Effects of Heat and Humidity on Running Pace

I ran a 5K last night at 9 pm.  For the previous three nights, it had been 100 degrees at 9 pm.  Thankfully, it had cooled off to a frosty 85 or so.  Still, that made it 15 degrees warmer than the race in which I set my recent PR back in April.

My PR pace was 6:11, but in last night’s race, I ran a 6:35 pace.  That got me to wondering about the impact of the heat and humidity.

How is running pace impacted by heat & humidity?

Heat

When you run strenuously in temperatures above 60°F, your core body temperature is raised.  Your body’s natural cooling system kicks in; some of the blood that would have been going to your running muscles gets diverted to the capillaries of your skin to cool it off.  That cooler blood then returns to your core and that brings your core temperature down.  What a great invention!  Unfortunately, that means less blood in your running muscles.  That means less oxygen.  It also means that your system of removing the waste byproducts from the muscles can get backed up as well.  Both of these result in a slower pace.

Temperature changes also alter the density of the air.  More dense, more oxygen with each intake of breath.  i.e. colder weather makes you faster.

Add it all up and you get:  cold weather facilitates a faster speed.  Heat slows you down.  This is one of the main reasons you see the majority of marathons in the Spring & Fall.

Humidity

Humidity also affects your core body temperature.  This is because sweating is one of the main functions of your body’s cooling system.  For sweat to do its job, it needs to evaporate and take some heat with it.  Unfortunately, high humidity means slower evaporation.  Slower evaporation of sweat means slower cooling.  That means that your body’s core temperature is not being lowered effectively.  Final result: high humidty leads to slower pace.

Conclusion

I have not found a formula that can tell us how much we will slow down, but you need to understand that you can’t run at your optimal pace in less than optimal conditions.  If you slow down a little in the heat & humidity, give yourself a break.  Your body is doing the right thing.

The good news is that even though your training might be slowed a great deal during the summer, your training will lead to MUCH faster times when things cool off again.

Have patience.  Forgive yourself for the slower pace.

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“Train smart, eat well, & enjoy the run!”  — P. Mark Taylor

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Check out these books by P. Mark Taylor for more advice on running:

The Gift of Running: A Book for Runners & Future Runners  Wise Running Book COVER mockup

&

Wise Running: Thoughts on Running and Life

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17 Comments.

  1. Running articles repeatedly mention that heat adds about 20 seconds per mile. Although, I’ve looked for and have never been able to find a reference for the statistic.

    • The estimate of 20 seconds per mile is pretty close to my experience. If I had pushed a tiny bit harder (and I could have), it would have been exactly right.

  2. Great write up on the heat and humidity! It’s actually been shown that training in the heat makes you race better when it’s cool out. There is one rough formula I know of that Schwartz made showing how much the heat will slow you down (based on the heat index) at a certain pace effort. Check it out here: http://206.214.217.133/articles/hic.pdf Based on this, it looks like your 6:35 effort at 85 degrees is roughly equivalent to 6:11 pace in the ideal 53-60 degrees. Enjoy!

  3. Excellent post. I shared it on my site: http://ddkurcfeld.com/blog/slow-down-its-hot, and on my FB page. Thanks so much!

  4. Excellent post. I shared it on my site: http://ddkurcfeld.com/blog/slow-down-its-hot, and on my FB page. Thanks so much!

  5. Don’t under estimate the mental factor of heat as well. If you spend all your time thinking about how damn hot it is it is going to be that much worse and you will beat yourself.

    I ran in a 4 mile race today, and while my performance was far worse than I wanted, worst time ever actually – people were dropping like flies around me. I am wiling to bet that I will have placed better than the previous year because of how unequipped other runners were.

    I read this article recently that had some good information as well, some good basic hydration information. http://runningtimes.com/Article.aspx?ArticleID=26451&PageNum=1

    Nice job on that pace in 85 degree temp!

  6. I live and train in flatland with heat and humidity and I’m thinking of trying a high altitude race in Colorado. Wondering if training in all this will be any help there.

    • It makes sense to me that it would make a difference ; the question is how much difference there will be in oxygen supply. Gotta factor that in…

      • Having done some trail runs in the last week or so at over 100 humid degrees, the oxygen supply seemed non-existent! It just seems that the difficulty of sucking in such humid air would give your lungs some kind of positive training for breathing in thinner air. But maybe they don’t relate. Guess I’ll find out!

  7. I’ve always known that heat/humidity have an effect on running performance, but never sat down to think about (or research) the reasons why they do. This clearly explains the effects of both !!!

    Congrats on a great race – despite the super hot temperatures!!

  8. I feel the near 100 degree temps we’ve been experiencing have not been letting me test my healing stress fracture because my body is shutting down before my leg is. Also, as I’m just out in the heat my leg has been getting achy, even if I haven’t really worked it very hard. I’m not sure how to deal with it.

    Thank you for such a timely & informative post.

  9. The Effects of Heat & Humidity on Running Pace | RunStJoe! - pingback on July 5, 2012 at 12:13 pm
  10. I went for two runs here in Utah last week and wanted to die on each run. I had to take walk breaks and such. I haven’t had to walk in a really long time especially on short runs. I blame the heat since each time it was over 100 degrees. My heart rate would shoot up as well starting out causing concern.

  11. This is very insightful information. As I live in the desert I run inthe AM, but even that can get hot quickly. Without having a gym membership can you suggest how to get a good workout run in that enables me to lose weight productively? Look forward to more articles. ~

  12. Linking this post to my blog where I complain a bit bitterly about running this summer in the heat. Thanks..

  13. Life’s Little updates… | Mizunogirl's Blog - pingback on July 21, 2013 at 3:45 pm

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