Pace Difference Between Running in Rain and Sunshine: Breaking Down the 5 Variables That Slow You Down 20 Seconds/km

Why Pace Slows in the Rain

Many runners observe that on the same route, with the same effort, they run 15–25 sec/km slower in the rain than on sunny days. This is not an illusion—it is the combined effect of five physical and physiological variables. This article breaks down each one and offers pace-adjustment advice for rainy days.

Variable 1: Reduced Shoe Traction

The friction coefficient of dry asphalt is about 0.7, but after rain it drops to 0.4–0.5. The impact on runners:

• Part of the push-off force is lost to “slipping”
• Ankle and knee stabilizer muscles need more control
• Stride length naturally shortens by 3–5%
• Pace loss: about 3–5 sec/km

Variable 2: Increased Clothing Weight

Clothing gains weight after absorbing water in the rain:

• Cotton T-shirt: 200g dry → 600g wet
• Shorts: 150g dry → 350g wet
• Socks: 50g dry → 150g wet

The extra load is about 700–900g, affecting pace by about 2–4 sec/km.

It is recommended to switch to quick-dry, moisture-wicking apparel, which absorbs less than one-third of what cotton does.

Variable 3: Reduced Visibility

Visibility drops in the rain, causing runners to:

• Subconsciously slow down to check the road surface
• Focus their gaze only 3–5 meters ahead, disrupting breathing rhythm
• Experience increased psychological tension, making muscles slightly stiff

Pace loss: about 2–3 sec/km.

Variable 4: Changes in Heat Dissipation

Many people assume rain means cooler weather and better performance, but that is not necessarily true:

Light Rain (18–22°C)

• Evaporative cooling efficiency decreases
• But it feels cool, and muscle temperature is moderate
• Pace is slightly faster by 2–3 sec/km

Cold Rain (10–15°C)

• Muscles get cold, power output drops
• More warm-up time is needed
• Pace slows by 5–10 sec/km

Heavy Rain (any temperature)

• Rain washing over the skin lowers core temperature
• Breathing becomes difficult (inhaling rain)
• Pace slows by 8–12 sec/km

Variable 5: Road Resistance and Obstacle Avoidance

Road conditions in the rain:

• Puddles: each step consumes 1–2% more energy
• Slippery roadside drain covers: require detours
• Leaves and debris accumulating on the road: obstacle avoidance increases stride variation
• Pace loss: about 3–5 sec/km

Sum of the Five Variables

Variable	Pace Impact
Reduced traction	-3–5 sec
Clothing weight	-2–4 sec
Visibility	-2–3 sec
Heat dissipation (neutral rain)	-1 to +3 sec
Road resistance	-3–5 sec
Total	-11–22 sec/km

In neutral rain, pace is about 15 sec/km slower; in cold rain or heavy rain, it can be 25 sec/km slower.

Pace Adjustment Recommendations for Rainy-Day Training

Easy Run

• Do not obsess over pace on the watch
• Maintain a perceived effort of Z2 (able to speak short sentences)
• It is normal to run about 20 sec/km slower than on sunny days

Threshold Run

• Use heart rate rather than pace as the benchmark
• Stay in the Z4 heart-rate zone (170–178 bpm)
• Accept that pace may shift from 4:50/km on sunny days to 5:05/km

Interval Run

• High-intensity intervals are not recommended in the rain
• Switch to a treadmill indoors or postpone by a day

Long Run (LSD)

• Maintain Z1–Z2 heart rate
• Keep the same hydration and fueling rhythm
• Focus on time rather than distance

Rainy-Day Gear Recommendations

• Quick-dry, moisture-wicking top (avoid cotton)
• Waterproof cap (to keep rain out of your eyes)
• Reflective vest or arm bands (for poor visibility)
• Waterproof phone pouch
• Non-slip running shoes (e.g., replace shoes with over 500K of mileage)

Should You Run in the Rain?

Decision logic:

• Neutral rain + moderate temperature: run as usual, treat it as mental toughness training
• Cold rain + strong wind: move indoors or rest
• Heavy rain + lightning: absolutely do not run—safety comes first

Conclusion

A slower pace in the rain is not your fault—it is a combined physical and physiological response. Understanding the magnitude of these five variables allows you to finish rainy-day training without anxiety or self-blame, using heart rate and time as the true indicators of training quality.

Related Reading

• The Complete Guide to Running in the Rain: Adjusting Shoes, Clothing, and Ground Friction
• Pace Adjustment in Hot Weather: How Many Seconds to Slow Down for Every 1°C Rise
• Temperature and Pace Adjustment: Recommended Pace Slowdown for Every 5°C Rise
• The Night-Running Pace Illusion: Why It Feels Faster but Is Actually Slower