def create_url(index, source):
    """
    Return the valid URL for download

    :param variable: string
    :param level: string
    :param date: datetime
    :return: sring
    """
    if source == 'NOAA':
        base_url = (
            'https://www.esrl.noaa.gov/psd/data/correlation/{index}.data')
    else:
        raise ValueError('Source not supported')
    base_url = base_url.format(index=index)
    return base_url
