Temperature-Humidity Index, or THI, a number used to indicate the discomfort caused by the combined effects of heat and moisture in the air. The formula used to calculate the index is designed to yield a value ranging from 70 to 80. In general, almost all persons are comfortable when the index is 70, and almost no one is when it is 80. In 1985, the National Weather Service officially adopted the Heat Index to replace the THI, but in some areas the THI is still given with summer weather data.