def _virtual_columns(self) -> Dict[str, str]:
        """
        Get the dict of virtual data columns
        """
        return {
            k: v
            for d in self._columns
            if isinstance(d, dict)
            for k, v in d.items()
            if isinstance(v, Callable)
        }

        Get the dict of virtual data columns
        
meteostat/interface/meteodata.py