def ps_weight_timemean(field, ps):
    """
    This takes the surface pressure time mean of atmos_fields
    input:
    field       xr.DataArray or xr.Dataset
    ps          surface pressure field with the same dimensions are field, it does not need the verical coordinates

    return
    same structure are field but time averaged

    """

    return (field * ps).mean('time') /ps.mean('time')