def relative_sunshine_duration(n_act, n_max):
    """
    Calculate the ratio of the actual duration of sunshine to the maximum
    possible duration of sunshine or daylight hours (Allen et al. 1998).

    This ratio expresses the cloudiness of the atmosphere.

    Parameters
    ----------
    n_act : float
        Actual duration of sunshine (hours).
    n_max : float
        Maximum possible duration of sunshine (hours).

    Returns
    -------
    float
        Relative sunshine duration (-).

    """
    return n_act / n_max