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

    Parameters
    ----------
    ea : ee.Image
        Vapor pressure [kPa].
    pair : ee.Image or ee.Number
        Air pressure [kPa].

    Returns
    -------
    ee.Image or ee.Number
        Precipitable water [mm].


    Notes
    -----
    w = pair * 0.14 * ea + 2.1

    """
    return ea.multiply(pair).multiply(0.14).add(2.1)