The heat index is a convenient way of expressing how hot we really feel when humidity is factored in.
Humidity matters because our body’s natural cooling process involves the evaporation of perspiration from our skin. High humidity slows the evaporation down which might lead to warmer body temperatures.
The heat index is calculated by knowing the temperature and the relative humidity. The chart above can then be used to arrive at a heat index value.
When using relative humidity, keep in mind that morning relative humidity is usually a lot higher then during the afternoon. Using a forecast high temperature and morning humidity will lead to some very large (and wrong) heat index values! Always use the current temperature and humidity when using the chart.