Calculating the heat index requires the temperature to be in Fahrenheit, so if working in Celsius, you need to convert it first: F = (C x 1.8) + 32

To convert Fahrenheit back to Celcius: C = (F - 32) / 1.8

Source of the formula to calculate heat index:

Calculator

Formula

The Calculator webpage mentions that heat index results below 80 may be meaningless if outside the range of the chart, so if it calculates a heat index below 80 then I just return the original temperature and ignore the heat index.

I've spot-checked the calculations in my code against their online calculator and they all came back the same. One slight difference is that I am using decimals (fractions) with the temperature that I send to the sub and the online calculator works with whole numbers (rounding up or down).

Wind speed can also be a factor in calculating the heat index, but I do not have that data in my project nor do I know how to factor that into the formula.

B4X:

```
Private Sub HeatIndex (dTempF As Double, dHum As Double) As Double
'Source: http://www.wpc.ncep.noaa.gov/html/heatindex_equation.shtml
Dim HI As Double, Adjustment As Double
'First get simplified heat index to see if it is at 80 F or higher
'HI = 0.5 * {T + 61.0 + [(T-68.0)*1.2] + (RH*0.094)}
HI = .5 * (dTempF + 61 + ((dTempF - 68) * 1.2)) + (dHum * .094)
'Per the equation and chart, if heat index is below 80 then calculating the heat index
'could produce meaningless results
If HI < 80 Then Return dTempF
'If heat index is 80 or higher, do a more thorough calculation
'HI = -42.379 + 2.04901523*T + 10.14333127*RH - .22475541*T*RH
' - .00683783*T*T - .05481717*RH*RH + .00122874*T*T*RH
' + .00085282*T*RH*RH - .00000199*T*T*RH*RH
HI = -42.379 + (2.04901523 * dTempF) + (10.14333127 * dHum) - (.22475541 * dTempF * dHum) _
- (.00683783 * dTempF * dTempF) - (.05481717 * dHum * dHum) + (.00122874 * dTempF * dTempF * dHum) _
+ (.00085282 * dTempF * dHum * dHum) - (.00000199 * dTempF * dTempF * dHum * dHum)
'Adjustments
'If the RH is less than 13% and the temperature is between 80 and 112 degrees F,
'then the following adjustment is subtracted from HI:
'Adjustment = [(13-RH)/4]*SQRT{[17-Abs(T-95.)]/17}
If dHum < 13 And dTempF > 79.9 And dTempF < 112.1 Then
Adjustment = ((13 - dHum) / 4) * Sqrt((17 - Abs(dTempF - 95)) / 17)
HI = HI - Adjustment
Return HI
End If
'If the RH is greater than 85% and the temperature is between 80 and 87 degrees F,
'then the following adjustment is added to HI:
'Adjustment = [(RH-85)/10] * [(87-T)/5]
If dHum > 85 And dTempF > 79.9 And dTempF < 87.1 Then
Adjustment = ((dHum - 85) / 10) * ((87-dTempF) / 5)
HI = HI + Adjustment
Return HI
End If
'If we get here, then no adjustments are necessary so return what we came up with in the
'long equation above
Return HI
End Sub
```