GetVapPresFromHumRatio Function

  function GetVapPresFromHumRatio(HumRatio, Pressure) result(VapPres)
    !+ Return vapor pressure given humidity ratio and pressure.
    !+ Reference:
    !+ ASHRAE Handbook - Fundamentals (2017) ch. 1 eqn 20 solved for pw

    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              ::  VapPres
      !+ Partial pressure of water vapor in moist air in Psi [IP] or Pa [SI]
    real              ::  BoundedHumRatio
      !+ Humidity ratio bounded to MIN_HUM_RATIO

    if (HumRatio < 0.0) then
      error stop "Error: humidity ratio is negative"
    end if
    BoundedHumRatio = max(HumRatio, MIN_HUM_RATIO)

    VapPres = Pressure * BoundedHumRatio / (0.621945 + BoundedHumRatio)
  end function GetVapPresFromHumRatio