Heat Index Information & Chart

Information provided by the National Weather Service Office in Tulsa

To help you prepare and be safe in this inevitable heat, the National Weather Service uses the “heat index” as a tool to measure how stressful the heat will be.

The heat index (HI) is an apparent temperature felt by the human body due to the combined effects of temperature and humidity. Most people understand that as the air temperature goes up, so does the heat index. But why does humidity play a role? It’s because the body’s perspiration cannot evaporate as well when the humidity increases. Therefore, the cooling effects of your sweat are reduced as the humidity rises, and your body is unable to cool itself naturally. Combine high heat and high humidity and you’ve got trouble!

Although it is convenient to use a single number (the heat index) to describe the apparent temperature your body feels, keep in mind that heat and humidity affect every body (and everybody) differently. Several assumptions are used to calculate the heat index. The heat index assumes that the body is:

If any of these factors change, e.g., more exertion, more clothing, and/or more weight, the heat index will change for that individual. For example, if you weigh 250 pounds, are wearing long-sleeved work clothes, and are working outside in the sun, the heat index value you hear reported on the radio is lower than what you are personally feeling.

The rules for minimizing the heat effects are simple:

If you heed the advice described above, the “hot” summer you have will apply to the fun times you had, as well as the temperature! Stay cool and enjoy your summer! (For more information on the heat index, see heat index chart below or the meteorological calculator.)

Lans P. Rothfusz
Meteorologist In Charge
NWS Tulsa
June 26, 1998

HEAT INDEX f (c)
  RELATIVE HUMIDITY (%)
Temp. 40 45 50 55 60 65 70 75 80 85 90 95 100
110
(47)
136
(58)
                       
108
(43)
130
(54)
137
(58)
                     
106
(41)
124
(51)
130
(54)
137
(58)
                   
104
(40)
119
(48)
124
(51)
131
(55)
137
(58)
                 
102
(39)
114
(46)
119
(48)
124
(51)
130
(54)
137
(58)
               
100
(38)
109
(43)
114
(46)
118
(48)
124
(51)
129
(54)
136
(58)
             
98
(37)
105
(41)
109
(43)
113
(45)
117
(47)
123
(51)
128
(53)
134
(57)
           
96
(36)
101
(38)
104
(40)
108
(42)
112
(44)
116
(47)
121
(49)
126
(52)
132
(56)
         
94
(34)
97
(36)
100
(38)
103
(39)
106
(41)
110
(43)
114
(46)
119
(48)
124
(51)
129
(54)
135
(57)
     
92
(33)
94
(34)
96
(36)
99
(37)
101
(38)
105
(41)
108
(42)
112
(44)
116
(47)
121
(49)
126
(52)
131
(55)
   
90
(32)
91
(33)
93
(34)
95
(35)
97
(36)
100
(38)
103
(39)
106
(41)
109
(43)
113
(45)
117
(47)
122
(50)
127
(53)
132
(56)
88
(31)
88
(31)
89
(32)
91
(33)
93
(34)
95
(35)
98
(37)
100
(38)
103
(39)
106
(41)
110
(43)
113
(45)
117
(47)
121
(49)
86
(30)
85
(29)
87
(31)
88
(31)
89
(32)
91
(33)
93
(34)
95
(35)
97
(36)
100
(38)
102
(39)
105
(41)
108
(42)
112
(44)
84
(29)
83
(28)
84
(29)
85
(29)
86
(30)
88
(31)
89
(32)
90
(32)
92
(33)
94
(34)
96
(36)
98
(37)
100
(38)
103
(39)
82
(28)
81
(27)
82
(28)
83
(28)
84
(29)
84
(29)
85
(29)
86
(30)
88
(31)
89
(32)
90
(32)
91
(33)
93
(34)
95
(35)
80
(27)
80
(27)
80
(27)
81
(27)
81
(27)
82
(28)
82
(28)
83
(28)
84
(29)
84
(29)
85
(29)
86
(30)
86
(30)
87
(31)
Category Heat Index Possible heat disorders for people in high risk groups
Extreme
Danger
130 or higher
(54 or higher)
Heat stroke or sunstroke likely.
Danger 105 - 129
(41 - 54)
Sunstroke, muscle cramps, and/or heat exhaustion likely. Heatstroke possible with prolonged exposure and/or physical activity.
Extreme
Caution
90 - 105
(32 - 41)
Sunstroke, muscle cramps, and/or heat exhaustion possible with prolonged exposure and/or physical activity.
Caution 80 - 90
(27 - 32)
Fatigue possible with prolonged exposure and/or physical activity.

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