How Humidity Affects Marathon Performance

How much does humidity really affect your marathon? This guide examines the research on humidity and marathon performance, drawing from major race datasets and lab studies.

Use our Running Humidity Calculator to get personalized pace adjustments.

The Research: Large-Scale Marathon Data

El Helou Study (2012)

Analyzed 1.8 million marathon performances across major races:

Optimal temperature: 3.8-9.9°C (39-50°F) for elite runners
Temperature was primary: Each 1°C increase above optimal = ~0.3% slowdown
Humidity modulated effects: High humidity amplified temperature effects above 18°C

Mantzios et al. (2022)

Analyzed 7,867 athletes across 1,258 races:

Optimal WBGT: 7.5-15°C (wet bulb globe temperature)
Slowdown: ~0.3-0.4% per 1°C WBGT outside optimal band
Humidity significant when temperature exceeded ~18°C (65°F)
Recreational runners: More affected than elites

Berlin Marathon Analysis

One of the most consistent big-city marathons, Berlin's data shows:

Ideal conditions: 8-12°C, low humidity
Warm + humid years: Average times 3-5% slower
DNF rates: Significantly higher in humid conditions

Lab Research on Humidity

Maughan et al. (Prolonged Exercise)

Protocol: Fixed warm temperature, varying humidity
Finding: Higher RH significantly reduced time to exhaustion
Mechanism: Impaired evaporative cooling raised core temperature

Jenkins et al. (Time Trial Performance)

Protocol: Cycling time trial, controlled temperature, elevated humidity
Finding: ~3.4% impairment when humidity was elevated
Key insight: Effect was independent of temperature changes

The Sweat Evaporation Mechanism

Laboratory work confirms:

Humidity impairs sweat evaporation
Core temperature rises faster
Heart rate increases at the same work output
Critical velocity (sustainable pace) decreases
Time to exhaustion shortens

Why Marathons Are Especially Affected

Duration Effect

Humidity effects compound over time:

30-minute run: Moderate impact
2-hour tempo: Significant impact
3-4 hour marathon: Maximum impact

The longer you're out there, the more cumulative heat stress builds.

Continuous Effort

Unlike interval training, marathons offer no recovery:

No rest periods for cooling
Sustained metabolic heat production
Progressive dehydration compounds issues
Glycogen depletion may be accelerated

Pacing Challenges

In humidity, traditional pacing strategies fail:

First half may feel fine despite dangerous conditions
Second half collapse when thermoregulatory system overwhelmed
Heart rate drift makes effort-based pacing unreliable

Dew Point Impact on Marathon Times

Based on research and race data:

Dew Point	Expected Impact	3:30 goal becomes	4:00 goal becomes
< 55°F (13°C)	None	3:30	4:00
55-60°F (13-16°C)	+1-2%	3:32-3:34	4:02-4:05
60-65°F (16-18°C)	+2-4%	3:34-3:38	4:05-4:10
65-70°F (18-21°C)	+4-6%	3:38-3:43	4:10-4:14
70-75°F (21-24°C)	+6-10%	3:43-3:51	4:14-4:24
75°F+ (24°C+)	+12%+	3:55+	4:29+

Case Studies: Famous Humid Marathons

Boston 2012 (Record Heat)

Conditions: 29°C (84°F), high humidity
Impact: Slowest Boston in 20 years
DNF rate: 2,124 runners (vs ~200 in normal years)
Medical tents: Overwhelmed

Chicago 2007 (Cancelled Mid-Race)

Conditions: 31°C (88°F), high humidity
Decision: Race shortened/cancelled at 3.5 hours
Death: One runner died from heat stroke
Lesson: Sometimes conditions are too dangerous

Houston Marathon (Consistently Variable)

January race: Humidity varies significantly year to year
Best years: Cool and dry, fast times
Worst years: Warm and humid, mass slowdowns
Strategy: Houston runners need weather backup plans

Strategies for Humid Marathon Racing

Before the Race

Monitor dew point, not just temperature
Have multiple goal paces based on conditions
Acclimate if possible (10-14 days minimum)
Pre-cool (ice vest, cold towels) if available

During the Race

Start conservatively—more conservative than you think
Monitor heart rate in addition to pace
Hydrate aggressively with electrolytes
Use cooling opportunities (water stations, sponges)
Accept the conditions—adjust expectations in real-time

Pacing Strategy

Negative split is unlikely in humid conditions. Better approach:

First half: Slower than goal pace by 10-15 sec/km
Second half: Try to hold steady (will feel progressively harder)
Final 10K: Survival mode—just finish

Hydration in Humidity

Humidity increases fluid needs:

Early and often: Don't wait until thirsty
Electrolytes matter: Sodium losses are high
Can't drink enough: You'll still lose net fluid; accept this
Watch for hyponatremia: Over-drinking is also dangerous

When to Adjust Your Goal (or DNS)

Yellow Zone: Dew Point 65-70°F

Adjust goal: Add 3-5 minutes
Plan B ready: Have a backup goal
Race smart: Very conservative start

Orange Zone: Dew Point 70-75°F

Adjust goal: Add 8-15 minutes
Consider: Is this worth risking injury/illness?
Alternative goal: Finish safely, no PR attempt

Red Zone: Dew Point 75°F+

Serious consideration: Should you start?
If racing: Purely for finish, not time
Monitor yourself: Be ready to drop out
DNF is okay: Health > one race

Elite vs Recreational Differences

Research shows recreational runners are more affected by humidity than elites:

Factor	Elite	Recreational
Running economy	Higher	Lower
Heat dissipation	More efficient	Less efficient
Pace sensitivity	Smaller slowdown	Larger slowdown
Finish time	2-2.5 hours	3-5 hours
Duration of exposure	Less	More

Implication: If you're a 4-hour marathoner, you'll be affected more than a 2:30 marathoner in the same conditions.

Historical Marathon Weather Data

Major marathons and typical humidity:

Marathon	Typical Month	Humidity Risk
Tokyo	February	Low-Moderate
Boston	April	Variable
London	April	Low
Berlin	September	Low
Chicago	October	Variable
New York	November	Low
Houston	January	Variable
Honolulu	December	High

Training for Humid Races

If racing in expected humidity:

10-14 Days Before Race

Train in heat/humidity 4-6 times per week
30-60 minutes per session initially
Build to longer efforts as adaptation develops
Stay very well hydrated

Expected Adaptations

Earlier sweating onset
More efficient sweating
Lower heart rate at same effort
Better psychological tolerance
~20-30% reduction in performance penalty

See our Heat Acclimation Guide for detailed protocol.

The Mental Game

Racing in humidity requires mental adjustment:

Accept slower times: This is not a failure
Redefine success: Finishing safely is an achievement
Stay present: Don't obsess over splits
Be ready to adapt: Rigid plans fail in variable conditions
Celebrate completion: Any finish in tough conditions is meaningful

Running Humidity Calculator - Personalized pace adjustments
Dew Point Running Chart - Complete slowdown tables
Dew Point vs Relative Humidity - Why dew point matters
Running Heat Calculator - Combined effects
Heat Acclimation Guide - Adaptation protocol

Conclusion

Humidity significantly impacts marathon performance, with effects becoming pronounced above 65°F (18°C) dew point. Research shows:

4-6% slowdown at moderate humidity (65-70°F dew point)
10-15%+ slowdown at high humidity (75°F+ dew point)
Longer races and slower runners are more affected
Acclimation helps but doesn't eliminate the effect

The key to racing in humidity is realistic goal adjustment, conservative pacing, aggressive hydration, and willingness to adapt to conditions.

For personalized calculations, use our Running Humidity Calculator.