Forget the Humidity Percentage — Watch the Dew Point Instead
- Liz Lichtenberger
- 2 days ago
- 4 min read
You've probably done this before: check the weather app before a run, see "65% humidity," and think, that's not so bad. Then you're a mile in, drenched, and gasping like you're running through soup. What happened?
The problem isn't that the forecast was wrong. It's that you were looking at the wrong number. Relative humidity is a percentage that depends on temperature, and it shifts throughout the day even when the actual moisture in the air hasn't changed at all. The number that actually predicts how your run will feel is the dew point. Here's why it matters, and how to use it to run smarter this summer.
Why Humid Air Makes Running So Much Harder
Running generates heat, and your body's main defense against overheating is sweat — but only when that sweat evaporates. Evaporation depends on a simple gradient: the surrounding air must be drier than your skin so that moisture can move from your body into the atmosphere. When the air is already close to saturated, that gradient collapses, and sweat just sits on your skin instead of pulling heat away with it.
This isn't a minor inconvenience. A 2025 study in the Scandinavian Journal of Medicine & Science in Sports found that the efficiency of sweat-based cooling drops dramatically as humidity rises — from around 50% efficient in dry conditions down to just 16% in very humid conditions, a two-thirds reduction in the body's ability to cool itself [1]. In that same research, power output fell 15% between low- and high-humidity conditions at the same air temperature. Your core temperature climbs faster, your heart works harder for the same pace, and eventually your nervous system forces you to slow down to protect you from overheating.
Why Dew Point Beats Relative Humidity
Relative humidity tells you how close the air is to being fully saturated relative to how much moisture it could hold at that particular temperature — and since warm air can hold more water vapor than cool air, that percentage swings around all day even if the actual amount of moisture stays fixed. An early-morning run might show 80% humidity at 65°F; by noon, the same air mass could read 45% at 85°F, despite containing exactly the same amount of water vapor.
Dew point sidesteps that confusion entirely. It's the temperature at which the air would become fully saturated. A 65°F dew point means the same thing at 6 a.m. as it does at noon, regardless of what the thermometer says. Because it tracks actual moisture content directly, dew point is a much more reliable stand-in for how hard it will be for your sweat to evaporate — and how hard your run will feel. You'll usually find it in the hourly or "feels like" section of most weather apps.
How Much Does It Actually Slow You Down?
More than most runners assume, and the effect scales with how long you're out there. Research examining results from seven major marathons over several decades found that as wet-bulb globe temperature — a heat-stress measure that factors in humidity — rose to its highest levels, even elite men finished about 4.5% slower than their personal bests, with mid-pack and back-of-pack runners slowing considerably more [2].
That's the elite end of the spectrum. Everyday runners tend to feel it more, largely because spending more time on the course means more time generating heat. A separate analysis of nearly 1.8 million marathon finishers across six major races confirmed that temperature is the single strongest environmental predictor of finishing time, and that the relationship isn't linear — the jump from 70°F to 90°F costs runners far more than the jump from 50°F to 70°F [3]. Practically speaking, a four-hour marathoner can lose 15–25 minutes on a high-dew-point race day, even while pacing perfectly.
A Rough Guide for Adjusting Your Effort
Use this as a general framework, not gospel — individual heat tolerance varies:
Dew Point | Effect |
Below 55°F | 🥰Little to no impact |
55–60°F | 🙂Easy runs feel normal; hard efforts a bit tougher |
60–65°F | 🤨Noticeably harder, especially at speed |
65–70°F | 😟Both easy and hard runs require real pace adjustment |
70–75°F | 😰Run by effort, not pace |
Above 75°F | 🥵Consider postponing hard sessions or moving indoors |
The core idea: below a dew point around 55°F, you can largely run your normal paces. Once you cross the mid-60s, both your easy days and your workouts need to be adjusted. Past 70°F, throw the pace chart out the window and run by feel — heart rate and perceived effort become your best guides.
Tips for Running Smart This Summer
Run by effort, not by GPS splits. On a humid day, hitting your usual pace numbers may mean pushing your body into an unsustainable thermal load. If your heart rate and effort match the intended workout, you're doing it right, even if the watch says you're slower.
Check the hourly forecast, not just the morning one. Dew point can actually be highest overnight and early morning, as moisture settles near the ground, and drop as the day goes on. A later start might occasionally be the smarter call.
Back off hard workouts during humid stretches. The cardiovascular toll of humid running adds up even on easy days. If you're in the middle of a sticky week, dropping to one quality session instead of two can preserve fitness without digging a fatigue hole.
Use the humidity to your advantage. Consistently training in heat and humidity drives real physiological adaptations — greater blood plasma volume, earlier sweat onset, and more efficient sweat composition — that pay off when temperatures drop for fall racing season.
The Bottom Line
Relative humidity is the number that's easiest to see, but dew point is the number that actually matters. Next time you check the forecast before a club run, glance past the humidity percentage and look for the dew point instead. It won't make a muggy morning feel any better, but it will help you set honest expectations, protect your effort, and know when to simply run by feel rather than chase pace.
Stay cool out there, and see you at the next group run.
References
Scandinavian Journal of Medicine & Science in Sports (2025). Study on sweating efficiency and power output across humidity levels. pubmed.ncbi.nlm.nih.gov/40107869
Analysis of marathon performance and wet-bulb globe temperature across seven major marathons. pubmed.ncbi.nlm.nih.gov/17473775
Analysis of environmental temperature and marathon finishing times across nearly 1.8 million finishers. pubmed.ncbi.nlm.nih.gov/22649525
Gaudette, J. How to Adjust Your Pace in Hot and Humid Weather. RunnersConnect. runnersconnect.net/dew-point-effect-running
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