The Heat Index compared to the old Temperature Humidity Index (THI)

Weather Blog

Dear Tom,
Today, it’s the heat index. When I was growing up, the term was THI. Are they the same?
—Bill Service
Dear Bill,
They are not. THI stood for Temperature-Humidity Index, which was used in the 1960s to quantify discomfort using temperature and dew point. An index of 70 or lower meant everyone was comfortable. At 71, 10% of the populace was deemed uncomfortable, 50% at 75 and 100% at 80. When the THI hit 84, federal workers in non-air conditioned offices were dismissed. The current heat index was introduced in the early 1980s, replacing the THI. It is a single, “apparent or feels like temperature” —how hot the air feels at a given temp and dew point or relative humidity. During the city’s tragic 1995 heatwave, respon- sible for more than 750 fatalities, the heat index at Midway Airport hit a city-record 125 on July 13 with a temp of 106 and a dew point of 81.

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