#define KELVIN          273.15
#define E               2.71828182845904523536

double psv_ASHRAE(double T)
{

// over ice
#define	ASH_C1	-5.6745359e+3
#define	ASH_C2	 6.3925247
#define	ASH_C3	-9.6778430e-3
#define	ASH_C4	 6.2215701e-7
#define	ASH_C5	 2.0747825e-9
#define	ASH_C6	-9.4840240e-13
#define	ASH_C7	 4.1635019

// over water
#define	ASH_C8	-5.8002206e+3
#define	ASH_C9	 1.3914993
#define	ASH_C10	-4.8640239e-2
#define	ASH_C11  4.1764768e-5
#define	ASH_C12	-1.4452093e-8
#define	ASH_C13	 6.5459673

	double lnpsw;

        T += KELVIN;

	if (T<KELVIN)                   // over ice
		lnpsw = ASH_C1/T
			+ ASH_C2
			+ ASH_C3*T
			+ ASH_C4*T*T
			+ ASH_C5*T*T*T
			+ ASH_C6*T*T*T*T
			+ ASH_C7*log(T);
	else		                // over water
		lnpsw = ASH_C8/T
			+ ASH_C9
			+ ASH_C10*T
			+ ASH_C11*T*T
			+ ASH_C12*T*T*T
			+ ASH_C13*log(T);

	return	pow(E,lnpsw);
}

// Peppers, 1998 (ASHRAE)
double dewPoint_ASHRAE(double T, double Rh)
{

// over water
#define ASH_C14 6.54
#define ASH_C15 14.526
#define ASH_C16 0.7389
#define ASH_C17 0.09486
#define ASH_C18 0.4569

//over ice
#define ASH_C19 6.09
#define ASH_C20 12.608
#define ASH_C21 0.4959

        double  al,pw,pws;

        pws     = psv_ASHRAE(T)/1000;
 //kPa
        pw      = (Rh/100)*pws;
        al      = log(pw);
        if (T < 0 )
                return ASH_C19
                        + ASH_C20*al
                        + ASH_C21*al*al;
        else
                return ASH_C14
                        + ASH_C15*al
                        + ASH_C16*al*al
                        + ASH_C17*al*al*al
                        + ASH_C18*pow(pw,0.1984);


}