GetRelHumFromVapPres Function

  function GetRelHumFromVapPres(TDryBulb, VapPres) result(RelHum)
    !+ Return relative humidity given dry-bulb temperature and vapor pressure.
    !+ 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)  ::  VapPres
      !+ Partial pressure of water vapor in moist air in Psi [IP] or Pa [SI]
    real              ::  RelHum
      !+ Relative humidity in range [0, 1]

    if (VapPres < 0.0) then
      error stop "Error: partial pressure of water vapor in moist air cannot be negative"
    end if

    RelHum = VapPres / GetSatVapPres(TDryBulb)
  end function GetRelHumFromVapPres