Heat (Stress) Index

The heat index (HI), also called the apparent temperature or comfort index, is a measure of how a given air temperature feels to the average person at a given relative humidity based on human physiology and on clothing science. The heat stress index (HSI) is a value mathematically calculated to determine the allowable exposure time to various environmental conditions. It takes into consideration the temperature, humidity, and activity level.

The HI takes into account the relative humidity as well as the air temperature. The HI temperature is measured in the shade and assumes a wind speed of 5.6 miles per hour (9.0 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 HI chart of the National Weather Service (NWS), if the air temperature is 70°Fahrenheit (21°C), the HI is 64°Fahrenheit (18°C) at 0 percent relative humidity and 72°Fahrenheit (22°C) at 100 percent relative humidity. At 95°Fahrenheit (35°C) and 55 percent relative humidity, the HI is 110°Fahrenheit (43°C). In very hot weather, humidity can raise the HI to extreme levels; at 115°Fahrenheit (46°C) and 40 percent relative humidity, the HI is 151°Fahrenheit (66°C). This is because humidity affects the body's ability to regulate internal heat through perspiration. The body feels warmer when it is humid because perspiration evaporates more slowly; thus, the HI is higher.

At higher temperatures, the air can hold more water vapor; thus absolute humidity and HI values increase as the atmosphere warms. Since the late nineteenth century, the mean annual surface temperature of the earth has risen. According to the National Aeronautics and Space Administration (NASA), nine of the ten warmest years in the modern meteorological record have occurred since the year 2000. The year 2011 had the ninth warmest global surface temperature recorded since 1880. These findings continue a trend in which the five-year mean temperature increased about 0.9°Fahrenheit (0.5°C) between 1975 and 1999, the fastest rate of recorded increase. Nighttime temperatures have been increasing twice as fast as daytime temperatures.

The HI is used to predict the risk of physiological heat stress for an average individual. Caution is advised at an HI of 80–90°Fahrenheit (27–32°C): fatigue may result with prolonged exposure and physical activity. An HI of 90–105°Fahrenheit (32–41°C) calls for extreme caution, as it increases the likelihood of sunstroke, muscle cramps, and heat exhaustion. Danger warnings are issued at HIs of 105–130°Fahrenheit (41–54°C), when sunstroke and heat exhaustion are likely and there is a potential for heat stroke. Category IV, extreme danger, occurs at HIs above 130°Fahrenheit (54°C), when heatstroke and sunstroke are imminent.

Greenhouse gases, including carbon dioxide, methane, nitrous oxide, ozone, and chlorofluorocarbons, increase the heat-trapping capabilities of the atmosphere. Evaporation from the ocean surfaces increased during the twentieth century, resulting in higher humidity that enhanced the greenhouse effect. It is projected that during the twenty-first century greenhouse gas concentrations will double or even quadruple from preindustrial levels. Increased urbanization also contributes to global warming, as buildings and roads hold heat. Climate simulations predict an average surface air temperature increase of 4.5–7°Fahrenheit (2.5–4°C) by 2100. This will increase the number of extremely hot days and, in temperate climates, double the number of very hot days, for an average increase in summer temperatures of 4–5°Fahrenheit (2–3°C). More heat-related illnesses and deaths will result.

The National Oceanic and Atmospheric Administration projects that the HI could rise substantially in humid regions of the tropics and subtropics. Warm, humid regions of the southeastern United States are expected to experience substantial increases in the summer HI because of increased humidity, even though temperature increases may be smaller than in the continental interior. Predictions for the increase in the summer HI for the southeast United States over the next century range from 8–20°Fahrenheit (4–11°C). This can lead to an increased risk of heat disorders such as heat cramps, heat exhaustion, and heatstroke, as well as increased respiratory distress for susceptible individuals.

See also Climate change ; Heat disorders .