GetRelHumFromTDewPoint Function

  function GetRelHumFromTDewPoint(TDryBulb, TDewPoint) result(RelHum)
    !+ Return relative humidity given dry-bulb temperature and dew-point temperature.
    !+ References:
    !+ ASHRAE Handbook - Fundamentals (2017) ch. 1 eqn 22

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

    if (TDewPoint > TDryBulb) then
      error stop "Error: dew point temperature is above dry bulb temperature"
    end if

    VapPres     = GetSatVapPres(TDewPoint)
    SatVapPres  = GetSatVapPres(TDryBulb)
    RelHum      = VapPres / SatVapPres
  end function GetRelHumFromTDewPoint