// Calculate the "e ^ (-mgh/RT)" term for pressure conversions:
static double calcPressureTerm(float tempF, float elevationFT)
{
double exponent;
double elevMeters = (double)wvutilsConvertFeetToMeters(elevationFT);
double tempKelvin = (double)wvutilsConvertFToC(tempF) + 273.15;
// e ^ -elevMeters/(tempK * 29.263)
exponent = (-elevMeters);
// degrees Kelvin (T)
exponent /= (tempKelvin * 29.263);
// e ^ (-mgh/RT)
exponent = exp(exponent);
return exponent;
}
// calculate altimeter pressure from station pressure
float wvutilsConvertSPToAltimeter (float SPInches, float elevationFT)
{
double magicEXP = 0.190284;
double inverseMagicEXP = 1 / magicEXP;
double elevMeters = (double)wvutilsConvertFeetToMeters(elevationFT);
double stationPressureMB = (double)wvutilsConvertINHGToHPA(SPInches);
double constantTerm;
double variableTerm;
double tempdouble;
double altimeter;
// Formula used: http://www.wrh.noaa.gov/slc/projects/wxcalc/formulas/altimeterSetting.pdf
// calculate the constant term
constantTerm = pow (1013.25, magicEXP);
constantTerm *= 0.0065;
constantTerm /= 288;
// calculate the variable term
tempdouble = stationPressureMB - 0.3;
tempdouble = pow (tempdouble, magicEXP);
variableTerm = elevMeters / tempdouble;
// compute main term
tempdouble = constantTerm * variableTerm;
tempdouble += 1;
tempdouble = pow (tempdouble, inverseMagicEXP);
// compute altimeter
altimeter = stationPressureMB - 0.3;
altimeter *= tempdouble;
// finally, convert back to inches
altimeter *= 0.0295299;
return (float)altimeter;
}
