GetVapPresFromRelHum Function

  function GetVapPresFromRelHum(TDryBulb, RelHum) result(VapPres)
    !+ Return partial pressure of water vapor as a function of relative humidity and temperature.
    !+ References:
    !+ ASHRAE Handbook - Fundamentals (2017) ch. 1 eqn 12, 22

    real, intent(in)  ::  TDryBulb
      !+ Dry-bulb temperature in °F [IP] or °C [SI]
    real, intent(in)  ::  RelHum
      !+ Relative humidity in range [0, 1]
    real              ::  VapPres
      !+ Partial pressure of water vapor in moist air in Psi [IP] or Pa [SI]

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

    VapPres = RelHum * GetSatVapPres(TDryBulb)
  end function GetVapPresFromRelHum