From Fellrnr.com, Running tips
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This chart gives an idea of what the perceived temperature is while running.
Normal heat indexes assume a person walking outdoors at 3 mph and generating 180 watts per square meter of skin. The original work states "higher levels of activity would increase the sensitivity to humidity… few people, however, sustain activity above the level considered here for long enough to reach a steady state". This is not very helpful to runners that are moving far quicker than 3 mph and generating a lot more heat. Ideally, I'd go back to the original perceived temperature work, recreate the mathematical models and plug in different values for the heat generation for runners. Sadly, I've not found an easy way of doing that. Instead, I have taken a simple approach based on the idea that in cold weather, the perceived temperature is about 20f warmer than the actual temperature. The chart below is based on the standard perceived temperature calculation, but with 10f added to the actual temperature. The chart below is a first pass at what running at a given temperature and humidity would feel like to someone walking. So running at 76f and 80% humidity would feel like walking in 100f temperatures.