Dew point vs humidity: why your weather app lies to runners

80% humidity means nothing

You check your weather app before a run. It says 80% relative humidity. Sounds miserable, right?

Maybe. Maybe not. 80% humidity at 8°C on a December morning feels crisp and pleasant. 80% humidity at 32°C in July feels like breathing through a wet towel. Same number, completely different experience.

This is the problem with relative humidity: it’s relative. It measures how much moisture the air holds compared to how much it could hold at that temperature. Cold air holds less moisture, so 80% of a small number is still a small number. Warm air holds a lot more, so 80% of a big number is a big deal.

Your weather app shows you the relative number. Your body feels the absolute one.

What dew point actually measures

Dew point is the temperature at which air becomes saturated and water starts condensing. Unlike relative humidity, it doesn’t swing when the air warms up or cools down during the day. If the moisture in the air stays the same, the dew point stays the same, whether it’s 22°C or 35°C outside.

For runners, this is way more useful than humidity.

A rough scale:

Below 10°C — dry. You won’t think about moisture.
10-15°C — comfortable. Sweat evaporates well.
15-18°C — getting sticky. You’ll notice it on long runs.
18-21°C — uncomfortable. Cooling system starts struggling, pace drops.
Above 21°C — oppressive. Heat illness risk climbs. Shorten the run or go earlier.

For context, most of Southeast Asia sits at 23-26°C dew point year-round. Houston and Miami regularly hit 22-24°C in summer. London rarely goes above 15°C.

Why your body cares about dew point, not humidity

When you run, your body dumps heat through sweat evaporation. That evaporation rate depends on how much moisture the surrounding air can still absorb. When the dew point is high, the air is already carrying a lot of water, so your sweat just sits on your skin instead of evaporating.

Your heart rate goes up because your body has to work harder to cool you down. Blood gets diverted from muscles to skin. Performance drops.

Ely et al. (2007) analyzed 36 years of marathon results in Medicine & Science in Sports & Exercise and found finishing times slowed progressively as WBGT rose above 10°C, with the effect consistent across all ability levels, including elites.

Relative humidity can’t tell you this because the same percentage maps to wildly different absolute moisture depending on temperature.

A real example

Let’s say you’re deciding between a morning run and an evening run on a summer day.

Morning, 6 AM: Temperature 22°C, relative humidity 83%, dew point 19°C. Evening, 7 PM: Temperature 30°C, relative humidity 53%, dew point 19°C.

Your weather app makes the morning look worse (83% vs 53% humidity). But the dew point is identical. Your body will struggle with moisture equally in both scenarios. The evening run is actually harder because you’re also dealing with 8 degrees more heat on top of the same moisture load.

Now compare the morning to a different day:

Morning A: Temperature 22°C, humidity 83%, dew point 19°C. Morning B: Temperature 22°C, humidity 55%, dew point 12°C.

Same temperature. The humidity numbers look different but don’t tell you the whole story. The dew point does: Morning B will feel dramatically better. Your sweat will evaporate, your heart rate will stay lower, and you’ll probably run faster without trying.

How we score it in GoWindow

In GoWindow’s scoring engine, we use dew point as the primary heat-moisture indicator for running and hiking. Relative humidity only comes in as a fallback when dew point data isn’t available, and as an input to our estimated WBGT calculation.

The thresholds come from exercise physiology research on heart rate drift and perceived exertion at different dew point ranges.

We deliberately left humidity out of the score. Adding it would double-count the moisture effect dew point already captures, and it would penalize cool, damp mornings that are actually great for running.

What to do with this

Next time you’re deciding when to run:

Find the dew point, not the humidity. Most weather apps bury it somewhere, or you can check weather.gov if you’re in the US.
If dew point is above 18°C, consider moving your run earlier or shortening it.
If you’re comparing two potential time slots, compare their dew points. The one with lower dew point will feel better, even if the temperature or humidity number looks similar.

Or just open GoWindow and tap “Running.” We already did the math.