def get_cd(r_n_metric, os):
    """
    Reference page number: 5
    Parameters
    ------------------------------
    r_n_metric: (``float``)
        Solar radiation in W/m^2
    os: (``bool``)
        Boolean which indicates whether to calculate G for short reference or tall reference
    Returns
    ------------------------------
    cd: (``float``)
        Denominator constant
    """
    cd = None
    daytime = r_n_metric > 5

    if os:
        if daytime > 5:
            cd = 0.24
            return cd
        else:
            cd = 0.96
            return cd
    else:
        if daytime > 5:
            cd = 0.25
            return cd
        else:
            cd = 1.7
            return cd