We here on the D.A.R.T. blog will never claim any sort of scientific knowledge about anything that involves more than counting our fingers and toes. However, we have been perplexed enough to wonder why, especially in the southern climes, it seems that running feels hotter than the air temperature indicates.
The short answer would be "it's not the heat, it's the humidity", but that's not all. For example, right now as this post is being written it's 87 degrees with 56% humidity. Obviously that's pretty warm for running, but maybe not horrible, due to the somewhat low humidity. However, running right now would really zap a person. That's reflected in another metric, the dew point.
The dew point is the temperature to which a given parcel of air must be cooled, at constant barometric pressure, for water vapor to condense into water. The condensed water is called dew. The dew point is a saturation point." (source: Wikipedia). The higher the dew point, the more uncomfortable it feels outside. That's because the high moisture content in the air interferes with one's sweat evaporation. Namely, when you sweat under high dew point conditions, the skin doesn't get cooled off adequately and makes you more susceptible to overheating.
So if the dew point today is 70 degrees, how does that feel? Generally speaking, a dew point 60-69 is perceived by most as somewhat uncomfortable, 70-74 as very uncomfortable, and above 75 as being extremely uncomfortable. So today, even though the humidity is only 56%, the dew point makes it feel pretty bad outside.
The next time you start to head out for a run, make sure to check the dew point in addition to the temperature and humidity. Just remember that the higher the dew point, the hotter you'll feel. If you start feeling bad, then stop. No use getting heatstroke.