def precipitable_water(temp, vap_press):
    """
    Calculate precipitable water after Prata (1996).

    Parameters
    ----------
    temp : numeric
        Air temperature (K).

    vap_press : numeric
        Vapor pressure (Pa).

    Returns
    -------
    precipitable_water : numeric
        Precipitable water (kg m-2).

    References
    ----------
    .. [1] Prata, A. J. (1996). A new long-wave formula for estimating downward
       clear-sky radiation at the surface. Quarterly Journal of the Royal
       Meteorological Society, 122(533), 1127–1151. doi:10.1002/qj.49712253306
    """
    vap_press_hpa = vap_press / 100
    u = 46.5 * vap_press_hpa / temp  # in g cm-2
    return u * (1e-3 * 1e4)