The NEW SUMMER SIMMER INDEX -

A COMFORT INDEX FOR THE NEW MILLENNIUM

 John W. Pepi *
Maynard, MA
(page 3)

 

to Hevenerís index, lending it further credibility.  However, like those indexes, it is an index relating to a moist, near 100% humidity environment; for the example of 90F and 50% humidity, the Humiture reads an index of 82F. And, of course, like the THI and ET, the Humiture exaggerates humidity effects at low temperatures and underestimates humidity effects at high temperatures.  Hevener first published his index (for public acceptance) as the average of temperature and relative humidity(rather than wet bulb temperature).  The same index in this form reads 70F; because it relates to a relative term that could be moist or dry depending on temperature, it has no logical meaning.

In 1960, noting deficiencies in the THI and Humiture, Lally and Watson (Lally, 1960) published a new index, also called Humiture, with the permission of Mr. Hevener.  This index corrects for the underestimation of humidity at high temperatures afforded by the earlier indexes.  The model is based on skin to atmosphere moisture differentials in the form of vapor pressure.  This pressure rises in a non linear way as the dew point approaches skin temperature, increasing at a higher rate.  Importantly, the authors recognized the need (for public acceptance) to relate the index to a dry environment, not  a moist one, and chose a dew point of 45F as the base level.  For our cited example of 90F and 50% humidity, the index reads 104F.  It is calculated by simple addition of air temperature in Fahrenheit and saturation vapor pressure in millibars, less 10F.  Based on sounder physiological grounds than the other indexes, and certainly more meaningful, the index still suffered from exaggerated effects of humidity at temperatures 90F and below, and subject to further scientific scrutiny.  

In 1979, also under the name of Humiture, Winterling (Winterling, 1979) published an index very similar to Lallyís, except he related it to a more moist environment , a dew point of 65F.  His equation subtracts 21F from the temperature and pressure addition described above (written for Floridians, they are more acclimatized to humid environments).  An approximation to the formula for temperatures in excess of



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

E-mail: information@ssi

 

95F was developed as the addition of air temperature  plus dew point minus 65, for values in Fahrenheit.  Like the other indexes, again, it suffered from exaggeration of humidity at lower temperatures.  It was widely used in both local and national forecasts, but again, relating to a moist environment, was an index hard to relate to in terms of how hot it feels for most people.

3.3 Category 3.

In 1971, recognizing the deficiencies of the ET scale, Gagge et al (op. cit.) , published the new Effective Temperature scale, called ET* (ASHRAE, 1972)..  This important work was based on regulatory sweating and skin wettedness and considered the physiological effects of body core pressure, thermal neutrality, thermal equilibrium, skin temperature, skin blood flow, convection, radiation, and conduction through clothing.  Zones of comfort and discomfort and effects of physiological heat stress, including heat exhaustion, heatstroke, and extreme danger, were developed.  The index rises rapidly at high temperatures and humidities until maximum skin wettedness is reached, while climbing more slowly with humidity at low temperatures.  Partly to allow for general public acceptance, the index chooses a 50% relative humidity as its base, rather than the 100% value used in the original ET.  Unfortunately, relative humidity is indeed relative, and a 50% value is quite moist at high temperatures and quite dry at low temperatures (there ought to be a campaign to stamp out relative humidity, one of the most widely misused concepts in history.).  Thus the index relates to a rather variable moisture environment.  For our cited example of 90F and 50% humidity, the index reads 90F.

In spite of the drawback of the index in terms of public acceptance, it nonetheless provided realistic data based on scientific principles.  Studies done  on human subjects at Kansas State University (Rohles, 1975) validated subjectively the numerical results of ET*.  Over 1600 college aged individuals were asked to record their sensations  of discomfort for various temperature and humidity combinations after an exposure of one hour or more.  The

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