Heat Index Information & Chart
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:
- 5' 7" tall.
- 147 pounds.
- Caucasian.
- At 98.6°.
- Clothed in long trousers and a short-sleeved shirt.
- In shade.
- Walking at a speed of 3.1 mph.
- In a breeze of 6 mph.
- Not dripping with sweat.
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:
- Monitor the NWS forecasts and advisories for periods of high heat indices.
- Take frequent breaks in the shade.
- Avoid prolonged exertion.
- Drink water often - and drink more than you think you need.
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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