The heat index (HI) is a measure that uses air temperature and relative humidity to determine how warm an average person feels (an apparent temperature) as a consequence of moisture in the air, when compared to the actual temperature measured by a thermometer at the same time and location. Generally speaking, the higher the relative humidity, the warmer the temperature will seem to be to a person. The reason for this relationship is that the human body normally loses heat through the process of perspiration (sweating). As the relative humidity rises, the rate at which perspiration occurs decreases, and the body loses heat less efficiently. Therefore, at high levels of relative humidity, the outside temperature appears to be higher than it actually is. The heat index is similar to the wind chill index, which measures the apparent temperature that is felt on exposed human skin due to air temperature and wind speed.
The heat index can be expressed as a graph with the true air temperature charted on the vertical axis and the relative humidity on the horizontal axis. On this graph, the apparent temperature perceived by a person is expressed as a sloping line that decreases from left to right (from 0 to 100% relative humidity). As an example, a true temperature of 100°F (38°C) is likely to be perceived by the human body as 90° F (32°C) at a relative humidity of 0%. But at 50% relative humidity, that same true temperature is perceived to be about 120°F (49°C), and at 100% relative humidity, well over 140°F (60°C).
One use for the heat index is as an early warning system for possible heat problems. The U.S. National Weather Service has defined four categories on the heat index graph with increasingly serious health consequences. The lowest category (IV) covers the range of perceived temperatures from 81 to 90°F (27 to 32°C). In this range, the average human can expect to experience some fatigue because of prolonged exposure and physical activity. In category I, covering temperatures greater than 130°F (54.4°C), heat stroke or sunstroke is regarded as imminent.