Heat Index: A quantity expressing the discomfort felt as a result of the combined effects of the temperature and humidity of the air.
The heat index tells you how hot it feels at a given humidity. Moist air feels hotter than dry air because it makes sweating less efficient. On a hot, dry day, your sweat will evaporate quickly and cool your skin; under humid conditions, sweat evaporates more slowly and doesn’t do as much. ~ Nat’l Weather Service