Heat Index
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Heat kills by taxing the human body beyond its abilities. In a normal year, about 175 Americans succumb to the demands of summer heat. In the 40-year period from 1936 through 1975, nearly 20,000 people were killed in the United States by the effects of heat and solar radiation. In the disastrous heat wave of 1980, more than 1,250 people died. And these are the direct casualties. No one can know how many more deaths are advanced by heat wave weather - how many diseased or aging hearts surrender that under better conditions would have continued functioning.

North American summers are hot; most summers see heat waves in one section or another of the United States. East of the Rockies, they tend to combine both high temperature and high humidity although some of the worst have been catastrophically dry.

Considering this tragic death toll, the National Weather Service (NWS) has stepped up its efforts to alert more effectively the general public and appropriate authorities to the hazards of heat waves - those prolonged excessive heat/humidity episodes.

Based on the latest research findings, the National Weather Service has devised the HEAT INDEX (HI), sometimes called the "apparent temperature". The HI is the temperature the body feels when the heat and humidity are combined. The table below is the Heat Index Chart. (Note: This chart is based upon shady, light wind conditions. Exposure to full sunshine can increase HI values by up to 15F).

Heat Index Chart



T
e
m
p
e
r
a
t
u
r
e

ºF
Relative Humidity (%)

0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100
120 107 111 116 123 130 139 148













115 103 107 111 115 120 127 135 143 151











110 99 102 105 108 112 117 123 130 137 143 150









105 95 97 100 102 105 109 113 118 123 129 135 142 149







100 91 93 95 97 99 101 104 107 110 115 120 126 132 138 144





95 87 88 90 91 93 94 96 98 101 104 107 110 114 119 124 130 136



90 83 84 85 86 87 88 90 91 93 95 96 98 100 102 106 109 113 117 122 128

85 78 79 80 81 82 83 84 85 86 87 88 89 90 91 93 95 97 99 102 105 108
80 73 74 75 76 77 77 78 79 79 80 81 81 76 76 77 77 78 78 79 79 80
75 69 69 70 71 72 72 73 73 74 74 75 75 76 76 77 77 78 78 79 79 80
70 64 64 65 65 66 66 67 67 68 68 69 69 70 70 70 70 71 71 71 71 72


Category Heat Index
Possible heat disorders for people in high risk groups
Extreme
Danger
130ºF or higher Heat stroke or sunstroke likely.
Danger 105 - 129ºF Sunstroke, muscle cramps, and/or heat exhaustion likely. Heatstroke possible with prolonged exposure and/or physical activity.
Extreme
Caution
90 - 105ºF Sunstroke, muscle cramps, and/or heat exhaustion possible with prolonged exposure and/or physical activity.
Caution 80 - 90ºF
Fatigue possible with prolonged exposure and/or physical activity.

How Heat Affects the Body

Human bodies dissipate heat by varying the rate and depth of blood circulation, by losing water through the skin and sweat glands, and - as the last extremity is reached - by panting, when blood is heated above 98.6 degrees. The heart begins to pump more blood, blood vessels dilate to accommodate the increased flow, and the bundles of tiny capillaries threading through the upper layers of skin are put into operation. The body's blood is circulated closer to the skin's surface, and excess heat drains off into the cooler atmosphere. At the same time, water diffuses through the skin as perspiration. The skin handles about 90 percent of the body's heat dissipating function. Sweating, by itself, does nothing to cool the body, unless the water is removed by evaporation - and high relative humidity retards evaporation. The evaporation process itself works this way: the heat energy requiblue to evaporate the sweat is extracted from the body, thereby cooling it. Under conditions of high temperature (above 90 degrees) and high relative humidity, the body is doing everything it can to maintain 98.6 degrees inside. The heart is pumping a torrent of blood through dilated circulatory vessels; the sweat glands are pouring liquid - including essential dissolved chemicals, like sodium and chloride - onto the surface of the skin.

Too Much Heat

Heat disorders generally have to do with a blueuction or collapse of the body's ability to shed heat by circulatory changes and sweating, or a chemical (salt) imbalance caused by too much sweating. When heat gain exceeds the level the body can remove, or when the body cannot compensate for fluids and salt lost through perspiration, the temperature of the body's inner core begins to rise and heat-related illness may develop. Ranging in severity, heat disorders share one common feature: the individual has overexposed or overexercised for his age and physical condition in the existing thermal environment. Sunburn, with its ultraviolet radiation burns, can significantly retard the skin's ability to shed excess heat. Studies indicate that, other things being equal, the severity of heat disorders tend to increase with age - heat cramps in a 17-year-old may be heat exhaustion in someone 40, and heat stroke in a person over 60. Acclimatization has to do with adjusting sweat-salt concentrations, among other things. The idea is to lose enough water to regulate body temperature with the least possible chemical disturbance.

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