Running in Humidity: Dew Point vs Relative Humidity Explained

Why do some humid days feel unbearable while others are tolerable? The answer lies in understanding dew point versus relative humidity. This guide explains why dew point is the better metric for runners and how to use it effectively.

Use our Running Humidity Calculator for personalized pace adjustments.

The Problem with Relative Humidity

Relative humidity (RH) is what most weather apps show prominently, but it's misleading for runners because it's temperature-dependent.

The Same RH, Completely Different Conditions

Scenario	Temperature	RH	Dew Point	How It Feels
Cool morning	15°C (59°F)	80%	12°C (54°F)	Comfortable
Hot afternoon	30°C (86°F)	80%	26°C (79°F)	Oppressive

Both have 80% relative humidity, but the hot day has nearly twice as much actual moisture in the air. That's why RH alone doesn't tell the whole story.

Why RH Changes Throughout the Day

Relative humidity is calculated as:

RH = (actual water vapor) / (maximum possible at that temperature) × 100%

As temperature rises during the day, the "maximum possible" increases, so RH drops even if actual moisture stays the same. This is why:

• Morning: Cool + high RH (80-90%)
• Afternoon: Hot + lower RH (40-60%)
• Evening: Cooling + rising RH again

But the actual moisture content may not change at all!

What Is Dew Point?

Dew point is the temperature at which air becomes saturated and dew forms. It directly measures the absolute amount of moisture in the air.

Key Properties of Dew Point

1. Temperature-independent: Unlike RH, dew point doesn't change with temperature
2. Directly measures moisture: Higher dew point = more moisture
3. Predicts "mugginess": The higher the dew point, the more oppressive it feels
4. Predicts sweat evaporation: Higher dew point = harder for sweat to evaporate

Dew Point Comfort Scale

Dew Point	Condition	For Running
< 50°F (10°C)	Dry	Perfect conditions
50-55°F (10-13°C)	Comfortable	Excellent
55-60°F (13-16°C)	Pleasant	Good, minimal impact
60-65°F (16-18°C)	Slightly humid	Noticeable, ~2% slowdown
65-70°F (18-21°C)	Humid	Sticky, ~4% slowdown
70-75°F (21-24°C)	Very humid	Difficult, ~6-7% slowdown
75-80°F (24-27°C)	Oppressive	Challenging, consider alternatives
> 80°F (27°C)	Extreme	Not recommended for long runs

The Science: Why Humidity Hurts Performance

How Your Body Cools During Running

Your body generates significant heat while running—up to 20 times your resting metabolic rate during hard efforts. You cool primarily through:

1. Sweat evaporation (most important, ~80% of cooling)
2. Convection (air movement across skin)
3. Radiation (heat radiating from body)

When Sweat Can't Evaporate

Sweat evaporates when moisture moves from your skin (high water concentration) into the air (lower water concentration). When dew point is high:

• Air is already saturated with moisture
• Sweat beads on skin instead of evaporating
• Cooling efficiency plummets

This creates a cascade of problems:

1. Core temperature rises faster
2. Heart rate increases at the same pace
3. Blood flow shifts to skin for cooling
4. Less blood available for working muscles
5. Sustainable pace decreases

Research Evidence

Maughan et al. found that in warm environments, increasing relative humidity significantly reduced time to exhaustion in prolonged exercise—even when participants were otherwise well-prepared.

Jenkins et al. reported approximately 3.4% impairment in cycling time trial performance when humidity was elevated, independent of temperature changes.

Temperature Gating: When Humidity Matters

Here's a critical insight: humidity only significantly affects running when it's warm.

Why Cool + Humid Is Okay

At cool temperatures (below ~18°C / 65°F):

• Your body produces less heat at the same pace
• Non-evaporative cooling (convection, radiation) is more effective
• You have more "thermal headroom" before overheating

Research Confirmation

Large marathon datasets (El Helou, Mantzios) show:

• Temperature is the primary driver of performance changes
• Humidity effects only emerge above ~18°C
• Below that threshold, even very high RH has minimal impact

Practical Implication

A cool, foggy morning with 95% RH and 12°C dew point? You'll run fine. A warm afternoon with 60% RH but 22°C dew point? Expect significant slowdown.

How to Find Dew Point

Weather Apps

Most weather apps show dew point if you know where to look:

iPhone Weather App:

1. Open Weather
2. Scroll down past hourly forecast
3. Look in "Humidity" section—dew point is often listed there

Weather.com:

1. View current conditions
2. Click "Today's Details" or similar
3. Find "Dew Point" in the expanded view

AccuWeather:

1. Current weather page
2. Look under "Current Weather" details

Running Apps:

• Garmin Connect often shows dew point in activity weather
• Some Strava premium features include it

Calculate from Temperature and RH

If your app doesn't show dew point, you can estimate it:

Temp	50% RH	60% RH	70% RH	80% RH
20°C	9°C	12°C	14°C	16°C
25°C	14°C	17°C	19°C	21°C
30°C	18°C	21°C	24°C	26°C

Or use our Running Humidity Calculator which handles the conversion automatically.

Adjusting Training for Humidity

Easy Runs

• DP < 60°F: Run normally
• DP 60-70°F: Accept slightly slower pace, stay hydrated
• DP > 70°F: Reduce duration or intensity

Tempo Runs

• DP < 60°F: Normal pacing
• DP 60-70°F: Slow target by 5-10 sec/km
• DP > 70°F: Convert to aerobic run or split into segments

Long Runs

• DP < 60°F: Normal distance and pace
• DP 60-70°F: Start early, carry extra fluids, accept slowdown
• DP > 70°F: Shorten significantly or move to treadmill

Intervals

• DP < 65°F: Normal workout
• DP 65-75°F: Extend recovery between reps
• DP > 75°F: Reduce total volume, longer recoveries

Race Day Strategy

One Week Out

Check the forecast for race morning:

• What will the dew point be at start time?
• Is it rising or falling throughout the race?
• Plan your pacing strategy accordingly

Morning Of

1. Check actual conditions, not just the forecast
2. Know your dew point threshold for goal adjustment
3. Have a backup pace plan if conditions are worse than expected

During the Race

• Start conservative in high humidity
• Hydrate aggressively with electrolytes
• Listen to your body—slow down if struggling
• Accept the conditions—a slower finish is better than a DNF

Humidity Acclimation

Does It Help?

Yes! Heat and humidity acclimation overlaps significantly. Adaptations include:

• Earlier and more efficient sweating
• Better cardiovascular response to thermal stress
• Lower core temperature at the same effort
• Psychological tolerance for discomfort

How to Acclimate

1. Run in humid conditions 4-6 times per week for 10-14 days
2. Start with easy efforts, 30-45 minutes
3. Progressively add duration and intensity
4. Stay very well hydrated throughout the process

Expected Benefits

After 10-14 days of proper acclimation:

• 20-30% reduction in humidity-related slowdown
• Better perceived comfort
• Lower heart rate at same effort

See our Heat Acclimation Guide for a complete protocol.

Common Questions

"Should I trust my watch's 'feels like' temperature?"

It depends. Some watches use heat index (combines temperature and RH), which is better than temperature alone but not as accurate as dew point for predicting running performance.

"Is morning or evening better for humid runs?"

Morning is usually better because:

• Temperature is lower → less total heat stress
• Dew point is roughly the same (it doesn't change much)
• Lower temperature + same moisture = more tolerable

"Do fans help during treadmill runs in humidity?"

Yes! Fans increase convection cooling and help with sweat evaporation by moving air across your skin. Position a fan directly facing you.

"Is indoor running better in high humidity?"

Only if the indoor space is climate-controlled. An un-air-conditioned gym can be worse than outside due to still air and no convection.

Related Resources

• Running Humidity Calculator - Personalized pace adjustments
• Dew Point Running Chart - Complete slowdown tables
• Humidity and Marathon Performance - Race research
• Running Heat Calculator - Combined temperature effects
• Heat Acclimation Guide - Adaptation protocol

Conclusion

Dew point is the single best metric for predicting how humidity will affect your running. Unlike relative humidity, it directly measures moisture content and accurately predicts how "muggy" conditions will feel.

Key takeaways:

1. Check dew point, not just RH
2. Humidity only matters when it's warm (above ~18°C / 65°F)
3. Adjust expectations and paces based on dew point ranges
4. Acclimate if you'll be racing in humidity

For personalized pace adjustments, try our Running Humidity Calculator.