Temperature-humidity index

From Glossary of Meteorology

temperature–humidity index

(Abbreviated THI; also known as discomfort index, effective temperature.) An index to determine the effect of summer conditions on human comfort, combining temperature and humidity.

Several equations have been used to calculate the index, dependent on the data availability:
where Td is the dry-bulb temperature in °F, Tw is the wet-bulb temperature in °F, Tdp is the dewpoint temperature in °F, and RH is the relative humidity in percent. In the equation, RH is used as a decimal; in other words, 50% relative humidity is indicated as 0.50. Studies have shown that relatively few people in the summer will be uncomfortable from heat and humidity while THI is 70 or below; about half will be uncomfortable when THI reaches 75; and almost everyone will be uncomfortable when THI reaches 79. There are portions of the United States in which the THI has reached values around 90.