def dsr_pb(D_eq):
    """
    Pruppacher and Beard drop shape relationship function.

    Arguments:
        D_eq: Drop volume-equivalent diameter (mm)
    Returns:
        r: The vertical-to-horizontal drop axis ratio. Note: the Scatterer class
        expects horizontal to vertical, so you should pass 1/dsr_pb
    """
    return 1.03 - 0.062*D_eq