John W. Pepi *
Maynard, MA
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1.       ABSTRACT

A new Summer Simmer Index is presented which provides a proven indicator of heat stress concerns and discomfort using meaningful equivalent temperature values for general public acceptance and awareness.  Based on sound, scientific principles, it is confirmed by independent physiological models and backed up by hundreds of tests on human individual subjects.

A review of existing literature over the past 75 years is made along with a summary of the various comfort indexes, with shortfalls noted.  The new Summer Simmer Index (SSI) is then presented. It is derived from studies by the American Society of Heating and Refrigeration Engineers (ASHRAE) and confirmed by tests and analyses done at Kansas State University.  Unlike the index derived from those studies, however, it meets all subjective and objective requirements while relating to a dry environment.  As such, the new SSI is the only temperature-humidity index that uses the results of proven physiological models and human tests and can be related to a dry environment for acceptance by the general public.  By doing so the index, like the wind chill factor used during the winter, provides a meaningful and realistic temperature equivalent that can be used not only as an indication as to how hot it feels, but also as a readily identifiable warning for individuals subject to the physiological dangers of heat exposure.

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

E-mail: information@ssi



Summer Comfort indexes seem to come around every 10 to 20 years or so, starting over 75 years ago when Houghten and Yaglou first published the Effective Temperature (ET) scale (Houghten, 1923). But, first, let me give a little background on my own involvement.

Like most weather enthusiasts, I am an amateur in this field, although my background as a Mechanical engineer has allowed me to work on several weather satellite programs for both the National Oceanic and Atmospheric Association (NOAA) and the National Aeronautics and Space Agency (NASA).  Always interested in the weather, I became intrigued about comfort indexes when I first read about the Temperature Humidity Index (THI) developed for  the US Weather Bureau (now the National Weather Service (NWS)) (Thom, 1959). The THI was based on a discovery by an engineer from a utility company who noted a direct correlation of temperature and humidity with cooling degree requirements for air conditioning systems.  This was very well correlated to the ET scale introduced 36 years earlier.  The ET scale was based on testing by ASHRAE with trained individuals who were asked to give their reactions of discomfort  by passing from room to room of varying temperature and relative humidity conditions.  Equations relating temperature and relative humidity (or dew point) were developed for the THI which, like the ET, were based on a moist, 100 percent humidity environment.  One could then look up the index, and relate it to a level of comfort or discomfort.  For example, a temperature  of 90 degrees F coupled with a relative humidity of  50 per cent ( dew point  near 70 degrees F) yielded an index of  81F, what it would feel like in a moist, 100% humidity environment.  This lies in a zone in which most everyone would feel

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