Heat index or HI is sometimes referred to as the "apparent Temperature". The
HI, given in degrees F, is a measure of how hot it feels when relative humidity
(RH) is added to the actual air temperature.
The following equation approximates the heat index. There are many
assumptions made produce this, far too many to list here. The equation was
obtain by multiple regression analysis and there is a ±1.3 degree °F error.
HI = -42.379 + 2.04901523T + 10.1433127R - 0.22475541TR -
6.83783x10 -3 T 2 - 5.481717x10 -2 R
2
+ 1.22874x10 -3 T 2R + 8.5282x10 -4
TR 2 - 1.99x10 -6 T 2 R 2
where
- T = ambient dry bulb temperature degrees Fahrenheit
- R = relative humidity
- The equation is only useful for temperatures 80 degrees or higher, and
relative humidities 40% or greater.
The previous table was outdated and has been replaced by the following.
Thanks to Lans Rothfusz, MIC at NWS Peachtree City, GA for the following table.
-
| 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°F or higher
(54°C or
higher) |
Heat stroke or sunstroke
likely. |
| Danger |
105 - 129°F
(41 - 54°C) |
Sunstroke, muscle cramps, and/or heat
exhaustion likely. Heatstroke possible with prolonged exposure
and/or physical activity. |
Extreme
Caution |
90 - 105°F
(32 - 41°C) |
Sunstroke, muscle cramps, and/or heat
exhaustion possible with prolonged exposure and/or physical
activity. |
| Caution |
80 - 90°F
(27 - 32°C) |
Fatigue possible with prolonged
exposure and/or physical
activity. | |
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