If you've ever checked the weather before a run and thought, "80 percent humidity? This is going to be brutal," you’re not alone. But what if I told you that humidity percentage isn't actually the best measure of how miserable the air will feel? If you're serious about understanding how weather affects your performance, it's time to shift your focus to dew point.
What’s the Difference Between Humidity and Dew Point?
Humidity percentage is a relative measure of how much moisture is in the air compared to the maximum amount the air can hold at a given temperature. The problem with relying on this number is that it changes based on temperature. A humidity reading of 80 percent on a cool morning will feel completely different from 80 percent on a hot afternoon. That’s because warm air can hold more moisture than cool air, so the same percentage can mean very different levels of actual water content in the air.
Dew point, on the other hand, is a fixed number. It tells us the temperature at which air becomes fully saturated and moisture starts to condense. The higher the dew point, the more moisture is in the air. Unlike humidity percentage, the dew point gives a more accurate picture of how humid and oppressive it actually feels outside.
Why Dew Point Matters More for Runners
When you run, your body cools itself through sweat. The problem? If the air is already holding a lot of moisture, that sweat doesn’t evaporate as efficiently, making it harder for your body to regulate temperature. This is why running in high dew point conditions can feel significantly harder, even if the actual humidity percentage isn’t off the charts.
Here’s a general guide to how different dew points feel:
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50°F or lower: Comfortable and great for running
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51-60°F: Manageable, but you may start to notice the humidity
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61-65°F: Getting uncomfortable, expect to feel sluggish
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66-70°F: Tough running conditions, increased risk of overheating
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70°F+: High risk of heat stress, running performance will take a hit
How to Adjust Your Runs for High Dew Points
If you see a high dew point in the forecast, adjust your expectations and plan accordingly. Hydrate well before, during, and after your run. Start slower than usual, listen to your body, and don’t be afraid to cut a run short if conditions feel overwhelming. If the dew point is pushing past 70°F, consider shifting your run to early morning or even taking it indoors.
The Bottom Line
Humidity percentage alone doesn’t tell you how oppressive the air will feel. Dew point is the number you should be paying attention to. The higher it climbs, the harder your body has to work to stay cool. If you want to set yourself up for success in training and racing, start checking the dew point before your runs. Your body will thank you.