Ultra running weather: plan the heat, cold and wind for a long day

What makes ultramarathon weather different from marathon weather?

Ultramarathon weather is a whole day of weather, not one forecast. A three-hour marathon meets one slice of the morning. A 100 km (62 mile) ultra or a 100-mile race meets the cold dark start, the afternoon heat, the evening and often a night, on courses that climb thousands of metres.

Three things change. Heat stress has hours to build, and the air can swing by 20 °C (36 °F) or more. You move slowly, making less breeze and less body heat. And the course crosses places with their own weather: a col, a canyon, a desert plain.

The answer is a pace-timed forecast: the weather for each stretch of your route at the time you'll reach it. This hub shows how to build one for a long day, and the free Map your run tool builds it from your route file and pace. Our four guides go deeper: Marathon des Sables weather in the Sahara, running an ultramarathon in the heat, mountain ultra weather and fastpacking weather.

Official race instructions and medical advice override anything here. This guide is not medical advice.

How much does the weather change during one ultra?

A lot, even at a race that isn't famous for heat. The Comrades Marathon in South Africa runs about 90 km (56 miles) between Pietermaritzburg and Durban, from 05:30 in the dark to a 12-hour cut-off.

Our own data follows an 11-hour finisher, at about 7:19 per km (11:46 per mile), through every Comrades from 2015 to 2024. It is ERA5 reanalysis, not a thermometer at the start line. There was no race in 2020 or 2021; 2022 ran in August.

In 2023 the feels like at running pace rose from 2.9 °C (37 °F) to 25.9 °C (79 °F), a swing of 23 °C (41 °F) in one run. Every year, heat stress reached Moderate between km 30 and 39 (mile 19 and 24): around halfway, just as you tire. Our Comrades Marathon weather guide goes race by race.

Why does the heat feel worse at ultra pace?

Because you make less of your own breeze, and less airflow means less cooling. We ran one still, sunny afternoon through the RunFast engine: 30 °C (86 °F) air, 40% humidity and strong sun.

The walker feels almost 4 °C (7 °F) hotter than the runner in the same air. The heat-stress number doesn't change with pace, because WBGT measures the place: here 29.0 °C (84 °F), Extreme on the bands big-city marathons use. See WBGT for runners and feels like temperature for runners.

Heat stress also builds with time. The American College of Sports Medicine (Armstrong et al., 2007) notes that fatigue comes faster as heat stress rises, and that heatstroke can strike even when it's relatively cool. Racinais and colleagues (2015) call heat acclimatisation, repeated exercise in the heat over 1–⁠2 weeks, the most important preparation.

Night works the other way. On the energy-cost figures our engine uses (Minetti et al., 2002), running costs about the same per kilometre at any pace. So shuffle at half your usual speed and you make roughly half the heat each hour.

How cold does it get at night and up high?

Colder than the start suggests. The air cools by about 6.5 °C for every 1,000 m you climb (3.6 °F per 1,000 ft), the average rate in NASA's standard-atmosphere model. Real days vary; on still, clear nights cold air can pool in valleys.

Take a valley at 13 °C (55 °F) on a summer night. A col 1,500 m (4,920 ft) higher would be about 10 °C (18 °F) colder, so about 3 °C (37 °F). Add a 25 km/h (16 mph) wind on the col and hike into it at 12:00 per km (19:19 per mile). The RunFast engine gives a feels like of −2.8 °C (27 °F).

The UTMB organisers say August weather around Mont Blanc can top 30 °C (86 °F) or fall to −10 °C (14 °F) above 2,000 m (6,560 ft), with rain, hail or even snow. The 2026 race covered 174 km (108 miles) with 9,700 m (31,800 ft) of climbing. It started at 19:45 on a Friday with a 45-hour 30-minute limit, so the back of the field spends two nights out.

Cold can kill in an ultra. In May 2021, 21 of 172 runners died in a 100 km (62 mile) mountain race in Gansu, China, after hail, freezing rain and gales struck around midday, NBC News reported. The county's low that day was 6 °C (43 °F), before wind chill. A wet, tired runner moving slowly in wind makes little heat and loses a lot.

The ACSM's cold position stand (Castellani et al., 2006) asks athletes to know the signs of hypothermia and to check up-to-date forecasts. See mountain ultra weather.

What should you check in the forecast for 10 to 40 hours on your feet?

Everything a marathoner checks, at every hour you'll be out and every height you'll reach.

The World Health Organization notes that ultraviolet levels rise with altitude, and that sand and fresh snow reflect more of it. The US National Weather Service says mountain thunderstorms typically develop in the early afternoon.

How much should you drink in a hot ultra, and do you need extra salt?

Drink to thirst, avoiding both large losses and overdrinking. Sweat rates differ too much for one number to fit everyone. The American College of Sports Medicine (Sawka et al., 2007) aims to prevent losing more than 2% of body weight to water deficit, with a personal plan built from your sweat rate.

Ultra research adds a warning about drinking too much. Hoffman and colleagues (2013) pooled five years of 161 km (100 mile) races in northern California. Of 669 finishers tested, 15.1% had exercise-associated hyponatraemia, from 4.6% to 51.0% by year, and it was more common in hotter years. Of those, 23.8% had gained weight, a sign of overdrinking, while 35.6% were dehydrated. The authors called aggressive drinking beyond thirst, and excessive sodium supplements, ill advised.

In a 161 km race that reached 39 °C (102 °F), runners lost 2.0–⁠3.1% of body weight on average, whatever their drinking strategy. Half the top 10 had lost more than 2% by 90 km (56 miles) (Hoffman and Stuempfle, 2014). Another study of a 161 km race at 39 °C (102 °F) found sodium supplements unnecessary to maintain hydration (Hoffman and Stuempfle, 2016).

Hew-Butler, lead author of the 2015 international consensus statement on hyponatraemia, advises drinking according to thirst in a 2017 update, adding that race sodium can't prevent it if you drink too much. The Wilderness Medical Society (Bennett et al., 2020) warns that its symptoms overlap with heatstroke, so go to the medical team rather than guess. See running in heat and humidity.

How do desert, canyon and mountain ultras differ?

Each puts the danger in a different hour: desert sun by day, canyon heat in the afternoon, mountain cold and storms.

Desert. Dry air helps sweat evaporate, but full sun drives heat stress up. Take 38 °C (100 °F) air, 12% humidity, a 15 km/h (9 mph) breeze and a high sun. The engine gives a dew point of 3.6 °C (39 °F) and a WBGT of 27.6 °C (82 °F): High. The Marathon des Sables organisers give a minimum speed of 3.5 km/h (2.2 mph) and an average of 5.5 km/h (3.4 mph), walking speeds that make little breeze.

Canyon. Western States climbs from 6,200 ft (1,890 m) to 8,750 ft (2,667 m) at Emigrant Pass, then drops into deep canyons. In 2021 Auburn, by the finish, reached 101 °F (38.3 °C), on the race's weather history. With 20% humidity, a 5 km/h (3 mph) breeze and high sun, both our assumptions, that air gives a WBGT of 32.1 °C (90 °F): Extreme.

How do you plan a stage race or a fastpack one day at a time?

Treat each day as its own run, then plan the nights between them.

1. Map each stage or day as its own route, with its own start time and pace.
2. Check each day as it enters the 10-day forecast, and again the evening before.
3. Plan the night in camp separately. The overnight low decides your sleeping kit and your first hour next morning.
4. Carry kit for the worse end of each day, not its average.
5. Move the next day's start if the heat or a storm shifts.

Our guides to Marathon des Sables weather and fastpacking weather work through both.

How do you build a weather plan for race day?

Start with history, switch to the forecast, and pack for three key hours.

1. Weeks out, study the range. Use race-day history, such as Comrades on our map or Western States' weather page.
2. Ten days out, map the forecast at your real pace. Include the walking: a 100 km (62 mile) race at 10:00 per km (16:06 per mile) takes 16 h 40 min.
3. Mark three hours: your coldest, your hottest and your wettest or windiest, and where you'll be in each.
4. Put kit in drop bags before the night and the high ground; see what to wear running at every temperature.
5. Plan fluids from your own sweat test, practised in training.
6. Read the final forecast the evening before, then follow the official flags and instructions on the day.

Our guide to weather along your running route explains reading it stretch by stretch.

What can RunFast show for an ultra, and what can't it?

It times the weather to every stretch of one long route at your pace; it can't yet plan a multi-day trip. Here is what Map your run does today.

• Your pace, walking included: 2:30 to 25:00 per km (4:01 to 40:14 per mile), adjusted for hills, on routes up to 250 km (155 miles).
• Each kilometre's weather: feels like at your pace, dew point, WBGT heat stress, rain and the wind at running speed.
• The weather cost: the time the weather adds, in words, with its main cause. At a WBGT of 25 °C (77 °F), the engine's model adds about 5.3 minutes to each hour at 10:00 per km (16:06 per mile).
• Fly the route in 3D, and get the final forecast emailed the evening before and two hours before.
• Race-day history for Comrades, every race day from 2015 to 2024.

The limits: forecasts reach 10 days ahead. Race-day history covers four races, and Comrades is the only ultra. One route is one run, so stage races and fastpacks go one day at a time; there's no multi-day tour yet. It times an even effort, so set a slightly slower pace to allow for aid-station stops. And it can't tell you how much to drink.