While several methods have been used to control fog on aircraft windows, the most common are electrical and pneumatic. Most of the high performance military aircraft, for which fog control requirements are most severe, use pneumatic systems. These systems may use bleed or ram air, or may use conditioned air from the aircraft environmental control system (ECS). With the introduction of high-pressure water extraction in the aircraft ECS, extremely dry air has become available for fog control. This allows increased design flexibility, making antifog systems, which operate continuously to prevent fog formation, more practical.