Heat Index

Equivalent heat index temperatures at various temperatures and corresponding relative humidity - in degrees Fahrenheit and Celsius

Heat Index in degrees Fahrenheit

The heat index can be calculated as

t_HI = -42.379 + 2.04901523 t + 10.14333127 φ

- 0.22475541 t φ - 0.00683783 t² - 0.05481717 φ²

+ 0.00122874 t²φ + 0.00085282 t φ² - 0.00000199 (T φ)² (1)

where

t_HI = heat index (^oF)

t = air temperature (^oF) (t > 57^oF)

φ = relative humidity (%)

heat index diagram fahrenheit

Apparent Temperature Heat Stress Index (^oF)
Relative Humidity %RH	Temperature (^oF)
Relative Humidity %RH	80	85	90	95	100	105	110	115	120	125	130	135	140
0	78	82	87	91¹⁾	94	97	100	103	105	107	108	110	110
5	78	82	86	90	94	98	102	106	110	114	117	121
10	78	82	86	90²⁾	95	100	105	110	116	121	128
15	78	82	86	91	96	102	108	115	122	130
20	79	82	86	91	97	104	112	121	130	140
25	79	82	87	93	100	108	117	127	138
30	79	83	88	94	102	112	122	134	148
35	80	84	89	96	106	116	129	143
40	80	84	91	99	109	121	136	152
45	80	85	92	102	114	127	143
50	81	86	95	105³⁾	118	134	152
55	81	88	97	109	124	141
60	82	89	100	113	129	149
65	82	91	103	118	136
70	83	93	106	123	143
75	84	95	109	128⁴⁾
80	84	97	113	134
85	85	99	117
90	86	102	122
95	86	104
100	87	107

T(^oC) = 5/9[T(^oF) - 32]

Sunstroke and heat exhaustion:

¹⁾ Caution - Fatigue is possible with prolonged exposure and/or physical activity

²⁾ Extreme Caution - Sunstroke, heat cramps and heat exhaustion are possible with prolonged exposure and/or physical activity

³⁾ Danger - Sunstroke, heat cramps and heat exhaustion are likely. Heat stroke is possible with prolonged exposure and/or physical activity

⁴⁾ Extreme Danger - Heatstroke/sunstroke is highly likely with continued exposure

Heat Index in degrees Celsius

heat index diagram celsius

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