What is the heat index?
Our bodies dissipate heat by varying the rate of blood circulation, losing water through
the skin and sweat glands—and, as a last resort, by panting—when the blood is heated
above 98.6 degrees Fahrenheit (37 degrees Centigrade), the average body temperature.
Sweating cools the body through evaporation. You can get the same feeling when you
put alcohol on your skin, because as the alcohol evaporates, the skin is cooled.
The heat index (HI) is an index that combines air temperature and relative humid-
ity to estimate how hot it actually feels. It is based on a mathematical concept called
the heat index equation, a long equation that includes the dry air temperature, rela-
tive humidity (in percent form), and many biometeorological factors too long to list
here. The resulting heat index table represents the apparent, or “feels like,” tempera-
ture. For example, if the air temperature is 90 degrees Fahrenheit, with the relative
humidity at 60 percent, it will feel like 100 degrees Fahrenheit.
Why do meteorologists want people to pay attention to the heat index? The major
reason involves how the body responds to high heat-value numbers: If the relative
humidity is high, it curtails evaporation on the skin, and the body is unable to effectively
cool itself (and a person will perceive that the air is warmer). When heat index values
grow higher, conditions exceed the level a body can remove heat, causing the body tem-
perature to rise. This can cause heat-related illnesses, such as sunstroke or heat exhaus-
tion. For example, according to the United States National Weather Service, exposure to
direct sunlight can increase the HI by up to 15 degrees Fahrenheit (9.4 degrees Centi-
grade). And when a heat index between a mere 90 degrees Fahrenheit (32.2 degrees
Centigrade) to 105 degrees Fahrenheit (40.6 degrees Centigrade) can cause possible sun-
stroke, heat exhaustion, and heat cramps, it is easy to see the meteorologists’ concerns.
The following table shows how the heat we actually experience changes with tem-
perature and humidity (humidity is expressed as a percentage; temperatures are in
degrees Fahrenheit).The Heat Index
Actual °F 90% 80% 70% 60% 50% 40%
80°F 85°F 84°F 82°F 81°F 80°F 79°F
85°F 101°F 96°F 92°F 90°F 86°F 84°F
90°F 121°F 113°F 105°F 99°F 94°F 90°F
95°F 133°F 122°F 113°F 105°F 98°F
100°F 142°F 129°F 118°F 109°F
105°F 148°F 133°F 121°F
110°F 135°FAccording to the National Weather Service, sunstroke, heat cramps, and heat
exhaustion are possible above 90 degrees Fahrenheit; temperatures above 105 degrees
can also lead to heat stroke; and above 130 degrees heat stroke is likely if exposure to
306 such temperatures is prolonged.