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<T[Deg.K]<maxT.*/
function PVS(T) {if (T<minT || T>maxT) return NaN;else if (T<C_OFFSET)return pvsIce(T);elsereturn 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);
}