code: def _temp_dew_dirint(temp_dew, times):
      
      if temp_dew is not None:
          # Perez eqn 4
          w = pd.Series(np.exp(0.07 * temp_dew - 0.075), index=times)
      else:
          # do not change unless also modifying _dirint_bins
          w = pd.Series(-1, index=times)
      return w
docs: Calculate precipitable water from surface dew point temp (Perez eqn 4),
      or return a default value for use with :py:func:`_dirint_bins`.