Question: What is Heat Index and How Can I Measure It?
By Test For Admin | February 26, 2010
Next we have an interesting question from Jan, a jogger planning to move to the South…
“Once again my family is moving because of my husband’s career in the military. This time he’s getting stationed at a base in South Carolina. This will be my first time ever living in the South and all my friends tell me I’m going to DIE in the Summer when I go for my daily jog. They say the ‘heat index’ is going to wipe me out. Can you tell me what that is and how I can measure it? Thank you. Jan.”
Your friends have obviously have your best interest at heart, Jan. The South has extremely hot temperatures in the Summer and packs an extra punch with its brutal humidity levels. Most people working outdoors or exercising outdoors in the Summer down here know to keep themselves hydrated and watch for signs of heat exhaustion.
With regard to your question about ‘heat index’,
“The heat index (HI) is an index that combines air temperature and relative humidity in an attempt to determine the human-perceived equivalent temperature — how hot it feels, termed the felt air temperature. The human body normally cools itself by perspiration, or sweating, which evaporates and carries heat away from the body. However, when the relative humidity is high, the evaporation rate is reduced, so heat is removed from the body at a lower rate causing it to retain more heat than it would in dry air.” ( source )
In a nutshell, heat index refers to what temperature a person’s body THINKS it is because of the actual temperature AND relative humidity. Therefore to measure heat index, one must accurately measure both temperature and humidity.
With both numbers in hand, one can look up the heat index on the appropriate chart… like the one below:
| 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) |
For those times when you just can’t get to a computer, though, you can always use a Heat Index Monitor which will measure the temperature and humidity for you and then calculate the heat index — with the push of a button.
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