How Much Does Humidity Slow a Run?

How much does humidity slow a run? I can answer that question in two words: a lot.

The anecdotal, unscientific response to how much humidity slows running paces is about 30 to 45 seconds per mile, based on my own current runs. Here in Houston, my morning runs are in 80+ degree weather with 80+ percent humidity. In addition to slower paces, I can’t run as long – probably due to the combination of heat and humidity. Some days, I’m lucky if I can get eight miles.

But I don’t expect you to take my word for it. Here’s what heat and humidity does to slow your running pace.

How Heat and Humidity Slow a Run

Humidity Traps Sweat

Normally, when you run, your body sweats. The sweat evaporates, and you’re able to keep going. It’s your very own personal cooling system. But the humidity traps sweat on your body, so it can’t evaporate.

As a result, your body has to work harder to cool off. Your heart rate rises along with the temperature, by up to 10 beats per minute in humidity levels between 60 and 90 percent. This makes even your easy runs feel like threshold pace runs.

How Much Humidity Slows a Run

Some calculations show that the heat and humidity can slow your running pace by 12 to 15 percent, when the temperature climbs to 80 degrees. For someone who normally runs an 8-minute mile in perfect (cool, low humidity) conditions, that can mean a 9:06 mile.

But this is just a rule of thumb. During the summer, I see a lot of runners beat themselves up over their paces (me included). And I understand how frustrating it is when you were crushing runs a few months ago and now feel like you’re slogging through mud.

Combating Heat and Humidity

Since we’re runners, we’re not going to stop running during the summer, right? Here’s what I do to keep the heat and humidity from becoming too much to bear.

  • I hydrate like crazy. I drink water before I run, bring a water bottle on every run, and drink water after I run. Sometimes I’ll bring an electrolyte replacement beverage instead of plain water on my run.
  • I try to run early in the morning. It’s a trade-off; it’s more humid in the morning, but it’s hotter in the afternoon. So I’ll go in the morning before the sun gets angry and hot.
  • I look for shady routes.
  • I try not to worry about my pace.

I’m not perfect at the last part, but I know that the paces I’m hitting now aren’t representative of what I’ll be hitting in January, when I hope to tackle the Chevron Houston Marathon again. (So far, there are no plans to cancel!)

Remember, winter PRs are forged in the summer heat. A slower pace now will help build up your cardiovascular system, and you’ll be in a much better position when temperatures drop to beat all your old records.

Just. Keep. Running.