The hot and sticky days of summer are here. Make sure that you are making some adjustments in your running. Most runners begin to slow down at 55 degrees and start suffering at 65 degrees. Of course, the body can adapt to heat stress and push the threshold up a bit, but you usually can't run as fast on a 75 degee day as on a 45 degree one. High humidity is also a major problem. It's like a wet blanket; it doesn't allow much evaporation or perspiration and your body heat builds up.
If you try to run too hard in hot or humid conditions you'll hit "the wall" sooner than expected. Trying to maintain a goal pace in heat is like going out too fast early in the race. Temperatures generally increase hour by hour; therefore you must adjust your pace for the temperature expected at the end of the race.
Above 85 degrees - Forget it... run for fun
* Note: This chart is based upon my own experience in the heat and talking to other runners. It has no scientific verification.