def rhum(dew, temperature):
    """
    Calculate relative humidity from dew temperature and ambient temperature.

    This uses the range of constants which yields results accurate in the range
    -20 to 50 Celsius.

    Parameters
    ----------
    dew : ndarray
        Dew point temperature (Celsius).
    temperature : ndarray
        Ambient temperature (Celsius).

    Returns
    -------
    rhum : ndarray
        Relative humidity (%).

    References
    ----------
    http://www.vaisala.com/Vaisala%20Documents/Application%20notes/Humidity_Conversion_Formulas_B210973EN-F.pdf

    """
    m = 7.59138
    Tn = 240.7263
    rhum = 100 * 10 ** (m * (dew / (dew + Tn) - temperature / (temperature +
        Tn)))
    return rhum
