Heat Acclimatization Before a Summer Ultra: Why You Can't Just Hope for Cool Weather

The competitive calendar for trail and ultra running concentrates its most demanding events in summer — a fact that seems straightforward until you examine what the heat environments of those events actually look like. Western States 100 sends runners from Olympic Valley to Auburn, California in late June on a course where temperatures in the American River canyon system regularly exceed 38°C. Hardrock 100 runs in Silverton, Colorado in mid-July with course high points above 4,300 metres where solar radiation at altitude hits harder than air temperature alone suggests. Leadville 100 sits at a base altitude above 3,000 metres through mid-August. UTMB starts in Chamonix in late August in a valley that bakes at 25–30°C before the course climbs to cols at 2,400–2,600 metres.

Athletes targeting summer ultras who spend their build phase training in cool, temperate climates — the UK, the Pacific Northwest, Scandinavia, upland Central Europe — arrive at race start physiologically unprepared for the thermal stress that will define their race. The fix is specific and learnable. But it requires deliberate planning, and leaving it to chance is a decision that shows up in the canyon sections.

What Acclimatization Does

The body's response to sustained heat exposure produces a cluster of physiological adaptations. The major ones, as understood from the sports science literature (and simplified here — consult a sports scientist for protocol design):

Plasma volume expansion: Within several days of consistent heat training, total blood plasma volume increases. This expands blood volume, which reduces the cardiovascular competition between supplying working muscles and supplying the skin for thermoregulation — the two simultaneous demands that make running in heat so much harder than running in cool conditions. Plasma volume expansion is considered one of the primary early adaptations and is measurable within the first week of heat exposure.

Improved sweating efficiency: Acclimatized athletes begin sweating at a lower core body temperature and produce more sweat per unit of heat stress. Sweating is the body's primary cooling mechanism during sustained exercise; a more responsive sweat system means more efficient heat dissipation at a given pace.

Lower working core temperature: After full acclimatization — generally understood to require roughly 10 to 14 days of consistent heat exposure — athletes complete a given work output at a lower core temperature than before. The physiological cost of running at race pace in the heat decreases.

These adaptations are real and performance-relevant, but they decay. Most of the benefit is lost within approximately three weeks of returning to cool-weather training. Heat acclimatization is not something to build in spring and expect to carry to a late-August race; the timing matters.

The Timeline

Begin deliberate heat acclimatization 10 to 14 days before race day, with daily or near-daily heat exposure across the full window. Full acclimatization takes roughly two weeks; starting earlier does not substantially increase the benefit and risks the adaptation partly decaying before race day. Starting later produces a partially-adapted athlete.

If a pre-race altitude camp, travel disruption, or taper structure removes you from regular heat exposure in the final 7 to 10 days, the acclimatization window needs to shift accordingly. Plan backward from the last day of consistent heat access, not from race start.

Active heat training is the most direct approach. Run or hike during the warmest part of the day, aiming for 30 to 60 minutes of sustained moderate-intensity effort — around 60 to 70 percent of maximum heart rate — in conditions above roughly 28°C. The goal is sustained elevated core temperature while continuing to move, not maximal effort. The key risk is piling high-intensity heat work on top of race-build volume: overreaching in the heat while also running big weeks is a meaningful illness and overtraining risk. Reduce the intensity of heat sessions if total training load is already high.

Post-exercise sauna sessions are the accessible alternative for athletes training in cool climates. Spending 20 to 30 minutes in a sauna (around 80°C) immediately after a training run — when core temperature is already elevated — has been shown in the research literature to produce measurable plasma volume expansion and thermal tolerance improvements. Repeated across the 10 to 14-day acclimatization window, post-run sauna appears to produce adaptations comparable in direction, if not always in magnitude, to outdoor heat training. This protocol makes heat prep accessible to athletes in northern Europe or the Pacific Northwest who cannot easily train in genuinely hot outdoor conditions. The caveats apply: the research continues to develop and individual responses vary; treat it as a useful tool, not a guaranteed substitute for the real thing.

Hot-bath immersion: Immersion in hot water (around 40°C) for 30 to 40 minutes after a training run shows similar effects to sauna in some research. Practically easier to access than a sauna but harder to regulate — water temperature drops during a long session and the bath needs reheating or topping up. The same post-exercise timing applies.

For all passive heat methods, do them after the training run, not instead of it. The combination of metabolic heat from the run and environmental heat from the sauna or bath is the stimulus; the sauna alone is a lesser version.

Altitude Complicates Everything

Several of the summer ultras combine heat with altitude, which is a more complex preparation problem than either alone.

Hardrock 100 sits almost entirely above 3,600 metres; the course high points exceed 4,300 metres. Leadville 100's start and finish is at 3,100 metres; Hope Pass peaks around 3,840 metres. UTMB crosses multiple passes above 2,400 metres while the valley running in Chamonix starts at roughly 1,000 metres.

Heat and altitude interact in ways that make separate preparation for each partially but not fully effective. Thin air at altitude reduces convective cooling — the mechanism by which moving air carries heat from the skin — meaning a given pace produces more thermal stress than at sea level, even when air temperatures feel cooler. Simultaneously, higher altitude means less atmospheric filtration of solar radiation, and direct sun exposure on an alpine ridge in July carries a significantly higher radiant heat load than sun at sea level. The body temperature cost of a comparable effort is higher at altitude even in cool ambient air.

The practical implication: if you are preparing for Hardrock or Leadville, altitude camp is the higher-priority adaptation, and heat work should be built around or before it. Athletes targeting UTMB — where the altitude range is lower and the valley heat is the more novel stressor for European athletes — can weight heat acclimatization more equally with altitude exposure.

For most age-group athletes, running both a meaningful altitude camp and a full heat acclimatization block in the same pre-race period is a resource and time constraint. Be specific about which stressor your race most demands, prepare for that one, and accept the partial compromise on the other.

Western States: What the Canyon Section Actually Requires

Western States 100 starts before dawn from Olympic Valley in the Sierra Nevada. The first 30 miles are cold. As the course descends into the American River canyon system — beginning roughly at mile 38 at Duncan Canyon — temperatures build through the morning. By early afternoon, when most of the field is working through Devil's Thumb, Deadwood Canyon, El Dorado Canyon, and the climb to Michigan Bluff, air temperatures regularly exceed 32°C and can reach 38°C in canyon bottoms. The terrain is rocky and offers minimal shade.

The race organisation operates mandatory weigh-ins at several checkpoints to screen for dangerous fluid loss, and ice is available at most aid stations from the canyon section onward. Athletes who arrive with working heat acclimatization use the ice to extend and sharpen performance; athletes who arrive unprepared use it to manage a crisis that proper preparation would have mitigated.

The canyon is where Western States is decided for most of the field, and where the race's medical team records the majority of its interventions. These two facts are directly connected. Improving your heat tolerance before arriving does not guarantee you will have a good day in the canyon — terrain difficulty, cumulative fatigue from the first 38 miles, and individual heat sensitivity all play roles — but it substantially shifts the odds.

Sodium and Fluid Adjustments

Heat acclimatization changes your electrolyte and fluid requirements. As sweat rate increases, total sodium loss per hour of exercise in the heat increases. Athletes who have calibrated their race-day nutrition strategy during cool-weather training will typically need to revise sodium intake upward for a summer race.

Sodium needs vary significantly between individuals — more than almost any other dietary race variable. Sweat sodium concentration testing, available through sports science labs and some sports nutrition services, gives an actual measurement of your individual loss rate. If testing is not available, use the heat training sessions as a rough calibration: increase sodium intake incrementally during the acclimatization runs and monitor cramping tendency and performance.

Gastrointestinal tolerance for fluid and calories while running in heat is also often lower than in cool conditions. Blood supply is diverted toward thermoregulation and working muscles; gut motility and absorptive capacity decrease. Practice ingesting fluids and calories at your planned race-day rates during the acclimatization sessions specifically — do not assume that what works in a cool spring long run will transfer.

The Decision Window

The structural problem with heat preparation is that it falls in the same window as race taper: the 14-day period before the race when training volume drops is also the 14-day period when heat acclimatization is most timing-efficient. The two are not incompatible — heat sessions at reduced intensity during taper still produce the cardiovascular adaptations — but they require explicit scheduling rather than defaulting to the standard taper template.

Athletes who have not planned the heat work by the time taper begins are, in practice, not going to do it. The decision needs to be made at the start of the build phase, 12 to 16 weeks out, when the preparation structure is being designed. The question to answer at that point is not "how do I taper?" but "where does my thermal environment preparation fit in this block, and what does it displace?"

Summer ultras are selective about who finishes comfortably and who suffers through the back half. Heat acclimatization is one of the few variables where the preparation gap between ready and unprepared athletes is wide, entirely addressable, and almost entirely a planning choice.