def name_sources(meta, mode):
    """
    From the meta data and the mode, create a filename fragment
    that names the sources used.
    """

    land_source = ''
    ocean_source = ''

    if mode in ('land', 'mixed'):
        land_source = '.GHCN'
    if mode in ('ocean', 'mixed'):
        ocean_source = '.' + meta.ocean_source.upper()

    return ocean_source + land_source