impact of heat and humidity on running

The problem with entering a race in October is the bulk of the training takes place during the summer months. If the weather regularly reaches over 30ºC and 90% humidity, it’s very unpleasant for being outdoors, let alone trying to run. I’m still not back at pre-holiday form, and add on the weather, recent outdoor runs have been poor:

  • 09-may: 9.5km 1.23.45hr 8.49min/km
  • 18-may: 10.02km 1.40.00hr 9.59min/km
  • 21-may: 5.01km 39.40min 7.55min/km
  • 25-may: 8.0km 1.16.45hr 9.36min/km *
  • 27-may: 6.36km 49.13min 7.44min/km

*wanted to do 10k, had to cut short because it was so hot, barely any shade.

It’s common sense, that heat and humidity affects performance. The explanation is fairly scientific too: when it’s hot, body temperature rises. Sweating is a mechanism for the body to cool down (thermodynamics: energy is required to convert liquid to gas, the energy in this case comes from body heat). When it’s humid, sweat can’t evaporate into the saturated atmosphere and stays on the body. Body temperature continues to rise causing heartrate to increase. The body compensates by diverting blood and oxygen from muscles to skin capillaries for cooling, and the body slows down.


The best condition for running is cold (around 10-12ºC) with a breeze and preferably with lots of shade or cloud cover. Running coach Jeff Galloway has a chart that correlates pace with temperature (I converted to ºC). It’s not scientific, but based on his and other runners’ experiences. It makes sense.


The folks at runontexas (via) has a table that shows the change with heat and humidity. This is in seconds per mile and assumes humid conditions means over 60%. It’s not hard to project that the effect when it’s over 90% will be even more pronounced.


As always, plotting data out in a chart makes it even easier to understand. The decrease in pace is exponential, once it gets to 30ºC and humid, expect to add 3 minutes per mile, that’s 2 mins per km.


Translating that to my own running. If I optimistically assume a base pace of 6.40min/km (10.45min/mi), then with current conditions, um, first of all it’s not recommended. If I did go running, then it’s a minimum of 8.32 pace. Since I’m nowhere near my base pace, I should expect to go even slower. This puts my recent runs into perspective, I haven’t been doing too badly on the shorter runs, need to build up endurance for longer runs. Wear a hat, find shade, hydrate, hydrate, hydrate. The latest run was decent, I stuck to shaded paths and drank about 1.5l of sports drink during the run, and more afterwards.

So this is all kinda depressing, but looking on the bright side, there is a silver lining:

when cooler weather returns you will be surprised at how you are in better condition and can run faster than you expect – often coinciding just in time for your goal fall race

I can only hope.