Educational - Heat index
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The heat index is an index that combines air temperature and moisture to give an apparent temperature (how hot it "feels").
Hot, humid air "feels" hotter than hot, dry air because the extra moisture in the air slows the evaporation of sweat from our skin. Evaporation of water always produces a cooling effect, since the extra energy needed to convert water from liquid to gaseous water vapor is taken from the air, thus reducing the temperature of the air.