def check_lonlat(lon, lat):
    """
    Raise a ``ValueError`` if ``lon`` and ``lat`` do not represent a valid 
    longitude-latitude pair.
    Otherwise, return nothing.
    """
    if not -180 <= lon <= 180:
        raise ValueError('Longitude {!s} is out of bounds'.format(lon))
    if not -90 <= lat <= 90:
        raise ValueError('Latitude {!s} is out of bounds'.format(lat))
