from - http://arduino.cc/playground/Main/DHT11Lib - the NOAA reference algorithm and the fast variation

// dewPoint function NOAA

// reference: http://wahiduddin.net/calc/density_algorithms.htm

double dewPoint(double celsius, double humidity)

{

double A0= 373.15/(273.15 + celsius);

double SUM = -7.90298 * (A0-1);

SUM += 5.02808 * log10(A0);

SUM += -1.3816e-7 * (pow(10, (11.344*(1-1/A0)))-1) ;

SUM += 8.1328e-3 * (pow(10,(-3.49149*(A0-1)))-1) ;

SUM += log10(1013.246);

double VP = pow(10, SUM-3) * humidity;

double T = log(VP/0.61078); // temp var

return (241.88 * T) / (17.558-T);

}

// delta max = 0.6544 wrt dewPoint()

// 5x faster than dewPoint()

// reference: http://en.wikipedia.org/wiki/Dew_point

double dewPointFast(double celsius, double humidity)

{

double a = 17.271;

double b = 237.7;

double temp = (a * celsius) / (b + celsius) + log(humidity/100);

double Td = (b * temp) / (a - temp);

return Td;

}