I checked the history and no one seems to have asked this particular question. I am a middle Georgia native and would consider myself acclimated to our unique form of summertime heat. I never really had a problem growing up because I was ALWAYS outside running, riding my bike, or playing whatever the seasonal sport was.
Now I'm 40 and have spent the last 20 years in the comfort of conditioned air and NO physical activity until this past Dec when I started training for the 2010 MCM. I finished the ING Ga 1/2 marathon in Feb in 2:29:38 when was exactly my goal (thanks again Mizuno pace team!).
My training plan says to adjust for heat and humidity by slowing 30 seconds every mile when the temp is over 60 degrees. Okay, I can do that. But...
When the relative humidty here in GA is 90%+ (and it is often), the air temp may only be 85, but it will feel like 102+.
Finally, here's my question:
Following the advice to slow down to prevent my ticker from stopping which would ruin my chances to complete my first marathon, do I use the heat index (my guess) or the actual air temp?
I over hydrate
I look to cut the milage back when the first hot weather hits.
I run earlier or later in the day
And finally, listen to the body. If the humidy it really taking it out of you on a particular day -- shut it down. Train hard but train smart.
After a while you'll get used to the humidity. Increase time and mileage as you feel comfortable