The short answer
Running an ultramarathon in the heat means carrying heat stress for 15 to 45 hours, not one hot hour. Western States says its course can pass 43 °C (110 °F); Death Valley's July highs average 47 °C (116 °F). Slow down early, cool with ice and water, drink to thirst, and know when your pace reaches the afternoon.
Key takeaways
- Western States finish rates averaged 73.8% in its six hottest years since 2010, 84.8% in the coolest five.
- When you walk, less air moves past you: the same afternoon feels hotter and costs more time.
- A 2015 Western States study blamed overdrinking, not low salt, for low blood sodium.
- Cool with ice and water at every chance, through the night too.
On this page
- What does an ultramarathon in the heat do to you?
- Why does a slower pace make the heat worse?
- Does heat change who finishes?
- How do you stay cool in a hot ultra?
- How much salt and water does a hot ultra need?
- What can runners elsewhere learn from these races?
- How do you plan the heat on your own ultra?
- Questions runners ask
- Sources and method
Words this guide uses
- WBGT (wet-bulb globe temperature)
- A heat-stress number combining air temperature, humidity, sun and wind; race medical teams set their flags by it.
- Exercise-associated hyponatraemia
- Blood sodium below 135 mmol/L during or up to 24 hours after long exercise, usually from drinking more than you lose.
- Pace-timed forecast
- The weather for each stretch of your route at the time you'll be there, at your pace.
What does an ultramarathon in the heat do to you?
An ultramarathon in the heat means carrying heat stress for 15 to 45 hours, not one hot hour. It builds each afternoon and, in a desert, eases little at night. Our ultra running weather hub covers cold, wind and the rest.
Two Californian races show the range: Western States 100, through the Sierra Nevada and gold-country canyons, and Badwater 135, across Death Valley.
| The races at a glance | Western States 100 | Badwater 135 |
|---|---|---|
| Month | Late June | July |
| Start | Olympic Valley, 05:00 | Badwater Basin, 85 m (280 ft) below sea level; waves 20:00 to 22:00 |
| Finish | Auburn, California | Whitney Portal, 2,530 m (8,300 ft) |
| Distance, km (miles) | 161.3 (100.2) | 217 (135) |
| Climb and descent, m (ft) | 5,514 (18,090) up, 7,001 (22,970) down | 4,450 (14,600) up, 1,859 (6,100) down |
| Time limit, hours | 30 | 45 |
| Heat, °C (°F) | Auburn race-day highs 23 to 38 (73 to 101), 2015–2026 | Furnace Creek in July: average high 47 (116), low 31 (88) |
Western States says temperatures on the run range from −7 °C (20 °F) to above 43 °C (110 °F). At an even 24-hour pace, you'd reach its canyons, from Last Chance at km 70 (mile 43.3), at about 15:20.
Why does a slower pace make the heat worse?
You spend longer in each hot kilometre, and less air moves past you to carry heat away. We ran both races through RunFast's engine as a pace-timed forecast: the weather for each stretch at the time you'd be there.
| Moment (ERA5) | Air, °C (°F) | Dew point, °C (°F) | WBGT, °C (°F), level | Feels like walking 10:00 per km (16:06 per mile), °C (°F) | Heat cost walking, min per hour |
|---|---|---|---|---|---|
| Western States canyons, 16:00, 26 Jun 2021 | 34.3 (94) | 9.1 (48) | 26.8 (80), High | 38.4 (101) | 6 |
| Badwater, July night, 02:00 | 36.7 (98) | 8.4 (47) | 24.1 (75), High | 36.7 (98) | 5 |
| Badwater, July afternoon, 15:00 | 45.5 (114) | 8.9 (48) | 33.6 (92), Extreme | 51.8 (125) | 10 |
On that Badwater afternoon, a runner at 6:00 per km (9:39 per mile) feels 50.0 °C (122 °F) against the walker's 51.8 °C (125 °F). The heat costs the runner 54 seconds a kilometre and the walker 98. Even at 02:00, heat stress stays High.
ERA5 is reanalysis, not a thermometer at the start line; its 31 km (19-mile) grid smooths out the hotter canyon floors. The heat cost is built on marathon studies (Mantzios et al., 2022; Ely et al., 2007): a rough guide in a 30-hour race. WBGT for runners explains the levels.
Does heat change who finishes?
At Western States, it seems to. In the race's table for 2010 to 2026, five races with Auburn highs below 29 °C (85 °F) averaged an 84.8% finish rate. Five at 29 to 34 °C (85 to 94 °F) averaged 76.1%; six at 35 °C (95 °F) or more, 73.8%.
Snow, rivers and fields vary too: an association, not proof. Illness follows the heat as well: pooling five years of 161 km (100-mile) races in northern California, Hoffman and colleagues (2013) found hyponatraemia in 4.6% to 51.0% of finishers by year, more in hotter weather.
How do you stay cool in a hot ultra?
Cool inside and out at every chance, after dark too. A 2017 review by Stevens and colleagues found cold drinks, ice slurry, and cooling the neck and face, including water poured on the head, all helped endurance in the heat.
The Racinais consensus (2015) adds iced towels, fanning, and about 1 litre (34 fl oz) of ice slurry. Western States suggests a light-coloured shirt and cap, kept wet. Death Valley's summer nights may only fall to 30 to 35 °C (85 to 95 °F), says the US National Park Service, so the ice keeps coming at night.
How much salt and water does a hot ultra need?
Drink to thirst, and don't count on salt for protection. The 2015 international consensus on exercise-associated hyponatraemia recommends thirst as the guide.
Western States research backs it. Hoffman and Stuempfle (2015) found 93.9% of runners took sodium supplements, and those who developed low sodium had taken them at the same rate. They concluded overdrinking was the main cause.
The Wilderness Medical Society (2019) warns that hyponatraemia can look like heat exhaustion or heat stroke, so a confused runner needs the medical team. Official race guidance and medical advice override this article, which is not medical advice; see the American College of Sports Medicine (2007) on heat illness.
What can runners elsewhere learn from these races?
That your finish time chooses your weather. On South Africa's 2016 Comrades Marathon down run, our ERA5 history shows a 9-hour runner meeting High heat stress from km 60 (mile 37). An 11-hour runner met it from km 48 (mile 30).
Air usually peaks in mid-afternoon, so the stretch you reach then is often your hottest. Plan your cooling there, in the Sahara's Marathon des Sables too, and read how hot is too hot to run to set your limits.
How do you plan the heat on your own ultra?
Map the route at your pace, find the afternoon stretches, and put your ice and slower pacing there. Map your run forecasts 10 days ahead and can email the final forecast the evening before and two hours before the start. Fly the route shows the course in 3D first. Race-day history covers only Comrades, Berlin, London and Boston, not these two.
Questions runners ask
How hot does Western States get in the canyons?
Above 43 °C (110 °F), the race warns, and its medical page records 46 °C (114 °F) radiating off the rocks in 1989. The canyons run from Last Chance, mile 43.3 (km 70), to Michigan Bluff, mile 55.7 (km 90), and a 24-hour runner reaches them at about 15:20.
How hot is Badwater 135?
Very hot, day and night. At Furnace Creek, mile 17.7 (km 28.5), the US National Park Service gives a July average high of 47 °C (116 °F) and low of 31 °C (88 °F). The race starts in evening waves, but nobody covers 217 km (135 miles) in one night.
How much weight should you lose in a hot 100-miler?
Western States tells runners to expect a 3 to 5% loss of body weight. Hoffman and colleagues (2013) found losses above 3% didn't seem to harm performance. Weight is a poor guide to blood sodium, though: in the 2009 race, half of those with hyponatraemia had lost 3 to 6%.
How long does it take to acclimatise for a hot ultra?
About two weeks. A 2015 consensus led by Racinais found 6 to 10 days bring near-complete heart and sweating adaptations, and two weeks optimise performance. Sessions should last at least 60 minutes a day and make you hot and sweaty. Western States gives the same two-week figure.
Can you map a 100-mile ultra in RunFast?
Yes. Map your run takes routes up to 250 km (155 miles) and paces as slow as 25:00 per km (40:14 per mile), so both races fit, walking included. It times the heat to every kilometre at your pace, from a forecast up to 10 days ahead. It has no race-day history for these two.
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
- Western States Endurance Run (2026). Weather history (race-day highs and lows in Auburn, and finish rates)
- Western States Endurance Run (2026). Course description
- Western States Endurance Run (2026). Medical and other risks
- Western States Endurance Run (2026). Aid stations
- AdventureCORPS (2026). Badwater 135
- US National Park Service (2026). Death Valley weather
- Medicine & Science in Sports & Exercise (2013). Exercise-associated hyponatremia and hydration status in 161-km ultramarathoners (Hoffman, Hew-Butler and Stuempfle)
- Medicine & Science in Sports & Exercise (2015). Sodium supplementation and exercise-associated hyponatremia during prolonged exercise (Hoffman and Stuempfle)
- Sports Medicine - Open (2015). Sodium intake during an ultramarathon does not prevent muscle cramping, dehydration, hyponatremia, or nausea (Hoffman, Stuempfle and Valentino)
- International Journal of Sports Physiology and Performance (2012). Hyponatremia in the 2009 161-km Western States Endurance Run (Hoffman et al.)
- Clinical Journal of Sport Medicine (2015). Statement of the Third International Exercise-Associated Hyponatremia Consensus Development Conference, Carlsbad, California, 2015 (Hew-Butler et al.)
- Wilderness & Environmental Medicine (2020). Wilderness Medical Society clinical practice guidelines for the management of exercise-associated hyponatremia, 2019 update (Bennett et al.)
- Sports Medicine (2017). Cooling during exercise: an overlooked strategy for enhancing endurance performance in the heat (Stevens, Taylor and Dascombe)
- British Journal of Sports Medicine (2015). Consensus recommendations on training and competing in the heat (Racinais et al.)
- American College of Sports Medicine (2007). Position stand: exertional heat illness during training and competition (Armstrong et al.)
- Medicine & Science in Sports & Exercise (2022). Effects of weather parameters on endurance running performance: discipline-specific analysis of 1258 races (Mantzios et al.)
- Copernicus Climate Change Service (ECMWF) (2023). ERA5 hourly data on single levels from 1940 to present
Race facts come from wser.org and badwater.com (Badwater's three evening waves and 45-hour limit as its race page lists them); Western States climbing adds the course description's 2,550 ft first climb to its further 15,540 ft. The 15:20 arrival is Last Chance (mile 43.3 of 100.2) at an even 24-hour pace from the 05:00 start, ignoring terrain. Finish rates by heat use the Western States weather-history table (Auburn highs from Weather Underground) for 2010 to 2026, with no race in 2020: five races below 85 °F, five from 85 to 94 °F and six at 95 °F or above, simple means of the listed finish rates. The worked example uses the RunFast engine: WBGT by Liljegren et al. (2008); feels like by Steadman's apparent temperature with sun, at the airflow of your pace plus a light crosswind; dew point by Magnus; heat cost from the engine's weather cost, built on Mantzios et al. (2022) and Ely et al. (2007), with the wind's own cost left out. Inputs are ERA5 reanalysis (Copernicus Climate Change Service) read through the Open-Meteo archive, which adjusts temperature to the height of the place asked for. Western States: 39.04° N, 120.77° W (near Michigan Bluff, read at 1,025 m, 3,360 ft) at 16:00 on 26 June 2021: 34.3 °C, dew point 9.1 °C, wind 13.6 km/h, cloud 4%. Badwater: Furnace Creek, 36.46° N, 116.87° W, the mean of every July day at 15:00 and at 02:00 from 2016 to 2025: 45.5 °C, dew point 8.9 °C, wind 8.3 km/h, cloud 34%; and 36.7 °C, dew point 8.4 °C, wind 4.3 km/h, dark. Sunshine is the engine's clear-sky value dimmed by cloud. Paces are on the flat with no stops: 10:00 per km (6 km/h) walking, 6:00 per km (10 km/h) running. The Comrades figures come from RunFast's ERA5 race history (9:00 and 11:00 finishes). Fahrenheit and imperial values are our conversions. RunFast is independent of both races.
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.