4.3: The Heat Index

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Page ID: 46740

Neel Desai and Alicia Mullens
De Anza College

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Heat Index Temperature

"It's not the heat, it's the humidity". That's a common phrase you may have heard in the summer, but it's actually both. The heat index, also known as apparent temperature, is the temperature a person feels when relative humidity is combined with air temperature. This has important implications for human comfort. When the body gets too hot, it sweats to cool itself. If perspiration cannot evaporate, the body cannot regulate its temperature. Evaporation is a cooling process. When perspiration evaporates from the body, it effectively lowers body temperature. When the atmospheric moisture content (i.e., relative humidity) is high, the rate of evaporation from the body decreases. In other words, the human body feels warmer in humid conditions. The opposite is true when relative humidity decreases, as the rate of perspiration increases. The body actually feels cooler in arid conditions. There is a direct relationship between the air temperature and relative humidity and the heat index, as shown in Table \(\PageIndex{1}\). As air temperature and relative humidity rise, the heat index rises.

Table \(\PageIndex{1}\): Heat Index Temperatures (°F) for various relative humidity values by NOAA. The higher the Heat Index, the greater the risk of Heat Stroke and Heat Exhaustion.
Relative Humidity (%) ↓ Temp (°F) →	80	82	84	86	88	90	92	94	96	98	100	102	104	106	108	110
40	80	81	83	85	88	91	94	97	101	105	109	114	119	124	130	136
45	80	82	84	87	89	93	96	100	104	109	114	119	124	130	137
50	81	83	85	88	91	95	99	103	108	113	118	124	131	137
55	81	84	86	89	93	97	101	106	112	117	124	130	137
60	82	84	88	91	95	100	105	110	116	123	129	137
65	82	85	89	93	98	103	108	114	121	128	136
70	83	86	90	95	100	105	112	119	126	134
75	84	88	92	97	103	109	116	124	132
80	84	89	94	100	106	113	121	129
85	85	90	96	102	110	117	126	135
90	86	91	98	105	113	122	131
95	86	93	100	108	117	127
100	87	95	103	112	121	132

Heat Index Risks

Meteorologists use the Heat Index to warn the public of health risks in the summer. Interestingly, the Heat Index values in Table \(\PageIndex{1}\) are not in direct sunlight, but in the shade. Exposure to full sunlight can increase the heat index by up to 14 °F. Table \(\PageIndex{2}\) lists the effects of different heat index values on our health.

Table \(\PageIndex{2}\): Effects of the heat index on the human body.
Heat index	Effects
81–90 °F	Caution: fatigue is possible with prolonged exposure and activity. Continuing activity could result in heat cramps.
90–106 °F	Extreme caution: heat cramps and heat exhaustion are possible. Continuing activity could result in heat stroke.
106–129 °F	Danger: heat cramps and heat exhaustion are likely; heat stroke is probable with continued activity.
over 129 °F	Extreme danger: heat stroke is imminent.

It is suggested that during high Heat Index days, you should drink plenty of water and reduce physical activity, especially outdoors. Other precautions include staying in the shade, using air conditioning when possible, and wearing lightweight, light-colored clothing.

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Relative Humidity (%) ↓ Temp (°F) →	80	82	84	86	88	90	92	94	96	98	100	102	104	106	108	110
40	80	81	83	85	88	91	94	97	101	105	109	114	119	124	130	136
45	80	82	84	87	89	93	96	100	104	109	114	119	124	130	137
50	81	83	85	88	91	95	99	103	108	113	118	124	131	137
55	81	84	86	89	93	97	101	106	112	117	124	130	137
60	82	84	88	91	95	100	105	110	116	123	129	137
65	82	85	89	93	98	103	108	114	121	128	136
70	83	86	90	95	100	105	112	119	126	134
75	84	88	92	97	103	109	116	124	132
80	84	89	94	100	106	113	121	129
85	85	90	96	102	110	117	126	135
90	86	91	98	105	113	122	131
95	86	93	100	108	117	127
100	87	95	103	112	121	132

Relative Humidity (%) ↓ Temp (°F) →	80	82	84	86	88	90	92	94	96	98	100	102	104	106	108	110
40	80	81	83	85	88	91	94	97	101	105	109	114	119	124	130	136
45	80	82	84	87	89	93	96	100	104	109	114	119	124	130	137
50	81	83	85	88	91	95	99	103	108	113	118	124	131	137
55	81	84	86	89	93	97	101	106	112	117	124	130	137
60	82	84	88	91	95	100	105	110	116	123	129	137
65	82	85	89	93	98	103	108	114	121	128	136
70	83	86	90	95	100	105	112	119	126	134
75	84	88	92	97	103	109	116	124	132
80	84	89	94	100	106	113	121	129
85	85	90	96	102	110	117	126	135
90	86	91	98	105	113	122	131
95	86	93	100	108	117	127
100	87	95	103	112	121	132

Relative Humidity (%) ↓ Temp (°F) →	80	82	84	86	88	90	92	94	96	98	100	102	104	106	108	110
40	80	81	83	85	88	91	94	97	101	105	109	114	119	124	130	136
45	80	82	84	87	89	93	96	100	104	109	114	119	124	130	137
50	81	83	85	88	91	95	99	103	108	113	118	124	131	137
55	81	84	86	89	93	97	101	106	112	117	124	130	137
60	82	84	88	91	95	100	105	110	116	123	129	137
65	82	85	89	93	98	103	108	114	121	128	136
70	83	86	90	95	100	105	112	119	126	134
75	84	88	92	97	103	109	116	124	132
80	84	89	94	100	106	113	121	129
85	85	90	96	102	110	117	126	135
90	86	91	98	105	113	122	131
95	86	93	100	108	117	127
100	87	95	103	112	121	132