Why Humidity Is a Runner's Kryptonite
Running on a hot, humid day can feel like you're exercising in a sauna. The reason it’s so much harder than running in dry heat comes down to one word: evaporation. Your body’s primary cooling system is sweat. As sweat evaporates from your skin, it releases
heat and lowers your body temperature. However, when the air is already saturated with moisture — as it is in high humidity — your sweat has nowhere to go. It drips off you instead of evaporating, meaning you lose the cooling benefit. This inefficiency forces your body to work overtime. Your core temperature rises faster, and your cardiovascular system goes into overdrive, pumping more blood to the skin in a desperate attempt to cool down. This process, sometimes called cardiovascular drift, means your heart rate will be significantly higher for the same pace you’d run on a cool, dry day.
The Danger of Chasing Pace
For many runners, the numbers on their GPS watch are a key measure of progress. But in high humidity, your pace lies to you. Trying to force your body to hit a specific speed target when conditions are tough is not only demoralising but also counterproductive and potentially dangerous. Pushing for your normal pace in humid weather forces your body into a much higher intensity zone than you intend. What should be an easy aerobic run can quickly become a high-stress workout, accumulating excess fatigue without the intended fitness benefit. This increases your risk of dehydration and heat-related illnesses like heat exhaustion or heatstroke, which can have serious consequences. Symptoms like dizziness, nausea, and confusion are signs that you’re pushing too hard and need to stop immediately.
The Solution: Train by Effort
Instead of battling your watch, the smartest approach is to shift your focus from an external metric (pace) to an internal one: effort. Training by effort means you listen to your body and run based on how you feel, ensuring you get the intended physiological benefit from your workout, regardless of what the clock says. This approach allows you to maintain consistent training stress, building fitness safely even when the weather is working against you. There are two simple and effective ways to measure your effort: the Rate of Perceived Exertion (RPE) and heart rate monitoring.
Using the Rate of Perceived Exertion (RPE)
Rate of Perceived Exertion, or RPE, is a subjective scale of how hard you feel you're working, typically rated from 1 to 10. It’s a powerful tool because it’s adaptable and requires no technology. A rating of 1 is sitting on the couch, while 10 is an all-out sprint you can only hold for a few seconds. Most training runs can be guided by this scale. An easy, conversational recovery run should feel like a 3-4 on the RPE scale. A steady-state or tempo run, where you can speak a few words but not hold a full conversation, would be around a 6-7. Hard intervals might push you to an 8 or 9. On a humid day, you will naturally have to slow down to keep your run in the 'easy' 3-4 RPE zone, and that's exactly the point. You're matching your effort to your goal, not to an arbitrary pace.
Training with Heart Rate Zones
For those who prefer a more data-driven approach, a heart rate monitor is an excellent tool for managing effort in the heat. Your heart rate zones don't change just because it's humid outside; what changes is the pace you can hold while staying within a specific zone. For example, if your target for an easy run is to stay in Zone 2 (your aerobic zone), you simply adjust your speed to ensure your heart rate doesn't climb higher. On a humid day, this will mean running significantly slower. This prevents you from over-exerting yourself and turning an easy day into an unintentionally hard one. By keeping your heart rate in the correct zone, you guarantee you're getting the right training stimulus, even if your pace feels sluggish.
