John W. Pepi *
Maynard, MA
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uncomfortable.  I wondered why an index would relate to a moist rather than dry environment, since the index was always lower than the actual temperature and hard to relate to.  No one goes out on a hot, humid 90 degree day and says “gee, it feels like 81 today.” In order to correct this perception, I took the equations developed for the THI and rewrote them for a dry environment at a relative humidity of 10 percent, as experienced often in the desert regions of the southwestern United States.  The new index, which I dubbed the Summer Simmer Index (SSI), now read 103 degrees at the example temperature of 90 F and humidity of 50% cited above - what it would feel like in a dry environment - rather than the 81F equivalent of the THI - what it would feel like in a moist environment.  It seemed like the time had come to publicize this data twenty years later, when the wind chill index was finally gaining popularity.  As the SSI relates to a dry environment, not a moist one, wind chill temperature relates to a calm environment, not a windy one.  I published the SSI in  a  summer edition of Weatherwise (Pepi, 1987).

Immediately after publication, I received many letters and phone calls from around the country and some from across the world.  A Florida radio station used the index during the summer of 1987 and a California station conducted an interview.  I received calls from professors at Kansas State University, who had developed a comfort index using college-aged individuals, and were interested in this approach.  They had been working with ASHRAE as well, who had developed a new effective temperature index (ET*) based on new physiological models (Gagge, 1971).   I received an additional letter noting works published earlier by others (under the title of Humiture) which gave similar results to the SSI.   At the same time, the Heat Index (HI) (or Apparent Temperature (AT)), was gaining popularity, and had become the norm adopted by the NWS.  All of these indexes were giving different numbers for identical combinations of humidity and temperature.  Which were correct? Which were sound? Which were based on dry environments, which on moist?  Swamped with data, I decided to

*Corresponding author address: John W. Pepi, 4 Nick Lane, Maynard, MA 01754;

E-mail: information@ssi


research the available literature more fully.  It would be prudent to review these other indexes based on that literature search, which lead to the new Summer Simmer Index described herein.


It might be best to categorize the various indexes in terms of similarity which are not necessarily chronological.  The first category lumps together the ET, THI, and SSI indexes; the second group involves Humiture in four different forms; the last involves the new ET (ET*), the Apparent Temperature(AT)(or Heat Index, HI), and the new SSI.

3.1 Category 1.

The Effective Temperature (1923), Temperature Humidity Index (1959) and Summer Simmer Index (1987) have been discussed above. Based on human subject tests for short periods and confirmed by cooling degree data, both the ET and virtually identical THI  were subjected to arguments that physiological principles were not considered.  The SSI eliminated the public perception of these indexes by relating to a dry environment, but would still be subject to the same arguments on a scientific basis.  While these indexes are rather accurate at moderate temperature and humidity levels, further efforts (Yaglou, 1947)revealed that the effects of humidity at low and moderate  temperatures  were exaggerated while the effects of humidity at higher temperatures were underestimated. This was due, in part, to instant reactions rather than prolonged subject exposure; in part, to heavier clothing; and, in part, to a lack of understanding of human regulatory response and heat flow parameters.  

3.2 Category 2.

Humiture was first coined as a word in 1937 by Osborn Fort Hevener (Hevener, 1959) in an attempt to combine the effects of temperature and humidity as to how hot one feels.  The index was published in 1946, and is computed by taking the arithmetic average of the dry bulb and wet bulb temperature. The equations developed for the THI and the ET were closely matched  

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