GetHumRatioFromSpecificHum Function

  function GetHumRatioFromSpecificHum(SpecificHum) result(HumRatio)
    !+ Return the humidity ratio (aka mixing ratio) from specific humidity.
    !+ Reference:
    !+ ASHRAE Handbook - Fundamentals (2017) ch. 1 eqn 9b (solved for humidity ratio)

    real, intent(in)  :: SpecificHum
      !+ Specific humidity in lb_H₂O lb_Air⁻¹ [IP] or kg_H₂O kg_Air⁻¹ [SI]
    real              :: HumRatio
      !+ Humidity ratio in lb_H₂O lb_Dry_Air⁻¹ [IP] or kg_H₂O kg_Dry_Air⁻¹ [SI]

    if (SpecificHum < 0.0 .or. SpecificHum >= 1.0) then
      error stop "Error: specific humidity is outside range [0, 1)"
    end if

    HumRatio = SpecificHum / (1.0 - SpecificHum)

    ! Validity check.
    HumRatio = max(HumRatio, MIN_HUM_RATIO)
  end function GetHumRatioFromSpecificHum