The primary instrument used to tell a pilot how high an aircraft is flying is the Altimeter. Most commercial aircraft use a Barometric Altimeter, which measures the outside atmospheric pressure through a "static port." As the plane climbs, the air pressure drops; the altimeter translates this pressure change into a height reading, usually in feet or meters above Sea Level (ASL). Because air pressure also changes with the weather, pilots must calibrate the altimeter to the local "altimeter setting" provided by air traffic control. In addition to the barometric version, planes use a Radio Altimeter (or Radar Altimeter) during landing. This device sends a radio pulse to the ground and measures how long it takes to bounce back, providing an extremely accurate "Height Above Ground Level" (AGL) reading, which is critical for the final moments of touchdown. In modern "Glass Cockpits," these readings are displayed as a moving "altitude tape" on the primary flight display directly in front of the pilot.