Posted on July 15th, 2026

 

 

Rising temperatures force your body to divert blood flow from your working muscles to the skin surface for cooling purposes.

 

This physiological shift reduces the amount of oxygen available for your stride and naturally increases your heart rate at any given speed.

 

We see runners struggle with these changes every year and knowledge the mechanics of heat stress helps you maintain progress without burning out.

 

The Science of Heat and Your Cardiovascular System

Your heart works harder during summer months because it manages two competing demands simultaneously. It must pump oxygen-rich blood to your legs to maintain your pace while also pushing blood toward your skin to release heat. This dual workload means your cardiovascular system reaches its limit much sooner than it does during a crisp autumn morning.

 

Core temperature regulation becomes the priority for your central nervous system as the sun beats down. When your internal heat climbs, your brain sends signals to slow your muscle contractions to prevent heatstroke or exhaustion. You feel this as a heavy sensation in your limbs and a sudden inability to hold your usual tempo paces.

 

Plasma volume also drops as you sweat to stay cool during a long session. Thicker blood is harder for your heart to move through your veins and arteries. We find that runners who ignore these biological signals often face prolonged recovery times or avoidable injuries during July and August.

 

Three Ways Humidity Makes Your Hardest Runs Feel Tougher

High humidity levels prevent your sweat from evaporating which is the primary method your body uses to cool itself. When the air is saturated with moisture, your perspiration stays on your skin rather than turning into vapour. This failure in the cooling process causes your core temperature to spike rapidly even at low intensities.

 

Your perceived exertion climbs significantly when the air feels thick and heavy. A pace that feels like a five out of ten in the winter can easily feel like an eight or nine in humid conditions. This disconnect between your watch and your effort levels creates mental fatigue and frustration for many athletes.

1. Evaporative cooling stops working efficiently when humidity exceeds sixty percent.
2. Heart rates climb by ten to twenty beats per minute compared to dry conditions.
3. Dehydration risks increase as you sweat more to compensate for the lack of cooling.

 

Breathing often feels more difficult because humid air is less dense than dry air. You might feel like you are gasping for air during a standard interval session. These factors combine to make every mile feel longer and more taxing on your nervous system.

 

Adjusting Your Expectations for Better Summer Training

Successful summer training requires a shift in focus from specific split times to effort-based running. If you try to force your winter paces during a heatwave, you risk overtraining and heat exhaustion. We encourage our runners to use a heart rate monitor or a scale of perceived exertion rather than relying on GPS pace data.

 

Running during the coolest parts of the day helps you maintain some intensity without the worst of the heat stress. Early morning sessions offer the lowest temperatures even if the humidity is occasionally higher than in the evening. Shifting your schedule allows you to hit your key workouts while the pavement is still cool.

"The heat is a physiological stressor that provides similar benefits to altitude training if you manage your intensity correctly."

 

Hydration strategies must extend well beyond the duration of your actual run. You need to consume electrolytes to replace the salts lost through heavy perspiration. Proper mineral balance supports muscle function and prevents the cramping that often plagues summer training cycles.

 

Start Run with Will's Online Coaching Program

Stop fighting the weather and start training smarter with a plan built for your environment.

 

Our expert coaches help you interpret your data so you know when to push and when to back off.

 

Reach your fitness goals with professional online coaching from Run with Will to keep your progress on track regardless of the weather.

 

Experience the difference that a structured and adaptable training programme makes to your performance.