Gill Instruments Ltd
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MaxiMet
Page 137
Issue 3
Doc. No. 1957-PS-021
May 2018
HEATIDX -
Heat Index
What the temperature feels like to the human body when relative humidity is combined with the air
temperature (also known as apparent temperature).
Simple Heat Index is defined as:
HI s = 0.5 * {T + 61.0 + [(T-68.0)*1.2] + (RH*0.094)}
where: HI s = Simple Heat Index expressed as an apparent temperature in degrees F.
T = MaxiMet Temperature in degrees F.
RH = MaxiMet Relative Humidity in percent.
Rothfusz Heat Index is defined as:
HI r = -42.379 + 2.04901523*T + 10.14333127*RH - 0.22475541*T*RH - 0.00683783*T*T -
0.05481717*RH*RH + 0.00122874*T*T*RH + 0.00085282*T*RH*RH -
0.00000199*T*T*RH*RH
See
http://www.wpc.ncep.noaa.gov/html/heatindex_equation.shtml
where: HI r = Rothfusz Heat Index expressed as an apparent temperature in degrees F.
T = MaxiMet Temperature in degrees F.
RH = MaxiMet Relative Humidity in percent.
Rothfusz Adjustment is defined as follows:
If the Relative Humidity is less than 13% and the Temperature is above 80 and below 112 degrees
F (above 26.66 and below 44.44 °C):
R Adj = -[(13-RH)/4]*SQRT{[17-ABS(T-95.)]/17}
where: R Adj = Rothfusz Adjustment,
T = MaxiMet Temperature in degrees F,
RH = MaxiMet Relative Humidity in percent,
ABS and SQRT are the absolute value and square root functions, respectively.
If the Relative Humidity is greater than 85% and the Temperature is above 80 and below 87 degrees F
(above 26.66 and below 30.55 °C).
R Adj = [(RH-85)/10] * [(87-T)/5]
where: R Adj = Rothfusz Adjustment.
T = MaxiMet Temperature in degrees F.
RH = MaxiMet Relative Humidity in percent.
If Relative Humidity and Temperature conditions are outside the two bands defined above:
R Adj = 0
where: R Adj = Rothfusz Adjustment
When HI s
≥ (HI r + R Adj), MaxiMet Heat Index shall be:
HI = HI s
where: HI = MaxiMet Heat Index expressed as an apparent temperature in degrees F,
HI s = Simple Heat Index expressed as an apparent temperature in degrees F
When HI s < (HI r + R Adj), MaxiMet Heat Index shall be: HI = HI r + R Adj
where: HI = MaxiMet Heat Index expressed as an apparent temperature in degrees F,
HI r = Rothfusz Heat Index expressed as an apparent temperature in degrees F,
R Adj = Rothfusz Adjustment.
Result e.g. +028 (°C default).
Note:
Heat Index is only reported when the temperature is above 75°F (approximately 23.88°C) and
both Relative Humidity and Temperature are functioning.
