Running in Heat: Performance Impact Chart - Complete Temperature Guide

How much does heat actually slow you down? This comprehensive guide provides research-backed charts showing the performance impact of temperature and humidity on running.

Use these tables alongside our Running Heat Calculator to get personalized pace adjustments.

The Science Behind Heat and Running

The data in these charts comes from two major research sources:

• Mantzios et al. (2022): Analyzed 7,867 athletes across 1,258 races
• Running Writings Model: 3,891 marathon performances across 754 races

Both studies found consistent patterns: optimal performance occurs between 2-13°C (35-55°F), with predictable slowdowns at higher temperatures.

Heat Performance Impact Chart (Temperature Only)

This chart shows approximate pace slowdown at various temperatures, assuming moderate humidity (50-60% RH).

Temperature	Heat Index	Speed Reduction	5:00/km becomes	Marathon Impact
10°C (50°F)	Optimal	0%	5:00/km	Reference
15°C (59°F)	Near optimal	1.5-2%	5:05/km	+3-4 min
18°C (64°F)	Comfortable	2.5-3%	5:08/km	+5-6 min
21°C (70°F)	Warm	3.5-4%	5:12/km	+7-9 min
24°C (75°F)	Hot	4.5-5%	5:15/km	+10-12 min
27°C (81°F)	Very hot	5.5-6.5%	5:20/km	+13-16 min
30°C (86°F)	Extreme	7-8%	5:24/km	+17-20 min
33°C (91°F)	Dangerous	8-10%	5:30/km	+21-26 min
36°C (97°F)	Extreme danger	10-12%	5:36/km	+27-33 min

Humidity Impact Chart

At temperatures above 18°C (64°F), humidity significantly amplifies heat effects.

At 27°C (81°F)

Relative Humidity	Feels Like	Additional Slowdown	Total Speed Reduction
30%	Dry heat	+0%	5.5%
50%	Moderate	+1%	6.5%
70%	Muggy	+2%	7.5%
85%	Very humid	+3%	8.5%
95%+	Tropical	+4%	9.5%

At 32°C (90°F)

Relative Humidity	Feels Like	Additional Slowdown	Total Speed Reduction
30%	Dry heat	+0%	7.5%
50%	Moderate	+1.5%	9%
70%	Muggy	+3%	10.5%
85%	Very humid	+4%	11.5%

Combined Temperature-Humidity Impact Table

This table shows total performance impact based on both factors:

Temp	40% RH	60% RH	80% RH
20°C (68°F)	3%	3.5%	4%
24°C (75°F)	4.5%	5%	6%
28°C (82°F)	6%	7%	8.5%
32°C (90°F)	7.5%	9%	11%
36°C (97°F)	10%	12%	14%+

Optimal Racing Conditions

Based on research data, here are the ideal conditions for race performance:

Race Distance	Optimal Temp Range	Notes
5K	5-12°C (41-54°F)	Shorter duration, can tolerate slightly cooler
10K	7-13°C (45-55°F)	Still benefits from cool conditions
Half Marathon	8-14°C (46-57°F)	Duration increases heat sensitivity
Marathon	2-13°C (36-55°F)	Longest duration, most heat sensitive
Ultra (50K+)	5-15°C (41-59°F)	Benefits from slightly cooler for extended effort

Marathon Time Impact Chart

How heat affects marathon finish times at different fitness levels:

Temp	3:00 goal	3:30 goal	4:00 goal	4:30 goal
10°C	3:00	3:30	4:00	4:30
18°C	3:05	3:36	4:07	4:38
24°C	3:09	3:41	4:12	4:44
30°C	3:14	3:47	4:19	4:51
33°C	3:18	3:52	4:25	4:58

Workout Type Adjustment Factors

Heat effects vary by workout type due to duration and intensity:

Workout Type	Heat Impact	Example at 28°C
Marathon race	100% effect	7% slower
Long tempo (60-90 min)	90% effect	6.3% slower
Easy/recovery run	80% effect	5.6% slower
Interval session	50% effect	3.5% slower

The reduced impact for intervals comes from recovery periods allowing cooling between efforts.

Heat Acclimation Impact

How acclimation status affects the performance penalty:

Acclimation Level	Heat Impact Modifier	At 28°C
Not acclimated (new to heat)	+20% worse	8.4% slower
Partially acclimated (1-2 weeks)	Reference	7% slower
Well acclimated (2+ weeks)	-30% better	4.9% slower

This shows why heat training before a summer race is valuable.

NWS Heat Index Categories for Runners

Heat Index	Category	Running Recommendation
<27°C (<80°F)	Normal	Full training with normal hydration
27-32°C (80-90°F)	Caution	Stay hydrated, expect 3-5% slowdown
32-39°C (90-103°F)	Extreme Caution	Reduce intensity 10-20%, shorten duration
39-51°C (103-124°F)	Danger	Consider indoor alternatives
>51°C (>124°F)	Extreme Danger	Do not run outdoors

Practical Application: Training Pace Chart

How to adjust training paces for heat (based on cool weather target):

Easy Run Pace Adjustments

Heat Index	6:00/km becomes	7:00/km becomes
Optimal	6:00/km	7:00/km
27°C	6:15/km	7:18/km
32°C	6:25/km	7:30/km
36°C	6:40/km	7:47/km

Tempo/Threshold Pace Adjustments

Heat Index	4:30/km becomes	5:00/km becomes
Optimal	4:30/km	5:00/km
27°C	4:45/km	5:15/km
32°C	4:52/km	5:25/km

Key Takeaways

1. Optimal marathon temperature: 2-13°C (36-55°F)
2. Slowdown rate: ~0.35% per degree Celsius above optimal
3. Humidity matters above 18°C: High humidity can double the heat effect
4. Heat acclimation helps: 2+ weeks of heat training reduces impact by 30%
5. Workout type matters: Intervals are less affected than continuous runs

How to Use These Charts

1. Check current conditions (temperature and humidity)
2. Find your row in the relevant chart
3. Apply the adjustment to your planned pace
4. Consider acclimation if you've been training in heat
5. Adjust for workout type (less impact for intervals)

For personalized calculations, use our Running Heat Calculator.

Related Resources

• Running Heat Calculator - Personalized heat-adjusted pace
• Heat Index for Runners Explained - Understanding heat index categories
• Heat Acclimation for Runners - How to adapt to hot weather
• Running Pace Calculator - Convert pace and speed
• Jack Daniels Running Calculator - Training pace zones

Conclusion

Heat significantly impacts running performance, with effects starting around 15°C and becoming severe above 30°C. Understanding these relationships helps you set realistic expectations, adjust training appropriately, and avoid dangerous conditions.

For exact pace adjustments based on your specific conditions, try our Running Heat Calculator.