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## Heat Index

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.

Calculate the heat index using the Meteorological calculator Javascript .

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 oF 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 Farhrenheit
R = relative humidity

 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 ExtremeDanger 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. ExtremeCaution 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.

References
Rothfusz, L. P., 1990:The heat index equation (or, more than you ever wanted to know about heat index). NWS Southern Region Technical Attachment, SR/SSD 90-23, Fort Worth, TX.