NWS Chicago: Heat Index/Wind Chill Charts

Heat Index Chart

In an average year only the winter's cold -- not lightning, hurricanes, tornadoes, floods, or earthquakes -- takes a greater weather-related death toll than the summer's heat and humidity. In an effort to alert you to the hazards of prolonged heat/humidity episodes, the National Weather Service devised the "heat index." The heat index (HI) is an accurate measure of how hot it really feels when the affects of humidity are added to high temperature.

The human body contains several mechanisms to maintain its internal operating temperature at 98.6° F. When threatened with above "normal" temperatures, the body will try to dissipate excesss heat by varying the rate and depth of blood circulation, by losing water through the skin and sweat glands, and, as a last resort, by panting. When weather conditions force the air temperature above 90° F and the relative humidity is high, the body is doing everything it can to maintain its normal temperature. Unfortunately, conditions can exceed the body's ability to cope with the combined affects of heat and humidity. At such times the body may sucumb to any of a number of heat disorders including sunstroke, heat cramps, heat exhaustion and heatstroke.

To use the heat index charts, find the appropriate temperature at the top of the chart. Read down until you are opposite the humidity/dewpoint. The number which appears at the intersection of the temperature and humidity/dewpoint is the heat index.

**Heat Index Chart (Temperature & Dewpoint)**
Dewpoint (° F)	Temperature (° F)
Dewpoint (° F)	90	91	92	93	94	95	96	97	98	99	100	101	102	103	104	105
65	94	95	96	97	98	100	101	102	103	104	106	107	108	109	110	112
66	94	95	97	98	99	100	101	103	104	105	106	108	109	110	111	112
67	95	96	97	98	100	101	102	103	105	106	107	108	110	111	112	113
68	95	97	98	99	100	102	103	104	105	107	108	109	110	112	113	114
69	96	97	99	100	101	103	104	105	106	108	109	110	111	113	114	115
70	97	98	99	101	102	103	105	106	107	109	110	111	112	114	115	116
71	98	99	100	102	103	104	106	107	108	109	111	112	113	115	116	117
72	98	100	101	103	104	105	107	108	109	111	112	113	114	116	117	118
73	99	101	102	103	105	106	108	109	110	112	113	114	116	117	118	119
74	100	102	103	104	106	107	109	110	111	113	114	115	117	118	119	121
75	101	103	104	106	107	108	110	111	113	114	115	117	118	119	121	122
76	102	104	105	107	108	110	111	112	114	115	117	118	119	121	122	123
77	103	105	106	108	109	111	112	114	115	117	118	119	121	122	124	125
78	105	106	108	109	111	112	114	115	117	118	119	121	122	124	125	126
79	106	107	109	111	112	114	115	117	118	120	121	122	124	125	127	128
80	107	109	110	112	114	115	117	118	120	121	123	124	126	127	128	130
81	109	110	112	114	115	117	118	120	121	123	124	126	127	129	130	132
82	110	112	114	115	117	118	120	122	123	125	126	128	129	131	132	133
Note: Exposure to full sunshine can increase HI values by up to 15° F

Heat Index Chart (Temperature & Relative Humidity)
RH
(%) Temperature (° F)

90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105

90 119 123 128 132 137 141 146 152 157 163 168 174 180 186 193 199

85 115 119 123 127 132 136 141 145 150 155 161 166 172 178 184 190

80 112 115 119 123 127 131 135 140 144 149 154 159 164 169 175 180

75 109 112 115 119 122 126 130 134 138 143 147 152 156 161 166 171

70 106 109 112 115 118 122 125 129 133 137 141 145 149 154 158 163

65 103 106 108 111 114 117 121 124 127 131 135 139 143 147 151 155

60 100 103 105 108 111 114 116 120 123 126 129 133 136 140 144 148

55 98 100 103 105 107 110 113 115 118 121 124 127 131 134 137 141

50 96 98 100 102 104 107 109 112 114 117 119 122 125 128 131 135

45 94 96 98 100 102 104 106 108 110 113 115 118 120 123 126 129

40 92 94 96 97 99 101 103 105 107 109 111 113 116 118 121 123

35 91 92 94 95 97 98 100 102 104 106 107 109 112 114 116 118

30 89 90 92 93 95 96 98 99 101 102 104 106 108 110 112 114

Note: Exposure to full sunshine can increase HI values by up to 15° F

**Heat Index Chart (Temperature & Relative Humidity)**
RH (%)	Temperature (° F)
90	91	92	93	94	95	96	97	98	99	100	101	102	103	104	105
90	119	123	128	132	137	141	146	152	157	163	168	174	180	186	193	199
85	115	119	123	127	132	136	141	145	150	155	161	166	172	178	184	190
80	112	115	119	123	127	131	135	140	144	149	154	159	164	169	175	180
75	109	112	115	119	122	126	130	134	138	143	147	152	156	161	166	171
70	106	109	112	115	118	122	125	129	133	137	141	145	149	154	158	163
65	103	106	108	111	114	117	121	124	127	131	135	139	143	147	151	155
60	100	103	105	108	111	114	116	120	123	126	129	133	136	140	144	148
55	98	100	103	105	107	110	113	115	118	121	124	127	131	134	137	141
50	96	98	100	102	104	107	109	112	114	117	119	122	125	128	131	135
45	94	96	98	100	102	104	106	108	110	113	115	118	120	123	126	129
40	92	94	96	97	99	101	103	105	107	109	111	113	116	118	121	123
35	91	92	94	95	97	98	100	102	104	106	107	109	112	114	116	118
30	89	90	92	93	95	96	98	99	101	102	104	106	108	110	112	114
Note: Exposure to full sunshine can increase HI values by up to 15° F

For the more ambitious who would like to calculate the heat index, here is the formula:

[Heat Index formula]

where: HI = heat index
T = temperature (° F)
RH = relative humidity (%)


where:		HI = heat index T = temperature (° F) RH = relative humidity (%)

Wind Chill Chart

How cold is it outside? Simply knowing the temperature doesn't tell you enough about the conditions to enable you to dress sensibly for all winter weather. Other factors including wind speed, relative humidity and sunshine play important roles in determining how cold you feel outside. A description of the character of weather known as "coldness" was proposed about 1940 by scientists working in the Antarctic. The "wind chill index" as developed to describe the relative discomfort/danger resulting from the combination of wind and temperature.

The wind chill index describes an equivalent temperature at which the heat loss from exposed flesh would be the same if the wind were near calm. For example, a wind chill index of -5 indicates that the affects of wind and temperature on exposed flesh are the same as if the air temperature were 5 degrees below zero eventhough the actual temperature is much higher.

The importance of the wind chill index is as an indicator of how to dress properly for winter weather. (Wind chill does not affect your car's antifreeze protection, freezing of water pipes, etc.) In dressing for cold weather an important factor to remember is that entrapped insulating air warmed by body heat is the best protection against the cold. Consequently, wear loose-fitting, lightweight, warm clothing in several layers. Outer garments should be tightly-woven, water-repellant and hooded. Mittens snug at the wrist are better protection than fingered gloves.

To use the chart, find the approximate temperature on the top of the chart. Read down until you are opposite the appropriate wind speed. The number which appears at the intersection of the temperature and wind speed is the wind chill index.

**Wind Chill Chart**
Wind (mph)	Temperature (° F)
Wind (mph)	35	30	25	20	15	10	5	0	-5	-10	-15	-20	-25
5	32	27	22	16	11	6	0	-5	-10	-15	-21	-26	-31
10	22	16	10	3	-3	-9	-15	-22	-27	-34	-40	-46	-52
15	16	9	2	-5	-11	-18	-25	-31	-38	-45	-51	-58	-65
20	12	4	-3	-10	-17	-24	-31	-39	-46	-53	-60	-67	-74
25	8	1	-7	-15	-22	-29	-36	-44	-51	-59	-66	-74	-81
30	6	-2	-10	-18	-25	-33	-41	-49	-56	-64	-71	-79	-86
35	4	-4	-12	-20	-27	-35	-43	-52	-58	-67	-74	-82	-92
40	3	-5	-13	-21	-29	-37	-45	-53	-60	-69	-76	-84	-92
Wind speeds above 40 mph have little additional chilling affect

In using the table above, values of wind chill below -10° F are considered bitterly cold. Values of wind chill below -20° F are extremely cold -- human flesh will begin to freeze within one minute!

If you would like to calculate the wind chill index for combinations of temperature and wind other than those given in the table above, you can use the formula:

WC = 91.4 - (0.474677 - 0.020425 * V + 0.303107 * SQRT(V)) * (91.4 - T)

where: WC = wind chill index
V = wind speed (mph)
T = temperature (° F)

WC = 91.4 - (0.474677 - 0.020425 * V + 0.303107 * SQRT(V)) * (91.4 - T)
where:		WC = wind chill index V = wind speed (mph) T = temperature (° F)