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?
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 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.
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.
“Train smart, eat well, & enjoy the run!” — P. Mark Taylor
Check out these books by P. Mark Taylor for more advice on running: