GetHumRatioFromTDewPoint Function

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

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

    VapPres   = GetSatVapPres(TDewPoint)
    HumRatio  = GetHumRatioFromVapPres(VapPres, Pressure)
  end function GetHumRatioFromTDewPoint