This time of year, a frequent feature of weather reports is the “heat index.” The heat index is a combination of the temperature and relative humidity. It is an attempt to indicate how we will perceive the outdoor environment. 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 cooling system works harder to maintain a desirable temperature level. High humidity slows evaporation making that even harder. That’s why you often hear the phrase “It’s not the heat; it’s the humidity”. 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 it the southwestern KS) the heat index is 100 °F, cooler than the actual air temperature.
Mary Knapp, Weather Data Library