Thermometers don't measure heat index values, just temperatures. The heat index is the temperature it "feels like" b/c the moisture in the air makes it difficult for your sweat to evaporate. Normally you sweat and when your sweat evaporates it cools you (it carries heat away from your body)... but since there is too much moisture already in the air the sweat can't evaporate as easily and it makes it "feel" like it's even hotter than the actual temperature.
On a side note, it poured at my place near the Medical Center yesterday... it was so great to see rain.... I ran around outside like a little kid... then ran back inside when I almost got struck by lightning.