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Heat Index and Humidity from Temperature and DewpointHeat Index and Dewpoint from Temperature and Humidity |
What Is Heat Index?
Heat index (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 to produce this, far too many to list here. The equation was obtained by multiple regression analysis and there is a ±1.3 degree °F error.
HI = -42.379 + 2.04901523T + 10.14333127R - 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
Thanks to Lans Rothfusz, MIC at NWS Tulsa, OK 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
Danger130°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
Caution90 - 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.