear skies is at its peak this month: beautiful days announce the coldest nights.
Humidity and DWR: strategies by massif
Northern Alps: priority on humidity resistance
In the Northern Alps, nocturnal condensation is intense and regular. A bivouac under the stars or a tarp in the Vercors in July frequently generates 50 to 100 grams of dew on the outer fabric of the bag. Routes like the Tour de Belledonne or the crossings of the Aravis regularly expose hikers to nocturnal fog, which completely saturates the outer fabric within a few hours.
For these conditions, three strategies are valid. First option: down with an active DWR treatment on the outer fabric, maintained with a Nikwax or Granger's spray each season. Second option: standard down + waterproof-breathable bivy bag (Gore-Tex or equivalent), which protects the bag from external condensation and maintains its performance over several nights. Third option: lightweight synthetic bag (retains 80-90% insulation when wet), relevant for routes under 7 days where the extra weight is acceptable.
Southern Alps: standard down is suitable
In the Southern Alps (Écrins, Queyras, Mercantour), nocturnal relative humidity is structurally lower. Condensation on the sleeping bag is much less intense, and a standard down bag without specific hydrophobic filling treatment stays dry for 4 to 7 consecutive nights without drying. For long crossings (Haute Route des Écrins, Tour du Queyras), 800 cuin down with active DWR fabric is sufficient without a systematic bivy bag in good weather, provided you compress the bag as soon as clouds appear.
Sizing recommendations by scenario
For a bivouac weekend in Belledonne or the Vercors in July (1,500-2,000 m): bag with 5-7°C comfort rating in 700-800 cuin down with active DWR fabric, or 10°C bag with silk liner. The Aegismax Leto trekking (642 g, 7°C, 700 cuin) covers this range with a sufficient margin.
For a one-week itinerary in the Écrins in July (2,000-2,800 m bivouacs): bag with 0°C to -2°C comfort rating in 800 cuin down, weight under 1,100 g. The Aegismax G2 trekking (1,052 g, -2°C, 800 cuin, 3.3 L compressed) is the consistent sizing for this scenario: the thermal margin up to -8°C limit covers the coldest clear nights of July at high altitude in the Écrins.
For a Tour du Mont-Blanc in August (1,600-2,600 m bivouacs): bag with 0°C to -2°C comfort rating. Nights in a tent above 2,000 m regularly feature minimums of 0-3°C in August, with nights at -2 to -4°C possible. Same sizing as for the Écrins.
For a September bivouac in the Queyras (2,000-2,500 m): bag with at least -5°C comfort rating. The Aegismax M3 (888 g, 0°C comfort) is just enough for these conditions: prefer the Aegismax G2 trekking (-2°C comfort) with a nighttime down jacket for the coldest nights.
Model featured in the article → 
Model featured in the article →
🚫 A sleeping bag rated for 10°C is not suitable for bivouacking above 2,000 m in the Alps. Clear nights in the Écrins or around Mont-Blanc drop below 0°C even in July.
Frequently asked questions
Which bag for the GR5 (crossing the Alps from Lake Geneva to Nice)?
The GR5 traverses very diverse altitudes (600 to 2,750 m) over 500 km. For a crossing from June to September, a bag with a 0°C comfort rating in 800 cuin down covers all scenarios without requiring additional layers. Stays below 1,000 m (lower Mercantour area) will be warm with this bag, but the open zipper allows for ventilation. Weighing under 800 g and rated for 0°C, the Aegismax G2 trekking (1,052 g) is the optimal versatile choice for this type of route.
Is a 0°C-rated bag really necessary for the Tour du Mont-Blanc in August?
Yes, if you are bivouacking above 2,000 m. Nights in a mountain hut or tent at 2,200–2,500 m in August regularly see minimums of 0-3°C, with cold nights reaching -3 to -4°C during rainy spells followed by clearing skies at night. A 5°C-rated bag with a silk liner covers 70-80% of nights, but not the 20-30% that are coldest.
Is condensation in the Northern Alps really a problem for down?
For a one- or two-night bivouac in a double-wall tent, no: the tent provides enough protection. For 5 nights or more in a light bivouac (tarp or bivy bag without a tent), yes. Down starts to accumulate moisture from the 3rd night without drying in the humid conditions of the Northern Alps. A Gore-Tex bivy bag or hydrophobic down is recommended in that case.
Can you bivouac comfortably in the Écrins in June?
In June, high-altitude passages in the Écrins (above 2,500 m) can see nights at -5 to -8°C. A bag with a -5°C comfort rating is the minimum sizing for these conditions. Residual snow makes bivouacking below the permanent snow line (which varies by year: 2,200 to 2,800 m in June) complicated. It is not recommended to bivouac on snow without a sleeping bag rated for at least -5°C.
What is the difference in conditions between the Northern and Southern Alps for a bivouac?
At the same altitude, the Northern Alps are more humid (more intense nighttime condensation, more frequent fog) and slightly milder in temperature thanks to more frequent cloud cover. The Southern Alps are drier (less condensation, standard down is sufficient), colder on clear nights (maximum radiative cooling), and conditions are more predictable. In terms of bag sizing, both ranges require the same thermal performance at equivalent altitudes, but the strategy for protection against humidity differs.
Conclusion
Bivouacking in the French Alps requires precise sizing of your sleeping bag based on real climate data, not approximations. The three essential variables are the altitude of the bivouac (a gradient of 0.65 to 1°C per 100 m), the mountain range (humid Northern Alps vs. dry and radiatively cold Southern Alps), and the season (July-August vs. June and September). The principle of a 5°C margin between the expected minimum and the certified comfort rating remains valid, increasing to 7°C for bivouacs above 2,500 m in the Southern Alps under clear skies. The trekking collection and the down collection in our shop allow you to directly identify the models suitable for these specifications.
⚡ Verdict: Northern Alps below 2,000 m in July-August: 5-7°C rated bag. Southern Alps above 2,000 m: 0°C to -2°C bag. September above 1,800 m: -5°C bag minimum. 5°C margin standard, 7°C for clear skies in the Southern Alps.
