def _find_longitude_range(coords):
    """Finds the start of the longitude range, assumed to be either 0,360 or -180,180
    :param coords: coordinates to check
    :return: starting value for longitude range or None if no longitude coordinate found
    """
    low = None
    for coord in coords:
        if coord.standard_name == 'longitude':
            low = 0.0
            min_val = coord.points.min()
            if min_val < 0.0:
                low = -180.0
    return low
