John W. Pepi *
Maynard, MA
(page 4)


results are markedly well correlated to the ET* scale.

In 1979, an ingenious work by Steadman was published (Steadman, 1979) , leading to the Apparent Temperature scale.  This index has been adopted by the National Weather Service  as its standard and is widely used in national  forecasts under the so-called Heat Index title.  The index, like ET*, corrects for the deficiencies of earlier indexes, but uses a somewhat different and complex physiological model, based on body heat transfer and moisture transfer to the environment, accounting for skin and clothing thermal resistance.  The model is part of a general model which includes the effects of wind, solar radiation, and atmospheric pressure as well.  The apparent temperature uses a constant vapor pressure as its base, corresponding to a dew point of 57F, an average value experienced in textile testing laboratories in the US.  While this relates to a constant moisture environment, that base is an average environment which is neither dry nor moist.  For the cited example of 90F, 50% humidity, the index reads 94F.  For most parts of the country, in general, the index does not read significantly higher than the air temperature.  This is important to note if the public is to be become aware of the dangers of heat stress.  For example, if the wind chill factor were related to a windy   environment rather than a calm one, it would soon be relegated to a curiosity factor rather than an indicator of the danger of frost bite. ( For example, suppose the temperature in winter is 10F and the wind is 25 mph.  The wind chill equivalent temperature is 29F (29 degrees below zero).  If  the wind chill factor were written as an effective temperature at 45 mph instead of a calm environment, the equivalent temperature would read 15F (15 degrees above zero), warmer than the actual temperature, and impossible to relate to.)


Enter, finally, the new Summer Simmer Index, the subject of this article. In moderate temperature and humidity levels, most of the indexes sited above are fairly accurate, although all use a different base. 

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

E-mail: information@ssi


For indexes which are accurate at all temperature and humidity levels, however, one must turn to ET* or the HI, both based on the most physiologically sound principles.  The author has chosen the ET* scale in order to derive this new index for a dry environment.  

This was done since it is based on 75 years of continuous research, is backed by tests of hundreds of individuals, and uses the results of regulatory sweating along with the equations of heat balance which relate metabolism, radiation, convection, conduction, clothing resistance, and evaporation.  Confirmed by subjective data, it eliminates the fears of those who do not like theory; using objective scientific and physiological theory, it eliminates the concerns of those who not like subjective data. Further, the index is derived from the standpoint of comfort and heat stress, rather than  simply thermal sensation.  It corrects for exaggeration of humidity at moderate temperatures by  correlating skin wettedness to mean skin temperature and humid operative temperature, well known predictors of discomfort using human tests and exact science of heat transfer for both sensible and insensible perspiration.  And, importantly, it corrects for underestimates of the effects of high humidity, using skin wettedness to predict heat stress and discomfort (rather than humid operative temperature and skin temperature), since this factor plays the most important role when the environment is unable to keep up with the body regulatory sweating, as blood flow and heart action increase.

Since all indexes refer to different moisture environments, I have reformulated the equations and charts of the ET* scale and related them to a constant dry environment.  This would give a much better public acceptance as to how hot it feels.  I chose a dew point of 35F as the dry equivalent base, for several reasons.  First, the environment had to be realistically dry, as could commonly be experienced in these United States.  Second, while dew points lower than 35F are common at lower summer temperature levels, the studies referenced above show such lower humidity effects inconsequential at these temperatures. 

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