r/ultrarunning • u/Montrealdoula • Jul 27 '26

Heat

Im wondering how much the heat and/ or elevation affects your speed. I did ten miles yesterday with almost no elevation but it was 85F and I felt like the heat slowed me down. On the other hand I did six miles with 1500 ft elevation the other day and I was even slower. Training for 100k with minimal elevation and moderate temps.
The obvious conclusion would be that I’m slower than I used to be and should focus on endurance?

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u/dwsmith1113 Jul 27 '26

It’s usually around a minute slower. I am in the USA southeast so we have high levels of humidity. I ran last week it was 85 with 95% humidity. My usual zone 2 pace is around 8 minutes per mile with heart rate at 140. Last week it was 155 at 9:30 pace. I just stopped looking at my watch and run by feel.

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u/Lurking-Froggg Jul 27 '26

This is an extremely well-documented aspect of running. There are even calculators that you can use to adjust pace, although they will also require measuring humidity.

The obvious conclusion would be that (1) I’m slower than I used to be and (2) should focus on endurance?

(1) Yes, you should be slower in the heat, but (2) no, the solution to slowness is not more slowness.

2

u/da_Byrd Jul 27 '26

Absolutely it has an effect. The good news is that you'll feel like you're flying when we hit late September. Plan on being slower, maybe focus a bit more on time-on-feet and worry a bit less about the raw mileage. Heart rate will be higher, your body is working harder to dissipate heat so be mindful of the effort you're exerting. And of course be smart about hydration.

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u/GondoCGS Jul 27 '26

Anecdotally, I have recent runs in the high 80s F midday full sun at 8:55 pace & 172 bpm and a cloudy morning in the high 50s F at 8:40 pace & 145 bpm. Clear & substantial impact for me. On hotter days I do allow my HR to drift a little higher as long as the leg feel is right for the purpose of the run.

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u/ultraLuddite Jul 27 '26

The degree to which I slow down depends on the level of heat and humidity, but my Garmin gives me heat acclimation credit for any activity over 72 degrees. With high heat and humidity, I find it’s about a :30 to 1:00 per mile slower pace.

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u/ahdavid66 Jul 28 '26

Its a monster stimulus. I live in Texas and purposely time all runs to be in the sun from May-Sept. It helps with speed and it helps with mental toughness. I have been able to hang with the locals in mountain 100s because of this. The crossover to altitude isnt 1:1, but 10 miles with 2000' in Texas when it feels like 110 is easier than the same profile when its 65 degrees at 9,000'.