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 Heat index, like wind chill, is a way to combine multiple factors, in this case temperature and humidity, to get a single number indicating what the air "feels like."  {{w|Heat index|This page}} gives a table, a formula, and lots more explanation.
-Human skin does not directly detect temperature - only the rate of heat gain or loss.  This is why a piece of metal feels cooler than a piece of plastic or wood at the same exact temperature (below body temperature, above it is the other way around) - the metal conducts heat away from the higher body temperature at a higher rate than a good insulator does.
+Human skin does not directly detect temperature - only the rate of heat gain or loss. If you put pieces of metal, wood, and plastic at the same room temperature (cooler than the human body) the piece of metal feels cooler to the touch than a piece of plastic or wood - the metal conducts heat away from the higher body temperature at a higher rate than a good insulator does.
 So in warm weather, it's not just the temperature that matters for comfort.  The humidity and wind speed also factor into it.  When humidity is high, sweat evaporation is less effective at cooling us off than in a "dry heat" with low humidity.

2026: Heat Index - Revision history