def _precipitable_water(pair, ea):
    """Precipitable water in the atmosphere (Eq. D.3)

    Parameters
    ----------
    pair : scalar or array_like of shape(M, )
        Air pressure [kPa].
    ea : scalar or array_like of shape(M, )
        Vapor pressure [kPa].

    Returns
    -------
    ndarray
        Precipitable water [mm].

    """
    return pair * 0.14 * ea + 2.1