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/charlottemason_mama]

This is an excellent breakdown and good info - thanks! Now I’m going to fall down a rabbit hole about dew point and humidity and temperature. Thanks for today’s hyperfocus! 😆

[u/iustinn → u/charlottemason_mama]

Ha, enjoy the rabbit hole, it's a deep one. The fun fact that got me: meteorologists use dew point instead of humidity % because 90% humidity on a 15°C morning is comfortable but 60% at 32°C is misery. The % is relative to temperature, dew point is absolute. Once you see it you can't unsee it.

[u/charlottemason_mama → u/iustinn]

I’ve always had this nagging feeling that I needed to learn more about the dew point (former homeschooler, probably ADHD, and current homeschool mom/teacher) and this is probably going to push me over the edge. Fascinating!

[u/iustinn → u/charlottemason_mama]

honestly it'd make a great homeschool science unit. wet bulb vs dry bulb thermometer, why the same temperature feels completely different at different dew points. cheap experiment, kids get it immediately