def total_cond(iq, cc, bg, wv):
    """
    Returns a weighting factor representative of the quality of conditions required to execute the observation.

    twcond = (1./cond['iq'])**3 + (1./cond['cc'])**3 + (1./cond['bg'])**3 + (1./cond['wv'])**3

    Parameters
    ----------
    iq : float
        observation image quality constraint percentile

    cc : float
        observation cloud condition constraint percentile

    bg : float
        observation sky background constraint percentile

    wv : float
        observation water vapour constraint percentile

    Returns
    -------
    twcond : float
        total conditions weight
    """
    return (1./iq)**3 + (1./cc)**3 + (1./bg)**3 + (1./wv)**3