The heat index (HI) or heat stress index—sometimes called the apparent temperature or comfort index—is a temperature measure that takes into account the relative humidity. Based on human physiology and on clothing science, it measures how a given air temperature feels to the average person at a given relative humidity. The HI temperature is measured in the shade and assumes a wind speed of 5.6 mph (9 kph) and normal barometric pressure.

At low relative humidity, the HI is less than or equal to the air temperature. At higher relative humidity, the HI exceeds the air temperature. For example, according to the National Weather Service's (NWS) HI chart, if the air temperature is 70°F (21°C), the HI is 64°F (18°C) at 0% relative humidity and 72°F (22°C) at 100% relative humidity. At 95°F (35°C) and 55% relative humidity, the HI is 110°F (43°C). In very hot weather...