A surface of constant atmospheric pressure that is related to a specific pressure datum (1013. 25 hPa) and is separated from other such surfaces by specific pressure intervals. It is conventionally given a numerical value to the nearest 1000 ft in units of 100 ft in accordance with the structure of the ICAO Standard Atmosphere. For example, the 500-hPa level is written as FL 180, the ICAO standard height being 18 289 ft.