Heat (Stress) Index

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Heat (stress) index

The heat index (HI) or heat stress indexsometimes called the apparent temperature or comfort indexis 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, humidity can raise the HI to extreme levels: at 115°F (46°C) and 40% relative humidity, the HI is 151°F (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.

The HI is used to predict the risk of physiological heat stress for an average individual. Caution is advised at an HI of 8090°F (2732°C): fatigue may result with prolonged exposure and physical activity. An HI of 90105°F (3241°C) calls for extreme caution, since sunstroke, muscle cramps, and heat exhaustion are possible. Danger warnings are issued at HIs of 105130°F (4154°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°F (54°C), when heatstroke and sunstroke are imminent.

Individual physiology influences how people are affected by high HIs. Children and older people are more vulnerable. Acclimatization (being used to the climate ) can alleviate some of the danger. However sunburn can increase the effective HI by slowing the skin's ability to shed excess heat from blood vessels and through perspiration. Exposure to full sunlight can increase HI values by as much as 15°F (8°C). Winds, especially hot dry winds, also can increase the HI. In general, the NWS issues excessive heat alerts when the daytime HI reaches 105°F (41°C) and the nighttime HI stays above 80°F (27°C) for two consecutive days; however these values depend somewhat on the region or metropolitan area. In cities, high HIs often mean increased air pollution . The concentration of ozone , the major component of smog , tends to rise at ground level as the HI increases, causing respiratory problems for many people.

The National Center for Health Statistics estimates that heat exposure results in an average of 371 deaths annually in the United States. About 1,700 Americans died in the heat waves of 1980. In Chicago in 1995, more than 700 people died during a five-day heat wave when the nighttime HI stayed above 89°F (32°C).

At higher temperatures, the air can hold more water vapor; thus humidity and HI values increase as the atmosphere warms. Since the late nineteenth century, the mean annual surface temperature of the earth has risen between 0.5 and 1.0°F (0.3 and 0.6°C). According to the National Aeronautics and Space Administration, the five-year-mean temperature increased about 0.9°F (0.5°C) between 1975 and 1999, the fastest rate of recorded increase. In 1998 global surface temperatures were the warmest since the advent of reliable measurements and the 1990s accounted for seven of the 10 warmest years on record. Nighttime temperatures have been increasing twice as fast as daytime temperatures.

Greenhouse gases , including carbon dioxide , methane , nitrous oxide , 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 pre-industrial levels. Increased urbanization also contributes to global warming, as buildings and roads hold in the heat. Climate simulations predict an average surface air temperature increase of 4.57°F (2.54°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 45°F (23°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 sub-tropics. Warm, humid regions of the southeastern United States are expected to experience substantial increases in the summer HI due to 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 820°F (411°C).

[Margaret Alic Ph.D. ]



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National Weather Service, National Oceanic and Atmospheric Administration, U. S. Department of Commerce, 1325 East West Highway, Silver Spring, USA 20910 , <http://www.nws.noaa.gov>

Physicians for Social Responsibility, 1875 Connecticut Avenue, NW, Suite 1012, Washington , DC USA 20009 (202) 667-4260, Fax: (202) 667-4201, Email: [email protected], <http://www.psr.org>

Union of Concerned Scientists, 2 Brattle Square, Cambridge, MA USA 02238 (617) 547-5552, Email: [email protected], <http://www.ucsusa.org>