WHY DEW POINT IS THE MOST POWERFUL WEATHER METRIC FOR ENDURANCE ATHLETES
By Samantha Murphy
Coach Samantha Murphy is the founder of Evolve Coaching Systems, one of the largest and most successful coaching groups in the Midwest. Today, she explains why athletes should worry about the dew point as much as, or more than, the temperature.
We are very much in the swing of summer training and racing in the Northern Hemisphere, and with many places reaching record-breaking temps over the last few weeks, we wanted to take a moment to discuss dew point and how it impacts the endurance athlete.
If you've ever checked the forecast, seen a "reasonable" temperature, and then blown up on the run leg anyway, you might have experienced the single biggest blind spot in race-day weather prep. It turns out, you were reading the wrong number.
Let’s start with some science. Temperature tells you how hot the air is. Humidity tells you how saturated it is, but only as a percentage relative to that specific temperature, which makes it nearly useless for comparing one day to another. Dew point tells you something neither of those can: how much moisture is actually in the air, in absolute terms, and therefore how well your body can cool itself.
For endurance athletes, that distinction drives pacing, fueling, and cooling. Knowing the dew point is the difference between a controlled effort and a forced march to the finish line.
The Problem With Relative Humidity
Relative humidity is relative to air temperature. That's the catch. 50% humidity at 60°F and 50% humidity at 90°F represent wildly different amounts of actual water vapor in the air, but the percentage looks identical on your weather app. This is why "it's humid" doesn't tell you much on its own, and why two race mornings with the "same humidity" can feel completely different to your body.
Dew point strips that ambiguity away. It's an absolute measure: the temperature at which air becomes saturated, and water starts to condense. A higher dew point always means more moisture in the air, regardless of the surrounding temperature. That consistency is exactly what makes it so useful for athletes trying to predict how a session or a race will actually feel and what their fueling and pacing strategy needs to be to assure success.
Why Moisture in the Air Matters More Than Heat Itself
A 75°F day with a 70°F dew point can be more physiologically dangerous than a 90°F day with a 55°F dew point
Your primary cooling mechanism during exercise is evaporative: sweat leaves the skin, and that phase change from liquid to vapor pulls heat away from your body. This process depends entirely on a vapor pressure gradient between your skin and the surrounding air.
When the air is already saturated with moisture — a high dew point — that gradient collapses. Sweat has nowhere to go. It pools and drips instead of evaporating, which means it's doing you no cooling good at all. You can be soaked and still overheating. And this is compounded when on the run, due to less airflow compared to riding a bike.
This is why a 75°F day with a 70°F dew point can be more physiologically dangerous than a 90°F day with a 55°F dew point. The second day is hotter, but your sweat can actually evaporate. The first day looks mild and quietly cooks you.
A Practical Dew Point Scale for Training and Racing
Sport scientists and coaches generally use bands like these to translate dew point into real pacing decisions:
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Below 50°F — Little to no impact on performance. Cooling is efficient.
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50–60°F — Noticeable but manageable. Most athletes feel slightly "heavier" in effort.
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60–65°F — Becoming oppressive. Expect measurable pace and power decline.
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65–70°F — Significant impairment. Cardiovascular drift accelerates.
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Above 70°F — Extreme. Even elite, heat-acclimated athletes see major performance loss and elevated health risk.
These bands matter more than any single temperature threshold, because they account for the mechanism that's actually limiting you.
What Dew Point Means for Pacing
On high dew point days, race and train by effort or heart rate, not by goal pace or power
When dew point climbs, your heart rate will drift upward at a given pace even if your perceived effort feels unchanged. This is a direct result of your body diverting blood flow to the skin for cooling while still trying to supply working muscle. This is cardiovascular drift, and it's real, not a fitness or willpower failure. This will show up in your post-workout analysis, so take a look at how your pace holds or drops while your heart rate goes up. The fitter you are and the more acclimated you are, the less this phenomenon will take place.
The practical takeaway for this is that on high dew point days, race and train by effort or heart rate, not by goal pace or power. A pace that is sustainable in cool, dry conditions may simply not be available to you when the air is saturated. Adjusting expectations before the gun goes off or you step out the door, rather than fighting the data or feeling down over a workout, is what separates athletes who fade in the final miles or give up on workouts from those who manage the day.
The Bottom Line
Temperature answers: "How hot is it?" Humidity answers: "How saturated is the air, relative to today's heat?" Only dew point answers the question that actually determines your performance: "Can my body cool itself right now?"
The next time you're checking conditions before a long run, ride, or race, skip the temperature and humidity percentage. Look up the dew point. It's the single number that tells you the most about what your body is about to go through, and it's the one that should be driving your pacing strategy.
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