I seem to recall reading some research that equated a <1% performance impact for a experienced runner to a ~3% performance impact for every 5*C over 10*C. So yes, there can be a huge performance impact. I think the negative performance impacts come from a variety of sources: greater stress / RPE, calories burned to cool body, dehydration, etc...

Edit
Some ways to handle:
1. Run in the heat more
2. Start out with cool body temp
3. More water! I remember reading dumping water on you can be a more efficient way to cool your body than drinking it (as long as you are already sufficiently hydrated)
4. Light colored and light weight clothing
5. Adjust pace expectations - use RPE

I live in GA and it is very hot in the summer here 90+ degrees F. I try to stay out of the heat for longer runs as I tend to overheat. Some other things that I do are:

• Run early in the morning even though the humidity is still 90+% as you mentioned
• Run a slower pace than normal, hard but I make myself when it is really hot
• Favorite thing is to do a trail run with tree coverage so I can minimize the direct sunlight
• Have a cold compress over my neck area and a cold cap to keep my head cooler if I am running in the blazing sun
• When I do bricks I will ride outside and then run inside on my treadmill
• Definitely slow down if you are feeling overheated and walk if you have to keep your HR down
• Keep up the hydration as the previous person mentioned

I used to do a hill climb race in Hawaii that started off at sea level and climbed up one of the volcanos. Very hot and humid at the start, but got cooler as you climbed. After overheating a couple times, I found that starting with ice cubes in my jersey pockets worked out great. It helps keep your core temperature down and I could stick a couple cubes in the vents of my helmet as well. There was a bit of a weight penalty, but by the time I had climbed up to slightly cooler temperatures, the ice had all melted for the last 6000 feet of climbing.

Just remeber it takes 80 calories to melt a gram of ice, but it only takes 1calorie to raise the temperature of that gram each additional degree.

All of my weekday runs happen at lunch. I've pretty much given up on running outside during the middle of the day during the summer here in Houston. High temps from mid May through Sept are above 32C, with the humidity starting around 90% in the morning and dropping to 60% in the afternoon. Any runs longer than about 15-20 minutes leave my brains feeling like scrambled eggs. Embrace the treadmill.

Acclimate, get out in the heat and do some training.

---

-Of course it's 'effing hard, it's IRONMAN!
Team ZOOT
ZOOT, QR, Garmin, HED Wheels, Zealios, FormSwim, Precision Hydration, Rudy Project

Yeah the first hot run is always a disaster. After a few weeks I'm usually acclimated. Slower in the heat for sure, but not a total disaster.

I'm a small, relatively hairless mammal, so the heat doesn't affect me too badly

That being said, If it's a Saturday afternoon in August and you go for a run around noontime ... If buzzards follow you as you go past the cornfield ... You might want to back it off, just a little bit

Trust me on this

---

"What's your claim?" - Ben Gravy
"Your best work is the work you're excited about" - Rick Rubin

I find a big difference between running in the heat... and running in the heat and mid day sun.

Covering up (hat, sun glasses, no bare shoulders if I can help it) helps

Keep soaking the hat in ice cold water from your bottle if you can. A few drops in the hat now and then is heavenly.

Slow down. Work up to performing hard in the heat. Nothing worse then overdoing it and having that groggy, sick feeling and pounding headache the next day.

---

Long Chile was a silly place.

I wrote this article (2 links, sorry) nearly 11 years ago now, but it still applies. The article focuses on cycling, running is even more impacted by temperature due to the lower speeds (and thus lower airflow over your body).

http://jbvcoaching.com/documents/60VN16.pdf
http://jbvcoaching.com/documents/61VN16.1.pdf

I agree with Bryan. Living in the hot desert, we can't do too much to avoid heat so most of our strategies are to learn to embrace it. There is a learning curve on how to throttle back your expectations when it gets warmer as mentioned by others. Also, learning to hydrate appropriately is important. One strategy I find very helpful is weighing myself pre and post workout after EVERY session (no matter if indoors or out). Doing this continually helps you to learn your direct losses for that particular workout, its conditions AND the natural variations your particular body endures for different modes and intensities for exercise. As an example, I've learned my personal tolerance to dehydration for both the bike and run, which of course have their own unique hydration challenges. I recall one summer's long bike ride, despite drinking as much as I possibly drink and carry, I lost 8lbs. Needless to say, I wasn't feeling the best after this, nor was I on my deathbed and could still manage a small run afterwards. Also knowing this loss helps to inform me on how actively I need to rehydrate to have a good shot of quality training the next session. Typically as it warms up in the desert, I'll see my weight tick up a few pounds as my plasma volume increases to compensate for the heat. It's a mistake to interpret this as weight that should be lost. Anyway, there are a few more things to think about.

I recall Lionel Sanders remarking that his heat training protocols before Kona really help him learn to embrace the feeling of his core temperature rising, but he did not plan for the feeling of the sun hitting the skin. That is another thing that anyone living in the desert gets used to, not just the baking feeling, but the radiant heat feeling of the sun hitting your skin. I can totally understand that if you are not used to that feeling on your skin, your brain would go into freak out mode not knowing how to handle this. I believe it is just a perception you have to get used to.

Russ Brandt wrote:

I recall Lionel Sanders remarking that his heat training protocols before Kona really help him learn to embrace the feeling of his core temperature rising, but he did not plan for the feeling of the sun hitting the skin. That is another thing that anyone living in the desert gets used to, not just the baking feeling, but the radiant heat feeling of the sun hitting your skin. I can totally understand that if you are not used to that feeling on your skin, your brain would go into freak out mode not knowing how to handle this. I believe it is just a perception you have to get used to.

I think there's also a good case for keeping the sun off your skin as much as possible, within reason. For example, I switched away from using a certain model of cycling jersey last year because the entire back was mesh and you felt the sun the same as you did on bare skin. Tradeoff was ventilation, but in general I think it was an improvement. I don't go as far as wearing arm coolers when I race, but I have experimented with them in training.

http://www.slowtwitch.com/...erformance_5870.html

You can try some arm coolers too. They help,. One reason is they keep the sun off the skin. There was an article I read last year where some testing was done, and they found that performance dropped greatly with the higher the sun index, even at the exact same temperature and humidity.

Makes sense! I LOVE the heat! I can run and bike in it, no problem. I swear the air must be thinner or something, because I cruise through that hot summer air! However, if I'm under direct sun, it drains me quick! completely different story under direct sunlight! Arm coolers and ice help. the first few runs in heat will suck, but after that, I'm loving it! (just keep to the shade as much as possible!)

I'll reciprocate and agree with what Russ said. We both train in pretty extreme conditions. Regarding arm coolers they are good when wet but if you need to keep soaking them in the bike you run out of water quickly. Better is to get a natural tan. In your A race you may sunburn a bit but having a tan already really helps.

---

-Of course it's 'effing hard, it's IRONMAN!
Team ZOOT
ZOOT, QR, Garmin, HED Wheels, Zealios, FormSwim, Precision Hydration, Rudy Project