Running in the Heat: It's Not Just About Temperature

Why is Heat Dissipation So Important?

Running inevitably generates heat. Every time your muscles contract, they convert chemical energy from nutrients into movement. However, this conversion isn't perfect; only a portion of the energy becomes mechanical work, while the rest is lost as heat. The more intense the effort, the more heat is produced. If the body can't eliminate this heat, its temperature will continue to rise.

And This is Where Sweat Comes In?

Exactly. But here's something that surprises many: sweat itself doesn't cool you down; its evaporation does. When sweat evaporates, it absorbs a significant amount of energy from your skin in the form of latent heat. This process is what allows our bodies to maintain a temperature compatible with performance. If, however, sweat stays on your skin or drips down your body, the cooling effect is much less effective.

So Sweating a Lot Doesn't Necessarily Mean You're Cooling Down Effectively?

That's right.
Many people return home completely drenched and think their body worked perfectly.
In reality, the opposite might have happened. They could have lost a lot of fluids and sodium without adequate cooling. The amount of sweat produced isn't the key metric; it's the amount of sweat that actually manages to evaporate.

This is Where Air Humidity Comes into Play.

Precisely. Air can only hold a certain amount of water vapor. When humidity is low, sweat evaporates easily. When humidity is high, the air is already rich in vapor, and evaporation slows down. The result is that you continue to sweat, but your primary cooling system becomes progressively less effective.

And This is Why Some Days Feel Much Harder Than Others, Even with Lower Temperatures?

Exactly. Consider two scenarios.
One day with 36°C and 25% humidity.
Another day with 30°C and 85% humidity.
Many would unhesitatingly choose the second option. However, it might actually be the more challenging one. In the first case, sweat evaporates easily; in the second, it struggles to evaporate, and the body dissipates less heat.
For a runner, how well they can cool down matters much more than the number displayed on the thermometer.

Is This Where the Famous Wet-Bulb Temperature Comes In?

Yes. The wet-bulb temperature is a parameter that combines temperature and humidity into a single value. Essentially, it tells us how effective cooling through evaporation is. The higher the wet-bulb temperature, the less our body can use sweat to dissipate heat. For this reason, it's a much more useful indicator than simple temperature when assessing thermal stress during a run.

What Happens to the Body When Cooling Becomes Difficult?

The body tries to compensate.
It sends an increasing amount of blood to the skin to aid heat dissipation.
Consequently, the amount of blood available for the muscles decreases.
To maintain adequate blood flow, the heart progressively increases its heart rate. This is why, on very hot and humid days, you often observe a higher heart rate even when running at the same pace. It doesn't necessarily mean you're out of shape; it's a normal physiological response to thermal stress.

There's Another Element Often Underestimated: Wind. How Important Is It?

Much more than people think. Many believe wind cools the body directly. In reality, its main effect is different.
A very thin layer of highly humid air constantly forms near the skin, produced by sweat evaporation.
If that air remains still, it quickly becomes saturated, and evaporation slows down. Wind continuously removes this layer and replaces it with relatively drier air.
This way, it facilitates new sweat evaporation and makes cooling much more efficient. This is why two workouts with the same temperature and humidity can feel completely different simply because one takes place with no wind and the other with a light breeze.

If You Had to Give One Piece of Advice to Runners Training in the Summer, What Would It Be?

Never fixate on the number of degrees.
The right question isn't: 'How hot is it?'
The right question is: 'Will my body really be able to dissipate the heat I will produce during my run?'
Because the answer to that question determines performance, fatigue, and, most importantly, safety.