In aviation and aviation meteorology, a flight level (FL) is an aircraft's altitude at standard air pressure, expressed in hundreds of feet. The air pressure is computed assuming an International Standard Atmosphere pressure of 1013.25 hPa (29.92 inHg) at sea level, and therefore is not necessarily the same as the aircraft's actual altitude, either above sea level or above ground level. Background Flight levels are used to ensure safe vertical separation between aircraft, despite natural local variations in atmospheric air pressure. Historically, altitude has been measured using a pressure altimeter, which is essentially a calibrated barometer. An altimeter measures ambient air pressure, which decreases with increasing altitude following the barometric formula. It then calculates and displays the corresponding altitude. If different aircraft's altimeters were not calibrated consistently, then two aircraft could be flying at the same altitude even though their altimeters appeared to show that they were at different altitudes. Flight levels solve this problem by defining altitudes based on a standard air pressure at sea level. All aircraft operating on flight levels calibrate to this setting regardless of the actual sea level pressure. To display true altitude above sea level, a pilot would need to calibrate the altimeter according to the local air pressure at sea level, to take into account natural variation of pressure over time and in different regions. Definition Flight levels are described by a number, which is the nominal altitude, or pressure altitude, in hundreds of feet, while being a multiple of 500 ft, therefore always ending in 0 or 5. Therefore, a pressure altitude of is referred to as "flight level 320". Flight levels are usually designated in writing as FLxxx, where xxx is a two- or three-digit number indicating the pressure altitude in units of . In radio communications, FL290 would be pronounced as "flight level two nine(r) zero." Transition altitude While use of a standardised pressure setting facilitates separation of aircraft from each other, it does not provide the aircraft's actual height above ground. At low altitudes, the altimeter is commonly set to show the altitude above sea level, which can be directly compared to the known elevation of the terrain. The pressure setting to achieve this varies with weather conditions. It is called QNH ("barometric pressure adjusted to sea level"), or "altimeter setting", and the current local value is available from various sources, including air traffic control and the local METAR-issuing