Chart of Heat Index

Find the heat index based on temperature and humidity. Protect yourself and your children from extreme heat.

About this chart
Source: various, NOAA
Also see Wikipedia

Heat Index

Legend:	Caution	Extreme caution	Danger	Extreme danger

Scroll sideways to see the whole table.

T e m p e r a t u r e°F	Relative Humidity																																	T e m p e r a t u r e°F
		40	42	44	46	48	50	52	54	56	58	60	62	64	66	68	70	72	74	76	78	80	82	84	86	88	90	92	94	96	98	100
	110	135	138																														110
	109	132	135	138																													109
	108	129	132	135	138																												108
	107	126	129	132	134	137																											107
	106	124	126	129	131	134	137																										106
	105	121	123	126	128	131	133	136	139																								105
	104	118	121	123	125	128	130	133	135	138																							104
	103	116	118	120	122	124	127	129	132	135	137																						103
	102	113	115	117	119	121	124	126	129	131	134	136	139																				102
	101	111	113	115	117	119	121	123	125	128	130	133	135	138																			101
	100	109	110	112	114	116	118	120	122	124	127	129	131	134	137	139																	100
	99	107	108	110	111	113	115	117	119	121	123	125	128	130	133	135	138																99
	98	104	106	107	109	111	112	114	116	118	120	122	124	127	129	131	134	136	139														98
	97	102	104	105	107	108	110	111	113	115	117	119	121	123	125	127	130	132	135	137													97
	96	100	102	103	104	106	107	109	110	112	114	116	118	120	122	124	126	128	130	133	135	138											96
	95	98	100	101	102	103	105	106	108	109	111	113	114	116	118	120	122	124	126	129	131	133	136	138									95
	94	97	98	99	100	101	102	104	105	107	108	110	111	113	115	117	118	120	122	125	127	129	131	133	136	138							94
	93	95	96	97	98	99	100	101	103	104	105	107	108	110	112	113	115	117	119	121	123	125	127	129	131	133	136	138					93
	92	93	94	95	96	97	98	99	100	102	103	104	106	107	109	110	112	113	115	117	119	121	122	124	127	129	131	133	135	138			92
	91	92	92	93	94	95	96	97	98	99	100	102	103	104	106	107	108	110	112	113	115	117	118	120	122	124	126	128	130	132	134	137	91
	90	90	91	92	92	93	94	95	96	97	98	99	100	102	103	104	105	107	108	110	111	113	114	116	118	120	121	123	125	127	129	131	90
	89	89	89	90	91	91	92	93	94	95	96	97	98	99	100	101	102	104	105	106	108	109	111	112	114	115	117	119	120	122	124	126	89
	88	87	88	89	89	90	91	91	92	93	94	95	96	97	98	99	100	101	102	103	104	106	107	108	110	111	113	114	116	117	119	121	88
	87	86	87	87	88	88	89	90	90	91	92	93	93	94	95	96	97	98	99	100	101	102	104	105	106	107	109	110	111	113	114	116	87
	86	85	85	86	86	87	87	88	89	89	90	91	91	92	93	94	95	95	96	97	98	99	100	101	103	104	105	106	107	108	110	111	86
	85	84	84	85	85	85	86	86	87	88	88	89	89	90	91	91	92	93	94	95	95	96	97	98	99	100	101	102	103	104	105	107	85
	84	83	83	83	84	84	85	85	86	86	86	87	88	88	89	89	90	91	91	92	93	93	94	95	96	97	98	98	99	100	101	102	84
	83	82	82	82	83	83	83	84	84	85	85	85	86	86	87	87	88	88	89	90	90	91	91	92	93	93	94	95	96	96	97	98	83
	82	81	81	81	82	82	82	83	83	83	84	84	84	85	85	86	86	86	87	87	88	88	89	89	90	90	91	92	92	93	93	94	82
	81	80	80	81	81	81	81	81	82	82	82	83	83	83	83	84	84	84	85	85	86	86	86	87	87	88	88	88	89	89	90	90	81
	80	79	80	80	80	80	80	80	81	81	81	81	82	82	82	82	82	83	83	83	83	84	84	84	85	85	85	85	86	86	86	87	80
		40	42	44	46	48	50	52	54	56	58	60	62	64	66	68	70	72	74	76	78	80	82	84	86	88	90	92	94	96	98	100
	Relative Humidity

About this chart

The heat index is an attempt to quantify the combined effect of temperature and humidity. It is a subjective estimate and cannot take into account all the variables that affect the impact of hot weather, such as body size and shape, clothing, wind, sunlight, etc.

However, it does provide a simple way of alerting people about dangerous conditions. Here are the National Weather Service's heat index levels:

Heat index	Effects on the body
80 to 89°F	Caution: fatigue possible with prolonged exposure and/or physical activity
90 to 102°F	Extreme caution: heat stroke, heat cramps, or heat exhaustion possible with prolonged exposure and/or physical activity
103 to 124°F	Danger: heat cramps or heat exhaustion likely, and heat stroke possible with prolonged exposure and/or physical activity
125°F or higher	Extreme danger: heat stroke highly likely

Note: Up to 15 degrees should be added for the effects of direct sunlight.

The formula was derived from a statistical analysis of a survey asking people how hot they felt under different temperature and humidity conditions. This is the formula, where T is the temperature and R is relative humidity: Heat Index = -42.4 + 2.0T + 10.1R - 0.2TR - 6.8x10^-3T² - 5.4x10^-2R² + 1.2x10^-3T²R + 8.5x10^-4TR² - 2.0x10^-6T²R². (The constants have been rounded off here.)