from math import exp, log as ln


def _rh_to_dewpoint_ardenbuck(rh, tc, b=18.678, c=257.14, d=234.5):
    """
    See https://en.wikipedia.org/wiki/Dew_point#Calculating_the_dew_point
    b = 18.678, c = 257.14 °C, d = 234.5 °C.
    """

    gam_m = ln((rh/100.)*exp((b - tc/d)*(tc/(c + tc))))
    return c*gam_m / (b - gam_m)