def ea_from_rhmax(e_tmin, rh_max):
    """
    Calculates actual vapour pressure [kPa] from maximum relative humidity
    using FAO equation (18).

    Arguments:
    e_tmin  - saturation vapour pressure at daily minimum temperature [kPa]
    rh_max  - maximum relative humidity [%]
    """
    # Raise exceptions:
    if (rh_max < 0 or rh_max > 100):
        raise ValueError, 'RH_max=%g is not in range 0-100' % rh_max

    return e_tmin * (rh_max / 100.0)