The temperature-humidity index, also known as the discomfort index or effective temperature, is a quantity that gauges the effect of summer conditions on human comfort. It is used to predict how much power will be needed to run air-conditioning systems. Several different equations have been used to evaluate the temperature-humidity index (THI), which take into account readings of temperature and humidity, dependent on the data available. One such equation is

 

    THI = 0.4(T₁ + T₂) + 15

 

where T₁ is the dry-bulb temperature (the temperature of the air as measured by a standard thermometer ) and T₂ the wet-bulb temperature in degrees Fahrenheit (see hygrometer). When the index is 70 almost everyone feels comfortable; at 80 or more, no one.