Charles Wax, head of the geosciences department at Mississippi State University, says people sometimes misunderstand the definitions of frequently used heat terms like humidity, heat index and heat lightning. Humidity affects how comfortable the air temperature feels. The moister the air, the hotter it feels. When humidity is high, perspiration can't evaporate from the skin, and the temperature feels much hotter. Perspiration is the body's natural way of regulating its temperature.
Wax defines heat index as the combination of temperature and humidity.
The heat index measures the apparent temperature, which is how the average person perceives temperature. As humidity increases, the apparent temperature rises as well. For instance, if the temperature of the air is 90 degrees and the relative humidity rises to 70 percent, the apparent temperature would be 106 degrees. However, if the humidity is low, the apparent temperature can feel cooler than the actual temperature. Although many people believe the flashes of light often seen on clear summer nights are caused by excessive heat, Wax says there is no such thing.