There are many studies out there that generally recognized that for every 10-degree increase in air temperature above 55 degrees, there's a 1.5 percent to 3 percent increase in average finishing time for a marathon. For an Ironman athlete looking to run a 3:30 marathon that is 3-6 min. This is compounding even more in my eyes for in triathlon as a lot of us are starting the Marathon in a more dehydrated state and I believe it is more like 8-12 min. The slow down occurs because heat impacts athletes at a physiological level through a few different means, including dehydration, increased heart rate and reduced blood flow to the muscles used for running.
Perspiration has a cooling effect on the body because it removes excess heat through evaporation. The rate of evaporation–and subsequently how well the body is cooled–changes depending upon humidity. When humidity is low, evaporation increases and we have a great cooling affect when humidity is high, the rate of evaporation decreases and the cooling affect decreases.
WHAT TO DO?
1) ADJUST YOUR PREPARATION.
We can adjust our preparation to become more adjusted to the humid heat by wearing a cap or long sleeves while training
2) ADJUST YOUR EXPECTATIONS.
When you are heading out into a Hot environment adjust our expectations of the work out and be sure to build into the effort so you don't drive your core body temp up to start
3) ADJUST YOUR MINDSET.
Mind set is a huge thing. If you know it is going to be hot and you don't have the above steps, having a positive mindset to deal with things is huge.
If you think it is hot running go stand in a port-o-potty for 60 sec. upon exit it will feel cool! It is all relative :)
See you at the races!
Chris McDonald, 6x Ironman Champion