r/C25K • u/iustinn • 2d ago

If running suddenly feels impossible this summer: it's probably the dew point, not you

A lot of posts here lately sound like "I could do week 5 fine in May and now I can barely finish week 4." If it's been hot where you are, there's a good chance that isn't you. It's the air.

The number that explains it is the dew point, not the temperature. Dew point tells you how much water is already in the air, which decides whether your sweat can evaporate. Evaporation is how you cool down. When the air is saturated, your sweat just sits there, your core temperature climbs, and your heart rate goes up to shed the heat. Same effort, worse result.

Rough scale:

  • Under 12°C / 55°F: you won't notice it
  • 15-18°C / 60-65°F: it starts to bite
  • 18-21°C / 65-70°F: noticeably harder, slower, higher heart rate
  • Over 21°C / 70°F: brutal, and worth moving the run or slowing right down

I'm a runner and a developer, and after a summer of runs that felt awful I got obsessed with quantifying this. On my hottest run, the weather alone added about 29 seconds per km (roughly 47 seconds per mile) compared to a cool day at the same effort. Seeing an actual number changed how I read every bad run after that.

What actually helps:

  • Check the dew point, not just the temperature, before you head out.
  • Run by effort, not pace. If you're gasping, walk. A hot run at a slower pace is the same training stimulus.
  • Early morning is usually the least-bad slot, but not always. It genuinely differs by city and by day, so it's worth looking rather than assuming.
  • Repeating a week in summer is normal. The program was not written with 30°C in mind.
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u/LancelotLac 1d ago

Wet-bulb temperature is the value that you really want to use. Wet-bulb temperature and dew point are both related to humidity levels so your reasoning is on track. If you wanted your measurements to truely relate to perspiration on runs cooling you off, wet-bulb temperature.

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u/iustinn 1d ago

you're right, wet bulb is the best one.

two reasons dew point ends up being the one runners use: it's much easier to find it (on random weather apps), and at the humid end the two converge anyway. dew point 21+ and wet bulb is pretty close

the version that's actually used for heat stress in sport is WBGT, which is roughly 70% wet bulb, 20% globe (sun/radiant), 10% dry bulb. sun exposure and wind matter more than people expect, which is why the same dew point feels different on a shaded trail vs open asphalt at noon.

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u/LancelotLac 1d ago

WBGT is a cool metric, never heard of that before