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float calc_dewpoint(float t,float h){

double logex;

double dew;

logex=0.66077+7.5*t/(237.3+t)+ (log10(h)-2);

dew=(logex-0.66077)*237.3/(0.66077+7.5-logex);

return (float)dew;

}

计算出来的值：

温度：5°，湿度：10%RH， 露点：-24.2,

与专业软件VAISALA Humidity Calculator的结果：-21.7有一定的差距，原因是没有进行分段修正。

解决办法：在网上找了个连接，计算出来结果和VAISALA一致，JS脚本如下方代码所示。（我已参照JS转换成C代码，用于ARM的设备里，经测试，0度以上与VAISALA软件完全一致，0度以下有一些偏差，可自行通过修订解决。C代码下载地址：https://download.csdn.net/download/shjhuang/13664809 ）

var minT = 173; // -100 Deg. C.

var maxT = 678;

/*

* Saturation Vapor Pressure formula for range -100..0 Deg. C.

* This is taken from

* ITS-90 Formulations for Vapor Pressure, Frostpoint Temperature,

* Dewpoint Temperature, and Enhancement Factors in the Range 100 to +100 C

* by Bob Hardy

* as published in "The Proceedings of the Third International Symposium on Humidity & Moisture",

* Teddington, London, England, April 1998

*/

var k0 = -5.8666426e3;

var k1 = 2.232870244e1;

var k2 = 1.39387003e-2;

var k3 = -3.4262402e-5;

var k4 = 2.7040955e-8;

var k5 = 6.7063522e-1;

function pvsIce(T) {

lnP = k0/T + k1 + (k2 + (k3 + (k4*T))*T)*T + k5*Math.log(T);

return Math.exp(lnP);

}

/**

* Saturation Vapor Pressure formula for range 273..678 Deg. K.

* This is taken from the

* Release on the IAPWS Industrial Formulation 1997

* for the Thermodynamic Properties of Water and Steam

* by IAPWS (International Association for the Properties of Water and Steam),

* Erlangen, Germany, September 1997.

*

* This is Equation (30) in Section 8.1 "The Saturation-Pressure Equation (Basic Equation)"

*/

var n1 = 0.11670521452767e4;

var n6 = 0.14915108613530e2;

var n2 = -0.72421316703206e6;

var n7 = -0.48232657361591e4;

var n3 = -0.17073846940092e2;

var n8 = 0.40511340542057e6;

var n4 = 0.12020824702470e5;

var n9 = -0.23855557567849;

var n5 = -0.32325550322333e7;

var n10 = 0.65017534844798e3;

function pvsWater(T) {

var th = T+n9/(T-n10);

var A = (th+n1)*th+n2;

var B = (n3*th+n4)*th+n5;

var C = (n6*th+n7)*th+n8;

var p = 2*C/(-B+Math.sqrt(B*B-4*A*C));

p *= p;

p *= p;

return p*1e6;

}

/**

* Compute Saturation Vapor Pressure for minT

*/

function PVS(T) {

if (TmaxT) return NaN;

else if (T

return pvsIce(T);

else

return pvsWater(T);

}

/**

* Compute dewPoint for given relative humidity RH[%] and temperature T[Deg.K].

*/

function dewPoint(RH,T) {

return solve(PVS, RH/100*PVS(T), T);

}

/**

* Newton's Method to solve f(x)=y for x with an initial guess of x0.

*/

function solve(f,y,x0) {

var x = x0;

var maxCount = 10;

var count = 0;

do {

var xNew;

var dx = x/1000;

var z=f(x);

xNew = x + dx*(y-z)/(f(x+dx)-z);

if (Math.abs((xNew-x)/xNew)<0.0001)

return xNew;

else if (count>maxCount) {

xnew=NaN;

throw new Error(1, "Solver does not converge.");

break;

}

x = xNew;

count ++;

} while (true);

}