Heat Index is the apparent temperature of exposed skin when exposed to air. Dry air allows evaporative cooling of the skin human body's surface. The higher the humidity, the less the skin can cool itself by evaporation.
This site intended to give the feeling of what is happening outside. The data is from a station standing out there - at the cold fronts and the heat waves. You can search in the archive for weather history or to ow when there was a very extensive weather (hot or cold).