code: def psychrometric_vapor_pressure_wet(dry_bulb_temperature, wet_bulb_temperature, pressure,
                                       psychrometer_coefficient=6.21e-4 / units.kelvin):
      r
      return (saturation_vapor_pressure(wet_bulb_temperature) - psychrometer_coefficient
              * pressure * (dry_bulb_temperature - wet_bulb_temperature).to('kelvin'))
docs: r"""Calculate the vapor pressure with wet bulb and dry bulb temperatures.
  
      This uses a psychrometric relationship as outlined in [WMO8-2014]_, with
      coefficients from [Fan1987]_.
  
      Parameters
      ----------
      dry_bulb_temperature: `pint.Quantity`
          Dry bulb temperature
      wet_bulb_temperature: `pint.Quantity`
          Wet bulb temperature
      pressure: `pint.Quantity`
          Total atmospheric pressure
      psychrometer_coefficient: `pint.Quantity`, optional
          Psychrometer coefficient. Defaults to 6.21e-4 K^-1.
  
      Returns
      -------
      `pint.Quantity`
          Vapor pressure
  
      Notes
      -----
      .. math:: e' = e'_w(T_w) - A p (T - T_w)
  
      * :math:`e'` is vapor pressure
      * :math:`e'_w(T_w)` is the saturation vapor pressure with respect to water at temperature
        :math:`T_w`
      * :math:`p` is the pressure of the wet bulb
      * :math:`T` is the temperature of the dry bulb
      * :math:`T_w` is the temperature of the wet bulb
      * :math:`A` is the psychrometer coefficient
  
      Psychrometer coefficient depends on the specific instrument being used and the ventilation
      of the instrument.
  
      See Also
      --------
      saturation_vapor_pressure