Environmental conditions—heat, cold, altitude, and humidity—significantly affect running performance and training adaptations. Understanding how the body responds to these stressors allows runners to train safely, adapt strategically, and adjust pacing appropriately for race-day conditions.
This article explains the physiological impacts of heat, cold, altitude, and humidity, outlines adaptation strategies, and provides practical guidelines for training and racing in varied environments.
Heat and marathon training
Physiological impact of heat
Challenges:
- Increased cardiovascular strain (diverted blood flow to skin for cooling)
- Elevated core temperature
- Greater sweat loss and dehydration risk
- Reduced blood volume available for working muscles
- Decreased running economy
Performance impact:
- Running pace slows by 10-30 seconds per mile in hot conditions
- Perceived effort increases at same pace
- Risk of heat illness (heat exhaustion, heat stroke) if precautions ignored
Heat adaptation (acclimatization)
Process: The body adapts to repeated heat exposure over 10-14 days:
- Increased plasma volume: More blood available for cooling and muscle work
- Earlier onset of sweating: Cooling begins sooner
- More efficient sweating: Greater sweat rate with lower electrolyte loss
- Reduced heart rate: Lower cardiovascular strain at given effort
- Lower core temperature: Better thermoregulation
Training approach:
- Run in heat for 60-90 minutes daily for 10-14 days
- Start with shorter, easier efforts and progress
- Hydrate well before, during, and after
Benefits:
- Improved performance in heat
- Potential performance boost in cool conditions (expanded plasma volume)
Strategies for training in heat
Timing:
- Run early morning or late evening to avoid peak heat
- Check temperature and humidity before heading out
Hydration:
- Pre-hydrate: 16-20 oz water 2-3 hours before running
- During: 4-8 oz every 15-20 minutes for runs over 60 minutes
- Post-run: Replace 150% of fluid lost (weigh before and after)
Clothing:
- Light-colored, moisture-wicking fabrics
- Hat or visor for sun protection
- Sunglasses
Pacing:
- Slow pace by 10-30 seconds/mile in heat
- Use perceived effort and heart rate rather than pace targets
- Accept slower training paces
Safety:
- Know signs of heat illness: dizziness, nausea, confusion, cessation of sweating
- Run with a partner or route with water access
- Stop and seek shade/water if feeling unwell
Cold weather training
Physiological impact of cold
Challenges:
- Greater energy expenditure (thermoregulation)
- Reduced muscle temperature (decreased performance until warmed up)
- Airway irritation from cold, dry air
- Increased injury risk if muscles are not warmed adequately
Potential benefits:
- Easier thermoregulation (less overheating)
- Often ideal racing conditions (40-50°F)
Strategies for training in cold
Layering:
- Base layer: Moisture-wicking fabric close to skin
- Mid layer: Insulation (fleece or light jacket)
- Outer layer: Wind and water-resistant shell
- Remove layers as you warm up; tie around waist if needed
Extremities:
- Gloves or mittens (mittens warmer)
- Hat or headband to cover ears
- Warm socks; consider gaiters in deep cold
Warm-up:
- Extend warm-up duration in cold conditions
- Start indoors if possible
- Gradual progression to running pace
Breathing:
- Breathe through nose initially to warm air
- Use a buff or scarf over mouth in extreme cold
- Cold air can irritate airways; listen to your body
Safety:
- Know signs of frostbite: numbness, white or pale skin
- Run into the wind at the start, with the wind on the return (avoid cooling when sweaty)
- Shorten runs in extreme cold (<0°F / -18°C)
Altitude training and racing
Physiological impact of altitude
Lower oxygen availability:
- Air pressure decreases with altitude
- Less oxygen per breath
- Reduced oxygen delivery to muscles
Acute effects (first days at altitude):
- Increased breathing rate
- Elevated heart rate at given pace
- Decreased performance (5-10% slower at moderate altitude)
- Fatigue, headache, nausea (altitude sickness in some individuals)
Long-term adaptations (weeks to months):
- Increased red blood cell production (more oxygen-carrying capacity)
- Improved oxygen delivery and utilization
- Enhanced buffering capacity
Training at altitude
Altitude categories:
- Low altitude: <3,000 feet (minimal impact)
- Moderate altitude: 3,000-8,000 feet (noticeable impact, adaptation possible)
- High altitude: >8,000 feet (significant impact, specialized training)
Adaptation timeline:
- Days 1-3: Acute altitude sickness possible, reduced performance
- Days 4-10: Gradual adaptation begins
- Weeks 2-4: Significant adaptation, performance improves
- Optimal altitude camp: 3-4 weeks at 6,000-8,000 feet
Training adjustments at altitude:
- Reduce intensity and volume initially
- Use heart rate or RPE rather than pace
- Expect 10-20% slower paces initially
- Hydrate more (increased fluid loss at altitude)
- Allow 7-10 days before resuming hard workouts
Live high, train low:
- Live at altitude (>6,000 feet) for red blood cell production
- Train at lower altitude for quality workouts
- Combines benefits of adaptation with ability to train hard
Racing at altitude
If you live at sea level and race at altitude:
- Arrive 1-2 days before race (before major altitude sickness sets in) or 2+ weeks before (to adapt)
- Reduce goal pace by 3-5% per 3,000 feet of elevation
- Hydrate aggressively
- Manage expectations
If you live at altitude and race at sea level:
- Potential performance boost from increased red blood cells
- Time race 2-3 weeks after descending for maximal benefit
- Some runners feel "heavy" or sluggish initially at sea level
Humidity and performance
Physiological impact of humidity
Impaired evaporative cooling:
- High humidity reduces sweat evaporation
- Core temperature rises faster
- Cardiovascular strain increases (similar to heat)
Dew point matters more than relative humidity:
- Dew point <50°F: Comfortable
- Dew point 50-60°F: Slightly uncomfortable
- Dew point 60-70°F: Very uncomfortable, significant performance impact
- Dew point >70°F: Oppressive, major performance decline
Strategies for high humidity
Adjust pacing:
- Expect 15-40 seconds/mile slower in high humidity
- Use RPE and heart rate rather than pace
Hydration:
- Even if not feeling thirsty, sweat rates are high
- Drink consistently throughout run
Clothing:
- Minimal, moisture-wicking clothing
- Light colors
Cooling:
- Pour water over head and body during long runs or races
- Seek shaded routes
Safety:
- Heat illness risk is high in hot, humid conditions
- Monitor for warning signs: dizziness, nausea, confusion
Wind and running performance
Impact of wind
Headwind:
- Increases energy cost significantly
- Pace slows by 5-20 seconds/mile depending on wind strength
Tailwind:
- Reduces energy cost
- Pace increases, but gains are smaller than headwind losses
Crosswind:
- Increases instability and effort
- Alters mechanics slightly
Strategies for windy conditions
Pacing:
- Run by effort, not pace
- Accept slower paces into headwinds
- Avoid surging too hard with tailwinds
Route planning:
- Start into the wind, return with the wind (avoid cooling when sweaty)
- Use buildings or trees as windbreaks when possible
Combining environmental stressors
Heat + humidity:
- Compounding effect, most challenging combination
- Reduce pace significantly, prioritize safety
Heat + altitude:
- Both increase cardiovascular strain
- Extra caution needed; adjust pace and hydrate aggressively
Cold + wind (wind chill):
- Dramatically increases cold exposure
- Layer appropriately, protect extremities
Practical guidelines for race-day conditions
Ideal marathon conditions
- Temperature: 40-55°F (4-13°C)
- Humidity: Low to moderate (dew point <50°F)
- Wind: Calm to light
- Cloud cover: Overcast (reduces sun exposure)
Adjusting race pace for heat
Temperature-based adjustments (starting at 60°F):
- 60-65°F: Slow by 1-3%
- 65-70°F: Slow by 3-5%
- 70-75°F: Slow by 5-8%
- 75-80°F: Slow by 8-12%
- >80°F: Slow by 12-20%+
Add 1-2% for high humidity (dew point >60°F)
Adjusting race pace for altitude
Pace adjustment per 3,000 feet elevation:
- 3,000 feet: Slow by 2-3%
- 6,000 feet: Slow by 5-7%
- 9,000 feet: Slow by 8-12%
Common misconceptions
"Training in heat makes you tougher, so don't adjust pace"
Reality: Training in heat without adjusting pace leads to excessive fatigue, poor recovery, and increased injury risk. Adjust pacing to maintain appropriate effort.
"Drinking cold water in heat causes cramps"
Reality: No evidence supports this. Cold water helps lower core temperature and is beneficial in heat.
"Altitude training benefits everyone"
Reality: Individual responses vary. Some runners adapt well; others struggle. Altitude camps are most beneficial for already well-trained athletes.
"You can't train effectively in extreme conditions"
Reality: With proper adjustments—pacing, hydration, clothing, safety—you can train effectively in varied conditions. Adaptation also provides resilience.
Summary
Environmental conditions significantly impact marathon training and performance. Heat increases cardiovascular strain, elevates core temperature, and reduces running economy; heat adaptation over 10-14 days improves thermoregulation and performance. Cold weather requires appropriate layering, extended warm-ups, and attention to extremities. Altitude reduces oxygen availability, slowing performance initially but potentially boosting red blood cell production with prolonged exposure. Humidity impairs evaporative cooling, compounding heat stress; dew point is a better predictor than relative humidity. Wind increases energy cost, especially headwinds. Runners should adjust pacing based on environmental conditions, prioritize hydration and safety, and allow for gradual adaptation when training or racing in extreme environments. Ideal marathon conditions include temperatures of 40-55°F, low humidity, and calm winds. Understanding and respecting environmental stressors allows safe, effective training and realistic race-day expectations.