def get_var_name(nc):
    """Guesses the variable_name of an open NetCDF file
    """
    non_variable_names = ['lat', 'lat_bnds', 'lon', 'lon_bnds', 'time',
        'latitude', 'longitude', 'bnds']
    _vars = set(nc.variables.keys())
    _vars.difference_update(set(non_variable_names))
    if len(_vars) == 1:
        return _vars.pop()
    return None
