A listener asked “How do you determine heat index”? The actual calculation is rather complex, but basically the heat index is a combination of the temperature and relative humidity. The higher the temperature and relative humidity, the higher the heat index. Because the human body cools itself through evaporation, factors that slow evaporation reduce the effectiveness of our natural cooling system. When temperatures are high, the body’s cooling system works harder to maintain a desirable temperature level. At 105 °F and 30 percent humidity, the heat index would be 114 °F; at 105 °F and 10 percent humidity (as might be seen in the desert Southwest) the heat index is 100 °F, cooler than the actual air temperature. Temperatures below 75 °F, don’t produce significant heat index values, and heat index values aren’t reported.
Mary Knapp, Weather Data Library