Hello from Mosquito Beachwear,
I was curious and looked up the wikipedia meaning for Heat Index:
The heat index (HI) is an index that combines air temperature and relative humidity in an attempt to determine the human-perceived equivalent temperature — how hot it feels, termed the felt air temperature. The human body normally cools itself by perspiration, or sweating, which evaporates and carries heat away from the body. However, when the relative humidity is high, the evaporation rate is reduced, so heat is removed from the body at a lower rate causing it to retain more heat than it would in dry air. Measurements have been taken based on subjective descriptions of how hot subjects feel for a given temperature and humidity, allowing an index to be made which relates one temperature and humidity combination to another at a higher temperature in drier air. More info than you probably wanted to know, ha!
Anyway, here is another link from ABC News regarding the Intense Heat with 17 states issuing heat warnings, watches & advisories!
Stay Cool and Have a Great Day!!!