This code is syntactically incorrect. I simplified it a little and added the necessary braces at the end, although I don't know if this calculates the correct value (I haven't studied the wikipedia article).

float tempC = 25.7;

float temp = (tempC -32) * 5 / 9;

float RH = 35.9;

float dewpoint = (237.7*(17.271*temp)/(237.7*temp+log(RH/100))/(17.271-(17.271*temp)/(237.7*temp+(log(RH/100)))));

This at least compiles. I took out the recurring calculation of the temperature in Fahrenheit and made it before the dewpoint calculation.

For this formula, the result I get is 0.00.

I used this formula, but I can not confirm if the values are correct.

float aux = (log(RH / 100) + ((17.27 * temperatureC) / (237.3 + temperatureC))) / 17.27;

float dewpoint = (237.3 * aux) / (1 - aux);

The values that I'm getting are:

24.22 *C 75.59 *F 43.69 %RH 11.09 dew point *C 18.66 Humidex

Can you confirm if I'm right?

You may also want to try:

const float

tempC = 25.7,

temp = (tempC - 32) * 5/9,

logRH = -1.024433, // ln(35.9/100)

a = 17.271,

b = 237.7,

y = a*temp/(b + temp) + logRH,

Td = b*y/(a - y);

In this calculation, it seems to me you were to ignore the value of the logarithm of %RH.

But I'm not sure.

thanks for your help,