Heat Index

 

Valid entries are, air temperatures greater than 80 °F ( 27 °C ), dew point temperatures greater than 60 °F ( 16 °C ), and relative humidities higher than 40 percent.


Heat Index and Humidity from Temperature and Dewpoint

in Fahrenheit

Enter in the air temperature ( Tair ) and the dew point temperature ( Tdp ) in degrees Fahrenheit, then click on Calculate HI to compute the heat index ( HI).

 
Tair in °F Tdp in °F RH =
in Celsius

Enter in the air temperature ( Tair ) and the dew point temperature ( Tdp ) in degrees Celsius, then click on Calculate HI to compute the heat index ( HI).
Tair in °C Tdp in °C RH =


Heat Index and Dewpoint from Temperature and Humidity

in Fahrenheit

Enter in the air temperature ( Tair ) in degrees Fahrenheit and relative humidity ( RH ) in percent ( without the % sign ), then click on Calculate HI to compute the heat index ( HI ).
Tair in °F RH Tdp =
in Celsius

Enter in the air temperature ( Tair ) in degrees Celsius and relative humidity ( RH ) in percent ( without the % sign ), then click on Calculate HI to compute the heat index ( HI ).

 
Tair in °C RH Tdp =

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
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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