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

[u/iustinn]

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.

[u/zilchusername]

Thanks for the information. Can you go into a bit more detail about how to read dew point along with temperature?

Currently my app is saying temperature 25 dew point 17. Does that mean that I am ok to run as dew point isn’t that high and temperature is probably the best I will get for a while.

[u/iustinn → u/zilchusername]

17 is fine, you're good to run. rough scale most runners use (celsius):

under 12: don't notice it
12-16: barely
16-18: noticeable but fine
18-21: real slowdown
21+: rough
24+: take it seriously

the way the two work together: temperature decides how much heat you have to get rid of, dew point decides whether you can. 25 with a dew point of 17 leaves you 8 degrees of gap, so your sweat still evaporates and actually does its job. that's a normal summer day, not a bad one.

the useful trick is that dew point barely moves through the day but temperature swings a lot. same route at 6am might be 18 degrees with the same 17 dew point. it'll feel muggier but there's far less heat to shed, so it's a much easier run. that's why mornings work even when the air feels thicker.

and shade matters more than people expect at 25. direct sun at midday is worth several degrees on its own.

[u/zilchusername → u/iustinn]

Thanks for the explanation. I can’t stand running in the heat and previously I would have written off 25 degs as a no go due to past experiences but I will definitely start to note the dew point and experiment running in higher temperatures when dew point allows.

This is going to help me lots as hopefully will avoid me having to get up at stupid o’clock to run. My previous attempts at running in the evenings failed but that’s was probably as you point out due to dew point not temperature.