A I R   T E M P E R A T U R E   (Fo)
  70 72 74 76 78 80 82 84 86 88 90 92 94 96 98 100 102 104 106 108 110 112 114 116 118 120  
0 64 66 68 70 72 73 75 77 78 80 83 84 86 88 90 91 93 95 96 98 99 101 103 105 106 107 0
5 64 66 68 71 72 74 76 77 79 82 84 86 87 89 91 93 95 97 98 100 102 104 106 108 110 111 5
10 65 67 69 71 73 75 77 79 81 83 85 87 88 91 93 95 97 99 101 104 105 107 109 112 115 117 10
15 65 68 70 72 74 76 78 80 82 84 86 88 90 93 95 97 99 102 105 107 108 110 115 117 120 123 15
20 66 68 71 73 75 77 79 81 83 85 87 89 92 95 97 99 102 105 107 110 112 115 120 124 127 130 20
25 67 69 72 74 76 77 80 82 85 87 88 91 94 96 98 101 105 108 110 114 117 121 125 130     25
30 67 70 72 75 77 78 81 83 86 88 90 93 95 98 101 104 107 110 115 120 124 129         30
35 68 70 73 75 77 79 82 84 86 89 91 95 97 100 104 107 111 116 120 126 130           35
40 68 71 73 76 77 79 82 85 87 90 93 96 99 103 107 110 116 121 127               40
45 69 72 74 76 78 80 84 86 88 91 93 96 99 106 110 115 121 127                 45
50 69 72 74 77 78 81 84 87 90 93 95 98 101 109 115 120 127                   50
55 70 72 75 78 79 82 85 88 91 95 96 100 105 114 120 125                     55
60 70 73 75 78 80 83 86 89 93 97 98 103 107 118 125                       60
65 70 73 76 78 81 84 87 91 95 99 103 109 115 124                         65
70 70 74 77 79 82 85 89 93 97 101 106 113 120 129                         70
75 70 74 77 79 82 86 90 95 99 105 110 118 125                           75
80 71 74 77 79 83 86 91 96 101 107 114 122 130                           80
85 71 75 77 80 85 87 94 98 105 111 117 127                             85
90 71 75 78 81 85 89 95 100 107 115 122                               90
95 72 75 78 82 83 90 97 103 110 118 125                               95
100 72 76 79 83 87 91 98 106 115 121 130                               100


 Very Warm: Fatigue possible with prolonged exposure
 Hot: Sunstroke, heat cramps, or heat exhaustion possible with prolonged exposure
 Very Hot: Danger: Sunstroke, heat cramps, or heat exhaustion likely with prolonged exposure
 Extreme Danger: Heat/sunstroke highly likely with continued exposure


The Heat Index

The heat index combines the effects of heat and humidity. When heat and humidity combine to reduce the amount of evaporation of sweat from the body, outdoor exercise becomes dangerous even for those in good shape. Key rules for coping with heat are to drink plenty of water to avoid dehydration and to slow down and cool off when feeling fatigued, a headache, a high pulse rate or shallow breathing. Overheating can cause serious, even life-threatening conditions such as heat stroke. The apparent temperature, which combines the temperature and relative humidity, is a guide to the danger. Below is the heat stress index based on the apparent temperature.



Apparent temperature heat stress index
CategoryApparent temperature Dangers
Caution80-90°FExercise more fatiguing than usual
Extreme caution90-105°FHeat cramps, exhaustion possible
Danger105-130°FHeat exhaustion likely
Extreme dangerGreater than 130°FHeat stroke imminent


Effects of heat illnesses:

Heat cramps: Exercising in hot weather can lead to muscle cramps, especially in the legs, because of brief imbalances in body salts. Cramps become less frequent as a person becomes used to the heat. Heat syncope or fainting: Anyone not used to exercising in the heat can experience a quick drop in blood pressure that can lead to fainting. As with heat cramps, the cure is to take it easy.

Heat exhaustion: Losing fluid and salt through perspiration or replacing them in an imbalanced way can lead to dizziness and weakness. Body temperature might rise, but not above 102 degrees. In some cases victims, especially the elderly, should be hospitalized. Heat exhaustion is more likely after a few days of a heat wave than when one is just beginning. The best defense is to take it easy and drink plenty of water. Don't take salt tablets without consulting a physician.

Heatstroke: In some cases extreme heat can upset the body's thermostat, causing body temperature to rise to 105 degrees or higher. Symptoms are lethargy, confusion and unconsciousness. Even a suspicion that someone might be suffering from heatstroke requires immediate medical aid. Heatstroke can kill.

©The USA TODAY Weather Book by Jack Williams

Formula to calculate Heat Index

If you know the relative humidity and the dry air temperature, then you can use the following equation to calculate the heat index.

Heat index (HI), or apparent temperature (AI) = -42.379 + 2.04901523(Tf) + 10.14333127(RH) - 0.22475541(Tf)(RH) - ((6.83783 x 10-3)(Tf2) - ((5.481717 x 10-2)(RH2) + ((1.22874 x 10-3)(Tf2)(RH)) + ((8.5282 x 10-4)(Tf)(RH2)) - ((1.99 x 10-6)(Tf2)(RH2))

Note: In order for the Heat Index formula to work correctly, you must use the relative humidity in percent form. In other words, if the relative humidity is 65%, use 65 for RH in the formula, not .65.

Summer Simmer Index

If you know the relative humidity and the dry air temperature, then you can use the following equation to calculate the summer simmer index.

Summer simmer index (SSI) = 1.98(Tf - (0.55 - 0.0055(RH))(Tf-58)) - 56.83

Tf= air temperature in degrees Fahrenheit, RH= relative humidity expressed as a whole number