See below for links to other science updates
Marathon performance and temperature
First - you need to understand what "WBGT" is. WBGT stands for Wet Bulb Globe Temperature and it is a method of evaluating environmental conditions that takes into account temperature, solar radiation (from the sun), humidity and wind speed. In other words it's a more accurate representation of how hot it feels, rather than just considering ONE of those factors like temperature or humidity.
In their article "Impact of Weather on Marathon Performance" published in the journal Medicine and Science in Sports and Exercise, author Matthew Ely and his co-authors examine marathon performances across a spectrum from a WBGT of 5.1 degrees Celsius (about 41 degrees F) to 25 degrees C (77 degrees F).
- 41 - 50 degrees F (5-10 degrees C)
- 50.1 - 59 degrees F (10.1 - 15 degrees C)
- 59.1 - 68 degrees F (15.1 - 20 degrees C)
- 68.1 - 77 degrees F (20.1 - 25 degrees C)
bottom line - wbgt does affect performance and it's not pretty! Two take aways:
- The higher the WBGT, the more effect it has on performance
- The slower the runner, the bigger the impact the WBGT has on performance.
To put that into perspective, the top runners (for example a 2:10 finisher) might see thier average pace per mile slow by only about 2.7 seconds per mile for each 5 degrees C increase in WBGT. This means their "ideal" of 2:10 in the lowest WBGT quartile slows to about a 2:14 finish in a WBGT of 77 degrees F (25C). On the other end of the spectrum, the 300th place runner (the 3:00 marathoner) could anticipate slowing by about 13.2 seconds a mile for each 5 degrees C increase in WBGT. That means their "ideal" 3:00 finish in the lowest WBGT quartile slows to a 3:23 in the highest WBGT quartile.
Although they didn't project out to higher temperatures or slower athletes - if you just assume that the performance of the 4:00, 4:30 and 5:00 marathoners is at least as impacted as it was for the 3:00 marathoner, you can use the same multiplier to come up with a reasonable estimate of performance in various temperatures. For Example:
- 4:00 marathon = 240 minutes. WBGT on race day is 68 degrees F (20 C). There's a 3.2% hit for each category level above 5C so multiply 3.2% by 3 (you're three catgories warmer) to get 9.6%. Then it's simple math: 240 minutes X 1.096 = 263 minutes
- 263 minutes equates to 4:23
Let's say your athlete has previously turned in a 3:00 marathon in cool temperatures (40 degrees F). Today's race day dawns with a WBGT temperature of 60 degrees F (15 C). Based on this research, it's reasonable to anticipate they'll be about 6% slower than they would be at around 40F. If you do the math for your 3:00 marathoner, you'll find it changes the anticipated finish time to about a 3:10. Pace per mile should be adjusted from the "planned" 6:52 to a more realistic 7:17. Failure to take the environmental conditions into account will likely come with a penalty later in the race!
Although not a perfect substitute for WBGT, you can use heat index as a starting reference. The National Weather Service heat index calculator can be found HERE and further explanation of WBGT can be found HERE
Medicine & Science in Sports & Exercise: March 2007 - Volume 39 - Issue 3 - pp 487-493.