def pb(D_eq):
    """Pruppacher and Beard Drop Shape relationship model.

    Implementation of the Pruppacher and Beard Drop Shape model given in [1]
    . This gives the ratio of the semi minor to semi major axis.

    Parameters
    -----------
    D_eq: float
        Volume Equivalent Drop Diameter

    Returns
    -------
    axis_ratio: float
        The ratio of the major to minor axis.

    See Also
    --------
    tb : Thurai and Bringi DSR
    bc : Beard and Chuang DSR
    brandes: Brandes DSR

    References
    ----------
    ..[1] Pruppacher, H. R. and Beard, K. V. (1970), A wind tunnel
      investigation of the internal circulation and shape of water drops
      falling at terminal velocity in air. Q.J.R. Meteorol. Soc., 96:
      247-256
    """
    return 1.03 - 0.062 * D_eq
