Heat Index for Runners Explained - Complete Safety Guide
Understand what heat index means for running. NWS danger categories explained, when to modify workouts, and how to stay safe running in hot weather.
What does heat index really mean for your running? This guide explains the heat index, how it's calculated, and what the different danger categories mean for your training.
Use this knowledge with our Running Heat Calculator to make smart decisions in hot weather.
What Is Heat Index?
Heat index, also called "apparent temperature" or "feels like" temperature, combines air temperature and relative humidity into a single number that represents how hot it actually feels to your body.
Why Humidity Matters
Your body cools itself primarily through sweating. When sweat evaporates from your skin, it carries heat away. But when humidity is high:
- Sweat evaporates slower or not at all
- Cooling efficiency drops dramatically
- Core temperature rises faster
- Performance suffers and health risk increases
A 30°C (86°F) day at 40% humidity is manageable. The same temperature at 90% humidity is dangerous.
The NWS Heat Index Equation
The National Weather Service uses the Rothfusz (1990) regression to calculate heat index. For temperatures above 26°C (80°F):
HI = -42.379 + 2.049T + 10.143RH - 0.225TR - 0.00684T² - 0.0548RH² + 0.00123T²RH + 0.000853TR² - 0.00000199T²R²
This polynomial accounts for the complex relationship between temperature, humidity, and perceived heat stress.
Heat Index Categories for Runners
Normal (Heat Index < 27°C / 80°F)
Risk Level: Low
Running Guidance:
- Normal training, all workout types
- Standard hydration practices
- Full intensity and duration
- No special precautions needed
What it feels like: Comfortable to warm. Sweating effective.
Caution (Heat Index 27-32°C / 80-90°F)
Risk Level: Moderate
Running Guidance:
- Expect 3-5% pace slowdown
- Increase hydration by 25-50%
- Monitor for early fatigue signs
- Still safe for most workouts
What it feels like: Noticeably hot. Heavy sweating. May feel sluggish.
Symptoms to watch: Unusual fatigue, elevated heart rate at normal pace.
Extreme Caution (Heat Index 32-39°C / 90-103°F)
Risk Level: High
Running Guidance:
- Reduce intensity by 10-20%
- Shorten workout duration
- Consider indoor alternatives
- Avoid prolonged continuous efforts
- Extra hydration and electrolytes essential
What it feels like: Oppressively hot. Sweating profuse but less effective. Difficult to cool down.
Symptoms to watch: Muscle cramps, headache, dizziness, nausea.
Heat cramps and heat exhaustion become likely with prolonged exposure.
Danger (Heat Index 39-51°C / 103-124°F)
Risk Level: Very High
Running Guidance:
- Consider postponing outdoor running
- If running: very easy effort only, short duration
- Early morning only (5-6 AM)
- Have a bail-out plan
- Run with a partner if possible
What it feels like: Dangerously hot. Sweating may be ineffective. Core temperature rises rapidly.
Symptoms to watch: All of the above plus confusion, cessation of sweating (emergency sign).
Heat exhaustion likely, heat stroke possible with extended exposure.
Extreme Danger (Heat Index > 51°C / 124°F)
Risk Level: Critical
Running Guidance:
- DO NOT run outdoors
- Use treadmill or cross-train indoors
- No exceptions—risk of heat stroke is extreme
Heat stroke highly likely even with short exposure.
Heat Index Chart
Quick reference showing heat index at common temperature/humidity combinations:
| Temperature | 40% RH | 60% RH | 80% RH |
|---|---|---|---|
| 24°C (75°F) | 24°C | 26°C | 28°C |
| 27°C (81°F) | 27°C | 30°C | 34°C |
| 30°C (86°F) | 31°C | 36°C | 42°C |
| 33°C (91°F) | 36°C | 43°C | 52°C |
| 36°C (97°F) | 43°C | 53°C | 63°C |
Notice how quickly heat index rises with humidity at higher temperatures.
Heat Index vs. WBGT
You may see WBGT (Wet Bulb Globe Temperature) used in sports science research. It's similar but more comprehensive:
| Measure | What It Includes | Use Case |
|---|---|---|
| Heat Index | Temperature + humidity | General public guidance |
| WBGT | Temperature + humidity + sun + wind | Athletic events, military, occupational |
WBGT is more accurate for outdoor athletic performance but requires special equipment. Heat index is a reasonable approximation available from any weather source.
How Heat Index Affects Performance
The performance impact by heat index category:
| Heat Index Range | Typical Slowdown | Marathon Impact |
|---|---|---|
| < 27°C | 0-2% | 0-5 minutes |
| 27-32°C | 3-5% | 6-12 minutes |
| 32-39°C | 6-10% | 13-25 minutes |
| 39-51°C | 10-15%+ | 25+ minutes |
Adjusting Workouts by Heat Index
Easy Runs
| Heat Index | Adjustment |
|---|---|
| < 27°C | Normal pace |
| 27-32°C | Slow by 15-20 sec/km |
| 32-39°C | Slow by 30-45 sec/km, consider shortening |
| > 39°C | Treadmill or postpone |
Tempo/Threshold Runs
| Heat Index | Adjustment |
|---|---|
| < 27°C | Normal pace |
| 27-32°C | Slow by 10-15 sec/km |
| 32-39°C | Convert to easy run or fartlek |
| > 39°C | Skip or move indoors |
Interval Sessions
| Heat Index | Adjustment |
|---|---|
| < 27°C | Normal |
| 27-32°C | Extend recovery between reps |
| 32-39°C | Reduce volume 25-50%, longer recovery |
| > 39°C | Skip or move indoors |
Long Runs
| Heat Index | Adjustment |
|---|---|
| < 27°C | Normal |
| 27-32°C | Start at dawn, carry extra fluids |
| 32-39°C | Shorten significantly or split into two sessions |
| > 39°C | Postpone |
Sun Exposure Adds to Heat Index
The official heat index assumes you're in shade. In direct sun:
- Add 3-4°C (5-8°F) to the heat index
- This can push a "Caution" day into "Extreme Caution"
- Wear light-colored, breathable clothing
- Consider routes with tree cover
Signs of Heat Illness
Heat Cramps
- Muscle cramps, usually in legs
- Heavy sweating
- Action: Stop, stretch, hydrate with electrolytes
Heat Exhaustion
- Heavy sweating OR pale skin
- Weakness, fatigue
- Nausea, headache
- Dizziness, confusion
- Action: Stop immediately, seek shade, cool down, hydrate
Heat Stroke (EMERGENCY)
- High body temperature (> 40°C / 104°F)
- Red, hot, DRY skin (sweating may have stopped)
- Rapid, strong pulse
- Confusion, agitation, slurred speech
- Loss of consciousness
- Action: Call emergency services, cool body immediately with water/ice
Strategies for Running in Heat
Before the Run
- Check heat index, not just temperature
- Run early morning (5-7 AM is coolest)
- Pre-hydrate with 500ml water 2 hours before
- Wear light, breathable clothing
- Plan routes with shade and water access
During the Run
- Start slower than normal
- Drink 150-250ml every 15-20 minutes
- Listen to your body—slow down if needed
- Take walk breaks if struggling
- Seek shade when possible
After the Run
- Cool down gradually
- Replace fluids (1.5x what you lost)
- Include electrolytes if sweating heavily
- Stay out of heat for at least 30 minutes
- Monitor for delayed symptoms
When to Cancel a Run
Consider cancelling outdoor running if:
- Heat index exceeds 39°C (103°F)
- You're not heat acclimated
- You feel any signs of illness
- You can't carry or access adequate fluids
- You're running alone in remote areas
Indoor Alternatives
When it's too hot outside:
- Treadmill: Adjust for reduced air resistance
- Gym running track: Climate controlled
- Pool running: Zero impact, cooling
- Cross-training: Cycling, elliptical, rowing
Related Resources
- Running Heat Calculator - Calculate your heat-adjusted pace
- Running in Heat Performance Chart - Temperature slowdown tables
- Heat Acclimation for Runners - How to adapt to heat
- Running Pace Calculator - Pace conversions
Conclusion
Heat index is the most practical measure for understanding heat stress during running. Understanding the categories and adjusting your training accordingly keeps you safe while still making progress through summer months.
When in doubt, run easier, run shorter, or run indoors. One missed outdoor run is better than heat stroke.
For personalized pace adjustments, try our Running Heat Calculator.