the psychrometric chart allows all the parameters of some moist air to be determined from any three independent parameters, one of which must be the pressure. changes in "state", such as when two air streams mix, can be modeled easily and somewhat graphically using the correct psychrometric chart for the location's air pressure or elevation relative to sea level. for locations at not more than 2000 ft (600 m) of altitude it is common practice to use the sea-level psychrometric chart.