Water boils at 100 °C at standard atmospheric pressure, but this temperature decreases with altitude. Since the amount of heat carried out of the engine is a function of coolant temperature and volume, the coolant has to be kept below boiling point and an increasing amount of fluid has to be used, along with an increasingly large radiator to cool it. The solution was to pressurise the entire cooling system, thereby not only preventing the decrease in cooling performance with altitude, but in fact increasing the boiling point even on the ground.