The short answer
Mountain ultra weather changes with height and with the clock. In the standard atmosphere, air cools about 6.5 °C for every 1,000 m you climb (3.6 °F per 1,000 ft), the highest passes may come at night, and storms build on afternoons. UTMB warns of anything from above 30 °C (86 °F) to −10 °C (14 °F).
Key takeaways
- UTMB's high cols sit about 1,300–1,530 m above Chamonix: roughly 8–10 °C (15–18 °F) colder on a standard day.
- In UTMB's 2026 timetable, the fastest runners crossed four of the course's five highest points in the dark.
- Hiking into a 30 km/h (19 mph) wind at 0 °C (32 °F) feels −6.9 °C (20 °F).
- Wet kit, wind and a slow pace together invite hypothermia; carry warm, waterproof layers.
On this page
- What is mountain ultra weather like?
- How much colder is it at the top of each climb?
- When will you be on the high passes in the dark?
- How cold does the wind make it at a hiking pace?
- Why is cold rain so dangerous on a mountain ultra?
- What should you do about thunderstorms above the tree line?
- How do you plan a mountain ultra around the forecast?
- Questions runners ask
- Sources and method
Words this guide uses
- Lapse rate
- How fast the air cools as you climb. The standard atmosphere uses about 6.5 °C per 1,000 m (3.6 °F per 1,000 ft); real days vary.
- Wind chill
- How cold moving air makes bare skin feel, from the air temperature and the wind speed.
- Pace-timed forecast
- The weather for each stretch of your route at the time you'll be there, from your start time and pace.
What is mountain ultra weather like?
Mountain ultra weather is three forecasts in one: the valley, the passes and the night. Height and darkness make the air colder, and storms build over high ground. Our ultra running weather hub covers the long day; this guide, the mountains.
UTMB, the Ultra-Trail du Mont-Blanc, is the worked example. Its equipment page warns that August there can bring more than 30 °C (86 °F) or −10 °C (14 °F) above 2,000 m (6,560 ft).
| UTMB at a glance | 2026 edition |
|---|---|
| Course | A loop around Mont Blanc through France, Italy and Switzerland |
| Distance, km (miles) | 174 (108) |
| Climbing, m (ft) | 9,700 (31,800) |
| Start | Chamonix, Friday 28 August, 19:45 |
| Time limit, hours | 45:30 |
How much colder is it at the top of each climb?
About 6.5 °C for every 1,000 m (3.6 °F per 1,000 ft), in the standard atmosphere NASA models. Real days vary: on clear, calm nights the air near the ground can be colder than the air above, an inversion, as the US National Weather Service explains.
The table applies 6.5 °C (11.7 °F) per 1,000 m from Chamonix, 1,035 m (3,396 ft) up, beside UTMB's 2026 timetable.
| Point (km, miles) | Height, m (ft) | Colder than Chamonix, °C (°F) | Fastest runners | Slowest runners |
|---|---|---|---|---|
| Col du Bonhomme (45, 28) | 2,329 (7,641) | 8.4 (15.1) | Fri 23:42, dark | Sat 04:43, dark |
| Col de la Seigne (63, 39) | 2,511 (8,238) | 9.6 (17.3) | Sat 01:46, dark | Sat 09:21, day |
| Col des Pyramides Calcaires (66, 41) | 2,565 (8,415) | 9.9 (17.9) | Sat 02:10, dark | Sat 10:11, day |
| Grand Col Ferret (106, 66) | 2,527 (8,291) | 9.7 (17.5) | Sat 07:19, day | Sat 21:47, dark |
So if Chamonix reads 12 °C (54 °F) at 22:00, expect about 2 °C (36 °F) on the highest col.
When will you be on the high passes in the dark?
Early: UTMB starts at 19:45, and the sun set at about 20:20 that weekend. Even the leaders crossed Bonhomme, Seigne and Pyramides Calcaires at night. The slowest met Bonhomme before dawn and Grand Col Ferret after dark on the second night. UTMB's kit must see you through one or two nights out.
How cold does the wind make it at a hiking pace?
Colder than the forecast air. At 20:00 per km (32:11 per mile) your own speed adds little airflow, so the wind does the chilling. The numbers are RunFast's engine: the wind chill formula on the air moving past you.
| Air, °C (°F) | Wind, km/h (mph) | Hiking into it, °C (°F) | Hiking with it behind, °C (°F) |
|---|---|---|---|
| 5 (41) | 30 (19) | −0.3 (31) | 0.5 (33) |
| 0 (32) | 0 | 0 (32) | 0 (32) |
| 0 (32) | 30 (19) | −6.9 (20) | −6.0 (21) |
Turning your back gains about 1 °C (2 °F). Our feels like temperature guide explains the method, and wind and hills shows what wind costs in time.
Why is cold rain so dangerous on a mountain ultra?
Because wet kit, wind and a slow pace together let your body temperature fall. The NHS lists wet clothes among the causes of hypothermia, and shivering, slurred speech and confusion among its signs. The American College of Sports Medicine (2006) asks athletes to know those signs and the forecast.
UTMB's basic kit is a cold-rain plan:
- a hooded waterproof jacket with taped seams
- waterproof overtrousers
- a warm long-sleeved layer of at least 180 g (6.3 oz)
- a beanie and warm, waterproof gloves
- a survival blanket of at least 1.40 × 2 m (4.6 × 6.6 ft)
UTMB can add items such as a third warm layer, announced at least 24 hours before the start. In 2012, forecast cold and snow cut the race to about 100 km (62 miles), all in France. Our what to wear running chart covers layering.
What should you do about thunderstorms above the tree line?
Be off ridges and passes before they build. The US National Weather Service says mountain thunderstorms typically develop in the early afternoon and that you are not safe outside in one: avoid hilltops and ridges, and spread out in a group.
Hardrock 100 in Colorado, US, shows the stakes. It covers 101.8 miles (163.8 km) and tops out at 14,048 ft (4,282 m) on Handies Peak, about 12.6 °C (22.7 °F) colder than Ouray, its low point, at the standard lapse rate. Its course page warns of the monsoon's afternoon thunderstorms, says several runners have had direct contact with lightning, and gives 48 hours partly so you can wait storms out in a valley.
How do you plan a mountain ultra around the forecast?
Time it to your pace. A pace-timed forecast gives the weather for each stretch at the hour you'll be there.
- Find the high, exposed sections and when you'll reach them.
- Cross-check with 6.5 °C per 1,000 m (3.6 °F per 1,000 ft): RunFast adds no altitude correction of its own.
- Pack for the coldest feels like, at night, wet.
- Move storm-prone ridges to the morning where you can.
RunFast maps routes up to 250 km (155 miles) at paces to 25:00 per km (40:14 per mile), so hiking works. It has no UTMB race-day history. Multi-day routes go one day at a time, as our fastpacking weather guide explains. The organiser's decisions and medical advice override this article, which is not medical advice.
Questions runners ask
How cold does it get at UTMB?
Colder than the valley suggests. UTMB's equipment page warns of down to −10 °C (14 °F) above 2,000 m (6,560 ft) in August, with rain, hail or snow. Its highest point, Col des Pyramides Calcaires at 2,565 m (8,415 ft), is about 10 °C (18 °F) colder than Chamonix on a standard day.
Was UTMB ever shortened because of the weather?
Yes. In 2012, with cold and snow forecast, the organisers cut the race to about 100 km (62 miles), all within France, National Geographic reported. UTMB's rules still let the organisers delay the start, change the course or cut-offs, or stop the race in bad weather. Their decisions override any plan you make from a forecast.
Does hiking instead of running change the wind chill?
Only a little. At a hiking pace your own speed adds little airflow, so the wind sets the chill. At 0 °C (32 °F) with a 30 km/h (19 mph) wind, hiking into it feels −6.9 °C (20 °F), and with it at your back still −6.0 °C (21 °F), by RunFast's engine.
What should you do if you start shivering in the rain on a mountain?
Act early. Put on your waterproof jacket and trousers, the warm layer, hat and gloves, eat, and head down or to the next aid station. The NHS lists shivering, slurred speech, tiredness and confusion as signs of hypothermia, a body temperature below 35 °C (95 °F). It is a medical emergency: tell a marshal or call for help.
Does RunFast warn you about lightning?
No. RunFast shows the chance and amount of rain for each stretch at the hour you reach it, from Apple Weather, plus the wind and the feels like, but it has no lightning alerts. For storms, read the local mountain forecast on race morning, follow the organiser's instructions, and be off exposed ridges before the afternoon.
See the heat at your pace.
Drop in your route (GPX, FIT or TCX), pick the day and your pace, and see the feels like, dew point and heat stress for every kilometre, timed to when you’ll be there. Free, in your browser, no sign-up.
Sources and method
- UTMB Mont-Blanc (2026). UTMB race page (2026 edition, course and timetable)
- UTMB Mont-Blanc (2026). Mandatory equipment
- UTMB Mont-Blanc (2026). 2026 regulations
- National Geographic (2012). Wintry conditions reroute the 10th Ultra-Trail du Mont-Blanc (Mary Anne Potts)
- NASA Glenn Research Center (2021). Earth atmosphere model, metric units
- US National Weather Service, Little Rock (2015). What is an inversion?
- US National Weather Service (2026). Wind chill chart
- US National Weather Service (2026). Lightning safety when you're outdoors
- Hardrock Hundred Mile Endurance Run (2026). The course
- Hardrock Hundred Mile Endurance Run (2026). About the run
- NHS (2023). Hypothermia
- American College of Sports Medicine (2006). Position stand: prevention of cold injuries during exercise (Castellani et al.)
UTMB facts (174 km, 9,700 m of climbing, the 19:45 Friday start on 28 August 2026, the 45-hour 30-minute limit, the countries) come from UTMB's race page; the passes' heights, their distances and the fastest and slowest passing times come from the course timetable on the same page, whose distances run to 176.9 km. Temperatures by height use the standard-atmosphere lapse rate of 6.5 °C per 1,000 m (NASA's model gives 6.49), applied from Chamonix at 1,035 m; real lapse rates vary and inversions reverse them. Dark or light uses the sun's height from RunFast's engine (NOAA's approximation) at each pass: sunset at 20:20 on 28 August and 20:18 on 29 August, sunrise at 06:48, local time, before any mountain shading. Wind chill is RunFast's feels like at running pace: the wind chill formula (the one the NWS and Environment Canada use) on the air moving past you, taken as two-thirds of the 10 m forecast wind at body height plus or minus your own speed, on flat ground. Pace conversions use 1 mile = 1.609344 km; Fahrenheit values are our conversions. RunFast is independent of UTMB and the Hardrock Hundred.
Wes Thomson
Wes Thomson builds RunFast, which times the heat, humidity and dew point to a runner's own pace along the route, and its sister Windline, live wind forecasts for cyclists. Both run on the same forecast engine. He writes these guides with one focus: what the weather will do to your run, kilometre by kilometre, and how to plan around it.