Heat loss and heat gain are important to human comfort. The concepts of "Heat Index" and "Wind Chill" provide indexes of two important aspects of heat balance. Heat or "energy" must be balanced because a person cannot long remain comfortable nor even long endure an out-of-balance environment. In other words, if you are becoming hotter by exercising or sitting in the sun, you must balance the heat gain by giving off more heat.

Heat index is a term that you may or may not have heard. Air temperature is the normal or usual measure of the atmospheric condition that influences our comfort, our well-being, and our happiness. Temperature is not the only factor that determines our comfort. In some cases, temperature is not even the primary environmental factor determining our comfort.

The heat index has various names. Sometimes it is called "apparent temperature." Sometimes it is called "humidity index." Occasionally it is referred to as the "temperature humidity index." Although all of these terms are apparently interchangeable, there are some intended differences by those who originated the terms. Apparent temperature normally refers to the measure of human comfort or discomfort due to the combination of humidity and temperature (Table 6.1).

Table 6.1 Heat stress index

The apparent temperature index (Table 6.1) is a measure of human comfort or discomfort due to the combined effects of heat and humidity. It was designed in 1979 by Dr. R. G. Steadman. The Heat Index existed prior to 1960, however, the 1979 version is the one announced regularly over the radio or TV during hot weather. Often it appears as a chart (Table 6.2).

Table 6.2 Apparent temperature index

The chart has the actual air temperature on the left with relative humidity across the top. Apparent temperatures and level of danger categories are indicated along the chart. If, for example, the temperature is actually 100°F (37.8°C) and the relative humidity is 60%, it will feel like 130°F (54.4°C). The heat index table gives values based on humidities from 10% to 90% and air temperatures from 65°F (18.3°C) to 120°F (48.9°C).

If the relative humidity is at 90% and the air temperature is at 80°F (26.7°C), the apparent temperature or the heat index is 88. In other words, it feels considerably warmer than the reported actual temperature. This is based on a neutral situation, assuming that at 80°F (26.7°C), 40% humidity would be typical. Conditions of 80°F (26.7°C) and 40% humidity have no impact on apparent temperature. If the humidity is very dry, perhaps 10%, the apparent temperature is cooler than the actual temperature of the air. In Idaho, Nevada, Utah, or most western states the apparent temperature will usually be cooler than the actual air temperature. In the Midwest, on average the true temperature and the apparent temperature are the same because during the heat of the day the mid-day humidity is usually near 40%. The exception is during extremely high temperatures or extremely humid periods.

Study Question 6.1
What is the apparent temperature if the air temperature is 80°F and relative humidity 20%?
Study Question 6.2
What is the apparent temperature if the air temperature is 90°F and relative humidity 65%?