GetTDewPointFromHumRatio Function

  function GetTDewPointFromHumRatio(TDryBulb, HumRatio, Pressure) result(TDewPoint)
    !+ Return dew-point temperature given dry-bulb temperature, humidity ratio, and pressure.
    !+ Reference:
    !+ ASHRAE Handbook - Fundamentals (2017) ch. 1

    real, intent(in)  ::  TDryBulb
      !+ Dry-bulb temperature in °F [IP] or °C [SI]
    real, intent(in)  ::  HumRatio
      !+ Humidity ratio in lb_H₂O lb_Air⁻¹ [IP] or kg_H₂O kg_Air⁻¹ [SI]
    real, intent(in)  ::  Pressure
      !+ Atmospheric pressure in Psi [IP] or Pa [SI]
    real              ::  TDewPoint
      !+ Dew-point temperature in °F [IP] or °C [SI]
    real              ::  VapPres
      !+ Partial pressure of water vapor in moist air in Psi [IP] or Pa [SI]

    if (HumRatio < 0.0) then
      error stop "Error: humidity ratio cannot be negative"
    end if

    VapPres   = GetVapPresFromHumRatio(HumRatio, Pressure)
    TDewPoint = GetTDewPointFromVapPres(TDryBulb, VapPres)
  end function GetTDewPointFromHumRatio